Re: [Haskell-cafe] [Haskell] ANN: Cabal v1.18.0 released
On Mon, Sep 16, 2013 at 10:16 AM, Henning Thielemann lemm...@henning-thielemann.de wrote: On Wed, 4 Sep 2013, Johan Tibell wrote: * GHCi support. It's now much easier to use ghci when developing your packages, especially if those packages require preprocessors (e.g. hsc2hs). That's a great feature! How can I configure Cabal to start ghci with certain options? I like to enable more warnings. I could not find a documentation for the Cabal config file. I assume a .ghci file, either in the project directory, or in your home directory, still works. See also http://www.haskell.org/ghc/docs/latest/html/users_guide/ghci-dot-files.html Regards, Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hoogle vs Hayoo
On Thu, Aug 22, 2013 at 9:23 PM, Mateusz Kowalczyk fuuze...@fuuzetsu.co.uk wrote: On 22/08/13 19:30, jabolo...@google.com wrote: Hi, I noticed Hayoo appears as a link in the toolbox of http://hackage.haskell.org and also that Hayoo seems to display better results than Hoogle. For example, if you search for 'PublicKey' in Hayoo, you will get several results from Hackage libraries, such as, 'crypto-pubkey' and 'crypto-api'. However, the same query in Hoogle displays no results. Is Hayoo the default Hackage search engine ? Is Hoogle deprecated ? What the status ? You could also try the Hoogle hosted by FPComplete guys, it indexes more stuff. Note that the 'normal' hoogle indexes all (?) of hackage. But by default it only searches the haskell platform. You can add a package with '+' to search in that package. E.g. PublicKey +crypto-api. Regards, Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Alternative name for return
On Thu, Aug 15, 2013 at 5:39 AM, Jason Dagit dag...@gmail.com wrote: Also, if anyone wants to look at prior art first, Idris supports applicative brackets. As does she [0]. Erik [0] https://personal.cis.strath.ac.uk/conor.mcbride/pub/she/idiom.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Value-weak hash tables in Haskell ?
I'm not sure, but there are weak pointer [0], though I have never used them. Erik [0] http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html On Sat, Aug 10, 2013 at 7:13 PM, Aleksey Uymanov s9gf4...@gmail.com wrote: Hello, haskellers. Is there any package implementing magic hash tables weak in value? I mean when the value is garbage collected, then this key+value authomatically removes from the hash table. -- Aleksey Uymanov s9gf4...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] deriving Data.HashTable - stack overflow
There is a ticket with discussion and a patch here [0].
Erik
[0] http://ghc.haskell.org/trac/ghc/ticket/7633
On Thu, Aug 8, 2013 at 8:11 PM, David Thomas davidleotho...@gmail.com wrote:
I do wish there was a compiler-checked way of specifying a minimum complete
definition.
On Thu, Aug 8, 2013 at 11:02 AM, Joey Adams joeyadams3.14...@gmail.com
wrote:
On Thu, Aug 8, 2013 at 12:22 PM, Lyle Kopnicky li...@qseep.net wrote:
...
So I went to the Data.Hashable page and looked up examples on how to
derive a Hashable instance for my datatype:
http://hackage.haskell.org/packages/archive/hashable/latest/doc/html/Data-Hashable.html
The problem occurs even when using the sample code on the page:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics (Generic)
import Data.Hashable
data Colour = Red | Green | Blue
deriving Generic
instance Hashable Colour
If I then type `hash Red` I get a stack overflow.
I am using the Haskell Platform, so I have hashable-1.1.2.5, but I notice
the docs are for hashable-1.2.0.10. If I install 1.2.0.10 though, other code
in my project breaks - seems like one part doesn't recognize the instances
from another part. So I'll stick with the platform version.
...
Generic support was added in hashable-1.2. Before then, the default
implementations for `hash` and `hashWithSalt` were written in terms of each
other:
hash = hashWithSalt defaultSalt
hashWithSalt salt x = salt `combine` hash x
Because you did not give an implementation for either of these, both
default implementations were used, leading to a loop.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Alternative name for return
What about `pure`? It's already used in applicative, and has the motivation that it's embedding a pure value in some context. Since I don't know the details of your project, I don't know if you need two names (one for the applicative version, and one for the monadic version). Erik On Tue, Aug 6, 2013 at 7:14 AM, J. Stutterheim j.stutterh...@me.com wrote: Dear Cafe, Suppose we now have the opportunity to change the name of the `return` function in Monad, what would be a better name for it? (for some definition of better) N.B. I am _not_ proposing that we actually change the name of `return`. I do currently have the opportunity to pick names for common functions in a non-Haskell related project, so I was wondering if there perhaps is a better name for `return`. - Jurriën ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: Non-recursive let
On Fri, Jul 26, 2013 at 6:44 PM, Andreas Abel andreas.a...@ifi.lmu.de wrote: mapSnd f = (id *** f) As a very small aside, this is just `second` from Control.Arrow. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] TH splicing and recompilation checking
There is a GHC ticket about implementing a DEPENDS pragma for doing this [0]. There are patches attached, but it looks like it isn't finished yet. Erik [0] http://ghc.haskell.org/trac/ghc/ticket/4900 On Tue, Jul 16, 2013 at 7:41 PM, Johannes Waldmann waldm...@imn.htwk-leipzig.de wrote: Hi. we are using template Haskell to splice in some code that is produced by reading and transforming the contents of another file. now, if this other file is touched (by editing), but not the main file, then ghc (and cabal) do not realize that the main file does need to be recompiled. is there a way to tell them about the dependency? (example main file: https://github.com/apunktbau/co4/blob/master/CO4/Test/Queens.hs) - J.W. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Correct way to catch all exceptions
Hi Michael, We do this as well. In addition to AsyncException, we ignore BlockedIndefinitelyOnSTM, BlockedIndefinitelyOnMVar and Deadlock. I'm not sure you can ignore Timeout, since the type is not exported from System.Timeout. I'm not sure how to classify these, though. They are in some sense non-recoverable: restarting whatever the thread was doing is not the right thing to do. Erik On Wed, Jul 10, 2013 at 8:28 AM, Michael Snoyman mich...@snoyman.com wrote: There's a pattern that arises fairly often: catching every exception thrown by code. The naive approach is to do something like: result - try someCode case result of Left (e :: SomeException) - putStrLn $ It failed: ++ show e Right realValue - useRealValue This seems perfectly valid, except that it catches a number of exceptions which seemingly should not be caught. In particular, it catches the async exceptions used by both killThread and timeout. I think it's fair to say that there's not going to be a single function that solves all cases correctly, but it is a common enough situation that people need to write code that resumes work in the case of an exception that I think we need to either have some guidelines for the right approach here, or perhaps even a utility function along the lines of: shouldBeCaught :: SomeException - Bool One first stab at such a function would be to return `False` for AsyncException and Timeout, and `True` for everything else, but I'm not convinced that this is sufficient. Are there any thoughts on the right approach to take here? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Correct way to catch all exceptions
On Wed, Jul 10, 2013 at 10:39 AM, John Lato jwl...@gmail.com wrote: I think 'shouldBeCaught' is more often than not the wrong thing. A whitelist of exceptions you're prepared to handle makes much more sense than excluding certain operations. Some common whitelists, e.g. filesystem exceptions or network exceptions, might be useful to have. You'd think that, but there are common use cases. For example, if you have a queue of work items, and a thread (or threads) processing them, it is useful to catch all exceptions of these threads. You can then log the exception, remove the item from the queue and put it in some error bucket, and continue on to the next item. The same goes for e.g. socket listening threads etc. The thing here is that you are *not* actually handling the specific exception, but instead failing gracefully. But you still want to be able to kill the worker threads, and you don't want to handle exceptions that you cannot recover from even by moving on to the next work item. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
The constraint on an instance never influences which instance is
selected. So as far as instance selection goes, 'instance Foo x' and
'instance C x = Foo x' are the same. The constraint is only checked
after the instance is selected.
Erik
On Fri, Jul 5, 2013 at 2:43 PM, Nicholls, Mark nicholls.m...@vimn.com wrote:
Hello,
I largely don’t know what I’m doing or even trying to do, it is a voyage
into the unknown….but….if I go…
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class Foo x y | x - y, y - x
instance Foo Integer Integer
That seems to work….and my head seems to say…your created some sort of
binary relation between 2 types…and made Integer,Integer a member of it…
Something like that anyway….
Then I go….
data Bar
instance Foo Bar x
Error!but I’m think I understand this….I can’t claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my
functional dependencies, because Bar,Integer is now a member of Foo..
Bad programmer…….
So how I naively go….
class NotAnInteger a
instance (NotAnInteger x) = Foo Bar x
I haven’t declared integer to be “NotAnInteger”….so (in a closed
world)….this would seem to exclude the contradiction….but…
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12
So
i)I clearly don’t understand something about the type
system.
ii) I don’t know how to restrict type variables in
instance declarations….i.e. how do I use the notion of “Foo” across
different combinations of types, without them colliding.
CONFIDENTIALITY NOTICE
This e-mail (and any attached files) is confidential and protected by
copyright (and other intellectual property rights). If you are not the
intended recipient please e-mail the sender and then delete the email and
any attached files immediately. Any further use or dissemination is
prohibited.
While MTV Networks Europe has taken steps to ensure that this email and any
attachments are virus free, it is your responsibility to ensure that this
message and any attachments are virus free and do not affect your systems /
data.
Communicating by email is not 100% secure and carries risks such as delay,
data corruption, non-delivery, wrongful interception and unauthorised
amendment. If you communicate with us by e-mail, you acknowledge and assume
these risks, and you agree to take appropriate measures to minimise these
risks when e-mailing us.
MTV Networks International, MTV Networks UK Ireland, Greenhouse,
Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions
International, Be Viacom, Viacom International Media Networks and VIMN and
Comedy Central are all trading names of MTV Networks Europe. MTV Networks
Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks
Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent,
London, NW1 8TT.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Catch multiple exceptions using 'Control.Exception'
Perhaps you can use `catches` [0]? Erik [0] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exception.html#v:catches On Wed, Jul 3, 2013 at 12:47 PM, Nikita Karetnikov nik...@karetnikov.org wrote: I'm trying to update a package that uses 'Control.OldException' (works with GHC 6.10.4). Here is a relevant (and simplified) part: import Control.OldException -- | A predicate matching synchronous exceptions. syncExceptions :: Exception - Maybe Exception syncExceptions (AsyncException _) = Nothing syncExceptions e = Just e throwAsync :: IO a throwAsync = throwIO $ AsyncException StackOverflow throwArith :: IO a throwArith = throwIO $ ArithException DivideByZero 'syncExceptions' is usually used like this: *Main tryJust syncExceptions throwArith Left divide by zero *Main tryJust syncExceptions throwAsync -- pass through *** Exception: stack overflow The above doesn't work with GHC 7.6.3 because 'Control.OldException' [1] was removed. And 'Control.Exception' doesn't have the 'Exception' type. Is there a way to adapt 'syncExceptions' to work with 'Control.Exception'? [1] http://hackage.haskell.org/packages/archive/base/4.1.0.0/doc/html/Control-OldException.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: haskell-names-0.1
Tom Lokhorst and myself worked on a tool to generate missing imports, both qualified and unqualified, at the Odessa hackathon. We created a working proof of concept [0]. I wasn't aware of fix-imports. Erik [0] https://github.com/haskell-suite/halberd On Tue, Jun 25, 2013 at 6:30 AM, Evan Laforge qdun...@gmail.com wrote: This is neat, it sounds like I could use this with fix-imports to find only modules that export the right function name, or even to add non-qualified imports. But since it's already 95% good enough for my use case, I probably won't get around to it any time soon. On Thu, Jun 20, 2013 at 8:13 AM, Roman Cheplyaka r...@ro-che.info wrote: I am pleased to announce the first public release of haskell-names, a name resolution library for haskell-src-exts AST. Namely, it can do the following: * for a module, compute its interface, i.e. the set of entities exported by the module, together with their original names. * for each name in the module, figure out what it refers to — whether it's bound locally (say, by a where clause) or globally (and then give its origin). Thanks to haskell-packages, this library is fully integrated with Cabal, so that you can easily generate name interfaces for any Cabalized package. See more details in the README: http://documentup.com/haskell-suite/haskell-names This library is based on the code written by Lennart Augustsson in 2010. Little of that code survived, but nevertheless it's been a great help. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Automating Hackage accounts
On Thu, Jun 13, 2013 at 4:22 PM, Tobias Dammers tdamm...@gmail.com wrote: On Thu, Jun 13, 2013 at 05:07:38PM +0300, Mihai Maruseac wrote: On Thu, Jun 13, 2013 at 5:02 PM, Tobias Dammers tdamm...@gmail.com wrote: On Thu, Jun 13, 2013 at 09:44:03AM -0400, Andrew Pennebaker wrote: Could we add an HTML form for creating new Hackage accounts? Right now, our community is small enough that emailing r...@soi.city.ac.uk and waiting for a manual response isn't too bad of a problem, but as we grow, it would be nice for these sorts of things to be handled by a server, like with RubyGems and NPM. IMHO, a more pressing issue is SSL uploads and package signing. As it stands, anyone with a Hackage account can upload a new version of any given package, and some wire-sniffing is enough to reveal a legit user's password. I'd try to solve the latest two things first before going into creating a specific form. On the other hand, maybe we can rig something up with Yesod or similar to solve all three points at the same time. I'm busy now with my masters disertation but I can attempt something in a month if it seems ok and no one else does it before that date. IIRC, there have been previous attempts, or at least a discussion. I can't remember what the result was, though. Either way, it'll take more than just a Yesod web application built over a weekend; signed packages would require package authors to, well, sign, so cabal would need features for that; you'd also have to extend it to *check* those signatures, and give the user options to refuse or allow unsigned packages. SSL should be relatively simple though, mostly a matter of updating cabal's configuration and installing a suitable certificate on the hackage server. There have been numerous discussions about this already. One of the tricky things is that cabal uses the HTTP package for http calls, and it doesn't support SSL. Adding it is non-trivial on windows, I believe. As for the user account creation and uploading packages you don't own, Hackage 2 (any day now) has fixes for both. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Automating Hackage accounts
On Thu, Jun 13, 2013 at 4:48 PM, Niklas Hambüchen m...@nh2.me wrote: As for the user account creation and uploading packages you don't own, Hackage 2 (any day now) has fixes for both. Does Hackage 2 have SSL at least for the web interface? I think it should be possible to set that up by proxying through e.g. Apache. You have to be careful to open up all urls 'cabal' accesses over http as well, but otherwise, I don't see a problem with that setup. I'm not quite sure what it would achieve, though. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: How to do automatic reinstall of all dependencies?
I think --reinstall only reinstalls the package you are actually installing, not the dependencies. You could try using a sandboxing tool, like cabal-dev. Then you just do 'cabal-dev install', and when you want to reinstall everything, you do 'rm cabal-dev/' to wipe the sandbox and start over. Regards, Erik On Thu, Apr 25, 2013 at 12:29 AM, capn.fre...@gmail.com capn.fre...@gmail.com wrote: -db - Forwarded message - From: Captain Freako capn.fre...@gmail.com Date: Tue, Apr 23, 2013 9:21 pm Subject: How to do automatic reinstall of all dependencies? To: haskell-cafe@haskell.org Hi all, Does anyone know why the following is not working, as an automatic way of reinstalling all dependencies?: dbanas@dbanas-lap:~/prj/AMI-Tool$ cabal install --only-dependencies --reinstall --force-reinstalls parsec Resolving dependencies... All the requested packages are already installed: Use --reinstall if you want to reinstall anyway. dbanas@dbanas-lap:~/prj/AMI-Tool$ Thanks, -db ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: [Haskell-beginners] Monad instances and type synonyms
On Sun, Apr 14, 2013 at 9:28 AM, Chris Wong chrisyco+haskell-c...@gmail.com wrote: On Sun, Apr 14, 2013 at 5:10 PM, Christopher Howard christopher.how...@frigidcode.com wrote: type Adjustment a = SaleVariables - a [...] instance Monad Adjustment where (=) = ... return = ... Essentially, you can't partially apply type synonyms. I don't recall the exact reasoning, but if this sort of thing was allowed it would probably poke funny holes in the type system. Also, Control.Monad.Instances already supplies a Monad instance for functions (r - a). So even if that did pass, you'd bump into overlapping instances anyway. The fact that that instance exists shows that you can define an instance like this (even though you don't have to, since it already exists). The trick is to define the type synonym partially applied. When you do that, you can define the instance: type Adjustment = (-) SaleVariables instance Monad Adjustment where Note that this needs the extensions TypeSynonymInstances and FlexibleInstances. Erik. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A question about data declaration
You could use a GADT:
{-# LANGUAGE GADTs #-}
data T a where
C1 :: Int - T Int
C2 :: Char - T Char
C3 :: T Char - T Char
This will allow you to put a C3 in a C3. If you want to prevent that,
just invent some other index, something like:
{-# LANGUAGE GADTs, EmptyDataDecls #-}
data Yes
data No
data T a where
C1 :: Int - T No
C2 :: Char - T Yes
C3 :: T Yes - T No
Not sure if this is a *better* way though. Your initial solution is
also ok, I guess.
Regards,
Erik
On Thu, Mar 21, 2013 at 1:48 PM, C K Kashyap ckkash...@gmail.com wrote:
Hi,
I have a situation where I need to define a data type T such that
data T = C1 Int | C2 Char | C3 T
However, I want to enforce a constraint that C3 only allows (C2 Char) and
not (C1 Int). That is
x = C3 (C1 10) -- should not compile - but my above definition will let it
compile
I was thinking of this -
data C1 = C1 Int
data C2 = C2 Char
data T = TC1 C1 | TC1 C2 | TC3 C2
Is there a better way to do it?
Regards,
Kashyap
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How does one create an input handle bound to a string instead of a file?
Is your parser impure? I would expect a function from String/Text/ByteString to Maybe (SExpr Pos). Then you have no need for a Handle. Regards, Erik On Thu, Feb 28, 2013 at 2:32 PM, John D. Ramsdell ramsde...@gmail.com wrote: I think I wasn't clear about my question. I want something that creates a value of type System.IO.Handle. You see, I have a high performance S-expression parser that I'd like to use in GHCi reading strings while at the command loop. Here is more details on my module SExpr that exports the SExpr data type and the load function. The desired function is called stringHandle. -- An S-expression data SExpr = S String -- A symbol | Q String -- A quoted string | N Int-- An integer | L [SExpr a] -- A proper list -- Read one S-expression or return Nothing on EOF load :: Handle - IO (Maybe (SExpr Pos)) In GHCi, I want to type something like: SExpr let h = stringHandle () SExpr load h Just (L []) SExpr load h Nothing SExpr It seems to me right now that I have to implement a duplicate parser that implements Read. At least S-expression parsing is easy. John On Thu, Feb 28, 2013 at 3:02 AM, Ganesh Sittampalam gan...@earth.li wrote: Hi, On 27/02/2013 20:38, John D. Ramsdell wrote: How does one create a value of type System.IO.Handle for reading that takes its input from a string instead of a file? I'm looking for the equivalent of java.io.StringReader in Java. Thanks in advance. http://hackage.haskell.org/package/bytestring-handle can make handles that read and write to ByteStrings. Cheers, Ganesh ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Nomyx 0.1 beta, the game where you can change the rules
Note that cookies are not the solution here. Cookies are just as user controlled as the url, just less visible. What you need is a session id: a mapping from a non-consecutive, non-guessable, secret token to the user id (which is sequential and thus guessable, and often exposed in urls etc.). It doesn't matter if you then store it in the url or a cookie. Cookies are just more convenient. Erik On Wed, Feb 27, 2013 at 3:30 PM, Corentin Dupont corentin.dup...@gmail.com wrote: Yes, having a cookie to keep track of the session if something I plan to do. On Wed, Feb 27, 2013 at 3:16 PM, Mats Rauhala mats.rauh...@gmail.com wrote: The user id is not necessarily the problem, but rather that you can impose as another user. For this, one solution is to keep track of a unique (changing) user token in the cookies and use that for verifying the user. -- Mats Rauhala MasseR -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) iEYEARECAAYFAlEuFVQACgkQHRg/fChhmVMu3ACeLLjbluDQRYekIA2XY37Xbrql tH0An1eQHrLLxCjHHBQcZKmy1iYxCxTt =tf0d -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to input Unicode string in Haskell program?
You can also set the locale encoding for a handle (e.g. System.IO.stdin) from code using `System.IO.hSetEncoding` [0]. Erik [0] http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-IO.html#v:hSetEncoding On Thu, Feb 21, 2013 at 12:07 PM, Alexander V Vershilov alexander.vershi...@gmail.com wrote: The problem is that Prelude.getLine uses current locale to load characters: for example if you have utf8 locale, then everything works out of the box: $ runhaskell 1.hs résumé 履歴書 резюме résumé 履歴書 резюме But if you change locale you'll have error: LANG=C runhaskell 1.hs résumé 履歴書 резюме 1.hs: stdin: hGetLine: invalid argument (invalid byte sequence) To force haskell use UTF8 you can load string as byte sequence and convert it to UTF-8 charecters for example by import qualified Data.ByteString as S import qualified Data.Text.Encoding as T main = do x - fmap T.decodeUtf8 S.getLine now code will work even with different locale, and you'll load UTF8 from shell independenty of user input's there -- Alexander On 21 February 2013 13:58, Semyon Kholodnov joker...@gmail.com wrote: Imagine we have this simple program: module Main(main) where main = do x - getLine putStrLn x Now I want to run it somehow, enter résumé 履歴書 резюме and see this string printed back as résumé 履歴書 резюме. Now, the first problem is that my computer runs Windows, which means that I can't use ghci :main or result of ghc main.hs to enter such an outrageous string — Windows console is locked to one specific local code page, and no codepage contains Latin-1, Cyrillic and Kanji symbols at the same time. But there is also WinGHCi. So I do :main, copy-paste this string into the window (It works! Because Windows has Unicode for 20 years now), but the output is all messed up. In a rather curious way, actually: the input string is converted to UTF-8 byte string, and its bytes are treated as being characters from my local code page. So, it appears that I have no way to enter Unicode strings into my Haskell programs by hands, I should read them from files. That's sad, and I refuse to think I am the first one with such a problem, so I assume there is a solution/workaround. Now would someone please tell me this solution? Except from Just stick to 127 letters of ASCII, of course. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Alexander ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How far compilers are allowed to go with optimizations?
On Wed, Feb 6, 2013 at 1:18 PM, Austin Seipp mad@gmail.com wrote: Now, on a slight tangent, in practice, I guess it depends on your target market. C programs don't necessarily expose the details to make such rich optimizations possible. And Haskell programmers generally rely on optimizations to make order of magnitude performance difference in constant factors, although perhaps not in direct big-O terms (and no, this isn't necessarily bad) I think some GHC optimizations do change at least space usage, even in big-O terms. For example, strictness analysis can make folds go from O(n) to O(1) space, I think. Regards, Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] hxt pickling question
There's showPickled [0] and unpickleDoc [1], maybe those help? Erik [0] http://hackage.haskell.org/packages/archive/hxt/latest/doc/html/Text-XML-HXT-Arrow-Pickle-Xml.html#v:showPickled [1] http://hackage.haskell.org/packages/archive/hxt/latest/doc/html/Text-XML-HXT-Arrow-Pickle-Xml.html#v:unpickleDoc On Thu, Jan 24, 2013 at 4:40 PM, bri...@aracnet.com wrote: Hi, Trying to understand how to write a document using a pickler but I can't make sense of the types. From the example: runX ( xunpickleDocument xpSeason [ withValidate no , withTrace 1 , withRemoveWS yes , withPreserveComment no ] simple2.xml processSeason xpickleDocument xpSeason [ withIndent yes ] new-simple2.xml ) So all I want to do is pickle a value directly instead of reading the value from a document. I expected to do something like: runX (someHXTPicklingFunction myValue xpickleDocument ...) but I can't seem to figure out what someHXTPicklingFunction should be, it's certainly nothing obvious like pickleDoc, because that generates the wrong value. Seems like I probably have a much more fundamental problem in that I really don't understand how the arrow part of this little example really works, but I was kind of hoping that doing something simple like this might shed some light on that. And then was immediately stuck in type hell. Thanks, Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to disable document in .cabal file?
You could do what I do: create an alias in bash. I have in my ~/.bashrc: alias ciq='cabal install --disable-documentation --disable-library-profiling --disable-executable-profiling' The alias 'ciq' stands for 'cabal install quick' and disables things I don't need during development, but do slow down the build. Regards, Erik On Wed, Jan 16, 2013 at 3:10 AM, Magicloud Magiclouds magicloud.magiclo...@gmail.com wrote: So the only way is to use param every time I build this certain project? Really hoping I could disable it in project.cabal On Wed, Jan 16, 2013 at 6:45 AM, Albert Y. C. Lai tre...@vex.net wrote: On 13-01-15 12:06 AM, Magicloud Magiclouds wrote: I have enabled document in .cabal/config, so I could get document every time I installed libraries or so. But when I compile my own applications, it also takes time on generating non-exist documents. How to disable it just for this project? If cabal install: add --disable-documentation. See also cabal install --help. If cabal configure and cabal build etc: nothing to do, just omit cabal haddock. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- 竹密岂妨流水过 山高哪阻野云飞 And for G+, please use magiclouds#gmail.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Example programs with ample use of deepseq?
On Wed, Jan 9, 2013 at 11:38 PM, Joachim Breitner m...@joachim-breitner.de wrote: Am Mittwoch, den 09.01.2013, 15:11 +0100 schrieb Erik Hesselink: We finally solved the problems by completely moving to strict map operations, strict MVar/TVar operations, and strict data types. do you mean strict by policy (i.e. before storing something in a [MT]Var, you ensure it is evaluated) or by construction (by `seq` or `deepseq`’ing everything before it is stored)? In the latter case: Seq or deeqseq? Again in the latter case: Do you worry about the performance of repeatedly deepseq’ing an already deepseq’ed and possibly large value? Strict by construction: we `seq` when storing in a var. This is similar to the primed functions in some places, and the new Data.Map.Strict. So repeatedly deepseq'ing isn't a problem: deepseq in our case is only used for debugging. You are not the first user of Haskell who ends up with that approach to lazy evaluation. I’m not sure what that means for Haskell, though. Well, we still use the 'normal' lazy approach in most places. Only where we have persistent shared state do we use the above approach of making everything strict. This means updaters pay the computational price, not readers, which seems like a sane idea. Regards, Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Example programs with ample use of deepseq?
On Tue, Jan 8, 2013 at 10:54 PM, Joachim Breitner m...@joachim-breitner.de wrote: Am Dienstag, den 08.01.2013, 13:01 -0800 schrieb Evan Laforge: surprisingly, deepseq is not used as much as I thought. http://packdeps.haskellers.com/reverse/deepseq lists a lot of packages, but (after grepping through some of the code) most just define NFData instances and/or use it in tests, but rarely in the „real“ code. For some reason I expected it to be in more widespread use. I've been using deepseq quite a bit lately, but for the purpose of debugging space leaks. If, when I deepseq a big structure, the space leak goes away, I can then apply it to some subset. After much trial-and-error I can find something which is insufficiently strict. Ideally I can then strictify that one thing and stop using the deepseq. I wish there was a more efficient way to do this. this is also a possible application of my approach, by providing a „I want this data structure to be fully evaluated now, please tell me how it currently looks, i.e. where in the data structure still thunks lie hidden.“ Do you have a nice, striking example where using your approach (using deepseq and comparing efficiency) you could make a difference, and where a tool as described above would make the analysis much easier? We've also used this approach to debug space-leaks, and would have loved such a tool. We used deepseq, and compared the heap profiles. We finally found the leaks this way, and fixed them using strictness annotations. Finally, we verified by running deepseq again at top level and observing that it didn't change anything anymore. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Example programs with ample use of deepseq?
On Wed, Jan 9, 2013 at 2:40 PM, Joachim Breitner m...@joachim-breitner.de wrote: Hi Erik, Am Mittwoch, den 09.01.2013, 14:23 +0100 schrieb Erik Hesselink: We've also used this approach to debug space-leaks, and would have loved such a tool. We used deepseq, and compared the heap profiles. We finally found the leaks this way, and fixed them using strictness annotations. Finally, we verified by running deepseq again at top level and observing that it didn't change anything anymore. same question to you: Would you have a suitable test case that can be used to test and demonstrate the usefulness of such tools? Sadly, no. This is a private, core part of our application that I cannot share. I can describe what we did, and also the structure of the data in broad terms: we have several Maps, where the keys are usually Text, and the values are custom data types. These contain keys into these maps again. The whole thing defines a graph with several indexes into it. We finally solved the problems by completely moving to strict map operations, strict MVar/TVar operations, and strict data types. If you have more questions, or tools you want to test, I'd be happy to help, though. Regards, Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Equality test between types that returns type-level Bool ?
If you're up for it, Oleg has a lot of interesting material about this subject [1]. Regards, Erik [1] http://okmij.org/ftp/Haskell/typeEQ.html On Sun, Nov 25, 2012 at 9:36 AM, Takayuki Muranushi muranu...@gmail.comwrote: Is it possible to write type family SameType a b :: Bool which returns True if a and b are the same type, and False otherwise? I encountered this problem when I was practicing promoted lists and tuples in ghc-7.6.1. One of my goal for practice is to write more modular version of extensible-dimensional calculations, and to understand whether ghc-7.6.1 is capable of it. http://hackage.haskell.org/packages/archive/dimensional/0.10.2/doc/html/Numeric-Units-Dimensional-Extensible.html Some of my attempts: https://github.com/nushio3/dimensional-tf/blob/master/attic/list-02.hs This fails because :==: is not an equality test between a and b, but is a equality test within a (promoted) kind. https://github.com/nushio3/dimensional-tf/blob/master/attic/list-03.hs This fails because type instance declarations are not read from top to bottom. (not like function declarations.) https://github.com/nushio3/dimensional-tf/blob/master/attic/map-03.hs I could define a lookup using class constraints, but when I use it, results in overlapping instances. So, will somebody teach me which of the following is correct? * We can write a type family SameType a b :: Bool * We cannot do that because of theoretical reason (that leads to non-termination etc.) * We cannot write SameType, but there are ways to write functions like 'filter' and 'merge' , over type-level lists, without using SameType. Always grateful to your help, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to determine correct dependency versions for a library?
On Wed, Nov 14, 2012 at 10:57 AM, Roman Cheplyaka r...@ro-che.info wrote: * Ivan Lazar Miljenovic ivan.miljeno...@gmail.com [2012-11-14 20:53:23+1100] Doesn't this prevent the error of this package won't build (even if the error message doesn't precisely say that)? Yeah, it replaces one error with another. How is it supposed to help me if I really want to build this package? Instead of fixing just the code, I now have to fix the cabal file as well! The error might be clearer, since it comes up right away, and points you to the right package, together with the reason (doesn't support the right base version). If it started to build instead, it might fail in the middle, with some error that you might not know is caused by changes in base. But the question comes down to numbers: how often do packages break with new base versions, how soon do people need to be able to use the new GHC without changing other packages, etc. Some might argue that packages 'usually' work, so we should leave out upper bounds, even if it gives worse errors. Others say the errors are so bad, or badly timed, that we should have upper bounds, and the work for maintainers, while greater, is not too large. I know what I think, but nobody has concrete numbers about breakages with new base versions, amount of time spent updating packages, unupdated packages etc. Some can be found with a grep over the hackage tarball, but most can't. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] local type denotation
You need to enable ScopedTypeVariables, and add a forall to introduce the
type variable at the top level. The local variable will then be the *same*
'a' instead of a fresh one:
{-# LANGUAGE ScopedTypeVariables #-}
data D a = D1 a | D2 a (a - a)
f :: forall a. Eq a = D a - a
f (D1 x) = x
f (D2 x g) = let y :: Eq a = a
y = g x
in if x == y then x else g y
main = putStr $ shows (f (D2 (1 :: Int) succ)) \n
On Wed, Nov 14, 2012 at 1:03 PM, Serge D. Mechveliani mech...@botik.ruwrote:
Please,
how to correctly set an explicit type for a local value in the body of
a polymorphic function?
Example (tested under ghc-7.6.1):
data D a = D1 a | D2 a (a - a)
f :: Eq a = D a - a
f (D1 x) = x
f (D2 x g) = let -- y :: Eq a = a
y = g x
in if x == y then x else g y
main = putStr $ shows (f (D2 (1 :: Int) succ)) \n
This is compiled byghc --make Main
Now I need, for a certain reason, to explicitly set the type for y in
`let', with the meaning:
this very `a' which is in the signature for f
(and I think that this type Haskell assignes to y in y = g x).
I need to declare this type in a separate line: y :: what ever it is.
Both `y :: a' and `y :: Eq a = a' are not compiled.
Please, copy the answer to mech...@botik.ru
Thanks,
--
Sergei
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to determine correct dependency versions for a library?
tl;dr: Breakages without upper bounds are annoying and hard to solve for package consumers. With upper bounds, and especially with sandboxes, breakage is almost non-existent. I don't see how things break with upper bounds, at least in the presence of sandboxes. If all packages involved follow the PVP, a build that worked once, will always work. Cabal 0.10 and older had problems here, but 0.14 and later will always find a solution to the dependencies if there is one (if you set max-backjumps high enough). Without sandboxes, cabal might have to reinstall a package with different dependencies, breaking other packages. It will currently warn against this. Future versions will hopefully tell you about the sandboxing features that can also be used to avoid this. In contrast, without upper bounds a packages is almost sure to fail to compile at some point. A user will then get compile errors outside his own code, somewhere in the middle of his dependency chain. Depending on his expertise, this will be either hard or impossible to fix. In particular, it is not clear that too lenient dependencies are the problem, and if it is clear, you do not know which ones. So I still see this as a tradeoff between the interests of package developers/maintainers (upper bounds give more testing/release work, and also, they want to use the latest versions) versus package users/companies (who want a build to work, but often don't care about the bleeding edge). Personnaly, I fall in both groups, and have experienced both problems. At Silk, we have (internal) packages with huge dependency chains (we depend on both happstack and snap). With cabal 0.10, this was a nightmare. Since 0.14, we've had no breakages, except from packages that do not specify upper bounds! We're fairly up-to-date with GHC versions: we're on 7.4 now, but with no immediate plans to switch to 7.6. Switching to a new GHC version is a bit of work, but we can decide when to do the work. Without upper bounds, our builds can break at any moment, and we have to fix it then and there to be able to continue working. If you do have to use the bleeding edge (or a packages uses really outdated dependencies) you can also use sandboxes to your advantage. Just 'cabal unpack' the problematic package, change the dependencies and add the resulting source package to your sandbox. This is what we do when we test out a new GHC version. We also try to contribute fixes back upstream. This is why I ask for people to specify upper bounds. They mean that packages keep working, and they prevent users from getting incomprehensible, badly timed build failures. Erik On Sat, Nov 10, 2012 at 5:16 PM, Roman Cheplyaka r...@ro-che.info wrote: * Erik Hesselink hessel...@gmail.com [2012-11-10 16:40:30+0100] On Fri, Nov 9, 2012 at 5:52 PM, Roman Cheplyaka r...@ro-che.info wrote: * Janek S. fremenz...@poczta.onet.pl [2012-11-09 17:15:26+0100] but I am aware that if the library were to be released on Hackage I would have to supply version numbers in the dependencies. The question is how to determine proper version numbers? With the current state of affairs, your best bet is not to specify any version bounds, or specify only lower ones. Upper version bounds much more often break things that fix things. I'd like to ask people not to do this. What you're doing then is moving the burden from the maintainer (who has to test with new versions and relax dependencies) to the users of the library (who will be faced with breakages when new incompatible dependencies are released). The trouble is, when things break, they break either way — so I simply propose to reduce the probability of things breaking. I know, I know, the theory is that Cabal magically installs the right versions for you. However, in practice it often can't do that due to various reasons (base libraries, package reinstalls etc.) I'm not trying to shift the burden from the maintainer — I simply know from experience that maintainers are not nearly as quick and responsible as would be required for such scheme to work, so as a *user* I'd prefer that they omit upper bounds — at least until the --ignore-constraints option is implemented. (https://github.com/haskell/cabal/issues/949) Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to determine correct dependency versions for a library?
On Mon, Nov 12, 2012 at 5:13 PM, Johan Tibell johan.tib...@gmail.comwrote: On Mon, Nov 12, 2012 at 1:06 AM, Erik Hesselink hessel...@gmail.comwrote: tl;dr: Breakages without upper bounds are annoying and hard to solve for package consumers. With upper bounds, and especially with sandboxes, breakage is almost non-existent. I don't see how things break with upper bounds, at least in the presence of sandboxes. If all packages involved follow the PVP, a build that worked once, will always work. Cabal 0.10 and older had problems here, but 0.14 and later will always find a solution to the dependencies if there is one (if you set max-backjumps high enough). The breakage people are talking about with regards to upper bounds is that every time a new version of a dependency comes out, packages with upper bounds can't compile with it, even if they would without the upper bound. For example, the version number of base is bumped with almost every GHC release, yet almost no packages would actually break to the changes that caused that major version number to go up. Yes, this is why I talk about living on the bleeding edge, and shifting the burden from package maintainers to package users. And I believe the last base changes included a change to 'catch' which would have broken a lot of packages. The Num changes also caused a lot of code changes, and there were probably more I don't remember. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to determine correct dependency versions for a library?
On Fri, Nov 9, 2012 at 5:52 PM, Roman Cheplyaka r...@ro-che.info wrote: * Janek S. fremenz...@poczta.onet.pl [2012-11-09 17:15:26+0100] but I am aware that if the library were to be released on Hackage I would have to supply version numbers in the dependencies. The question is how to determine proper version numbers? With the current state of affairs, your best bet is not to specify any version bounds, or specify only lower ones. Upper version bounds much more often break things that fix things. I'd like to ask people not to do this. What you're doing then is moving the burden from the maintainer (who has to test with new versions and relax dependencies) to the users of the library (who will be faced with breakages when new incompatible dependencies are released). Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Security] Put haskell.org on https
While I would love to have hackage available (or even forced) over https, I think the biggest reason it currently isn't, is that cabal would then also need https support. This means the HTTP library would need https support, which I've heard will be hard to implement cross-platform (read: on Windows). However, I guess providing https as an option is still a huge step forwards compared to the current situation. Erik On Sun, Oct 28, 2012 at 1:20 AM, Niklas Hambüchen m...@nh2.me wrote: (I have mentioned this several times on #haskell, but nothing has happened so far.) Are you aware that all haskell.org websites (hackage, HaskellWiki, ghc trac) allow unencrypted http connections only? This means that everyone in the same Wifi can potentially - read you passwords for all of these services - abuse your hackage account and override arbitrary packages (especially since hackage allows everybody to override everything) I propose we get an SSL certificate for haskell.org. I also offer to donate that SSL certificate (or directly create it using my Startcom account). Niklas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Security] Put haskell.org on https
I think it is only needed for 'cabal upload'. So if you upload via the web only, you'd never send your password over plain HTTP. Erik On Sun, Oct 28, 2012 at 1:38 PM, Petr P petr@gmail.com wrote: Erik, does cabal need to do any authenticated stuff? For downloading packages I think HTTP is perfectly fine. So we could have HTTP for cabal download only and HTTPS for everything else. Best regards, Petr Pudlak 2012/10/28 Erik Hesselink hessel...@gmail.com: While I would love to have hackage available (or even forced) over https, I think the biggest reason it currently isn't, is that cabal would then also need https support. This means the HTTP library would need https support, which I've heard will be hard to implement cross-platform (read: on Windows). However, I guess providing https as an option is still a huge step forwards compared to the current situation. Erik On Sun, Oct 28, 2012 at 1:20 AM, Niklas Hambüchen m...@nh2.me wrote: (I have mentioned this several times on #haskell, but nothing has happened so far.) Are you aware that all haskell.org websites (hackage, HaskellWiki, ghc trac) allow unencrypted http connections only? This means that everyone in the same Wifi can potentially - read you passwords for all of these services - abuse your hackage account and override arbitrary packages (especially since hackage allows everybody to override everything) I propose we get an SSL certificate for haskell.org. I also offer to donate that SSL certificate (or directly create it using my Startcom account). Niklas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: cabal-install-1.16.0 (and Cabal-1.16.0.1)
On Wed, Oct 3, 2012 at 6:06 PM, Johan Tibell johan.tib...@gmail.com wrote: On the behalf of the many contributors to cabal, I'm proud to present cabal-install-1.16.0. Congratulations and thanks to all those involved! To install: cabal update cabal install cabal-install-1.16.0 Cabal-1.16.0.1 It seems that hackage no longer has a preferred-version for cabal, so that a simple 'cabal update cabal install cabal-install' will do the trick. Was this intentional? With 0.14, it took quite a while for it to become the preferred version. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Copying a syntax tree
You could add {-# LANGUAGE DeriveFunctor #-}, and then add 'deriving
Functor' to all your data types (or you could of course manually
define your functor instances). Then what you want is just 'fmap
show'.
Erik
On Tue, Oct 2, 2012 at 9:55 AM, Sergey Mironov ier...@gmail.com wrote:
Hi! I have a syntax tree defined like this:
data A a = A Int Int (B a)
data B a = B String String (C a)
data C a = C Int Int (D a)
and so on, all the data are parametrized with a type variable. This variable
is actually used as a field in the very end of a hierarchy:
data D a = D Int a
Now I have to write a function which would copy (A Int) to (A String). Is it
possible to do so using TH/syb without writing
copyA (A i1 i2 b) = A i1 i2 (copyB b)
copyB = ...
copyC = ...
..
copyD (D i a) = D i (show a)
?
Could you provide me with a hint?
Thanks,
Sergey
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 64-bit vs 32-bit haskell platform on Mac: misleading notice on Platform website?
On Wed, Sep 26, 2012 at 10:58 AM, Johan Tibell johan.tib...@gmail.com wrote: On Wed, Sep 26, 2012 at 7:44 AM, Carter Schonwald carter.schonw...@gmail.com wrote: To the best of my knowledge there is absolutely no reason to use the 32bit haskell on OS X (aside from memory usage optimization cases which likely do not matter to the *typical* user), and the community should probably update the recommendation to reflect this. The source of the recommendation are the benchmark results presented here: http://mtnviewmark.wordpress.com/2011/12/07/32-bits-less-is-more/ The problem of missing 32-bit C libraries might be a good reason for us to recommend 64-bit though and leave the 32-bit recommendation to people who know what they are doing. We switched to a 64bit GHC recently for this exact reason. The 64bit libraries are either already installed, or can easily be installed through e.g. brew. For 32bit libraries, we sometimes had to compile from source, passing all kinds of flags. The downside for us is doubling the memory usage, but that's more easily solved (with more memory). I haven't noticed the performance reduction. So in short, I think the 64bit version should be the default recommendation on OS X. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Transforming a ADT to a GADT
On Fri, Sep 14, 2012 at 2:01 PM, Sean Leather leat...@cs.uu.nl wrote: On Fri, Sep 14, 2012 at 12:45 PM, Florian Lorenzen wrote: I'd like to transform a value of an ADT to a GADT. Suppose I have the simple expression language data Exp = Lit Int | Succ Exp | IsZero Exp | If Exp Exp Exp and the GADT data Term t where TLit :: Int - Term Int TSucc :: Term Int - Term Int TIsZero :: Term Int - Term Bool TIf :: Term Bool - Term ty - Term ty - Term ty that encodes the typing rules. Now, I'd like to have a function typecheck :: Exp - Maybe (Term t) typecheck exp = ... that returns the GADT value corresponding to `exp' if `exp' is type correct. It's not pretty, but it should still be safe... import Control.Applicative import Unsafe.Coerce tcInt :: Exp - Maybe (Term Int) tcInt (Lit i) = pure (TLit i) tcInt (Succ e) = TSucc $ tcInt e tcInt (If c e1 e2) = TIf $ tcBool c * tcInt e1 * tcInt e2 tcInt _ = empty tcBool :: Exp - Maybe (Term Bool) tcBool (IsZero e) = TIsZero $ tcInt e tcBool (If c e1 e2) = TIf $ tcBool c * tcBool e1 * tcBool e2 tcBool _= empty typecheck :: Exp - Maybe (Term t) typecheck e = forget $ tcInt e | forget $ tcBool e where forget :: Term a - Term b forget = unsafeCoerce I don't think this is safe. What will happen if you evaluate typecheck (Lit 1) :: Maybe (Term Bool) In general, I think you have to work inside an existential. So you hide the type of the parsed Term inside an existential. If you want to apply functions to this Term, you unpack, call the function, and repack. I don't think there's a way around this, since the type parameter to Term _is_ existential. You know there is some type, but you don't know what it is. If you make it polymorphic, the _called_ can choose it, which is the opposite of what you want. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Transforming a ADT to a GADT
On Fri, Sep 14, 2012 at 2:27 PM, Erik Hesselink hessel...@gmail.com wrote: In general, I think you have to work inside an existential. So you hide the type of the parsed Term inside an existential. If you want to apply functions to this Term, you unpack, call the function, and repack. Maybe I should expand what I mean by this. Let's say you have: data SomeTerm where SomeTerm :: Term a - SomeTerm Your typecheck function goes: typecheck :: Exp - SomeTerm and you want to apply: transform :: Term t - Term t You should do something like: f (SomeTerm t) = SomeTerm (transform t) Or, more generally: onSomeTerm :: (forall t. Term t - Term t) - SomeTerm - SomeTerm onSomeTerm f (SomeTerm t) = SomeTerm (f t) Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How Type inference work in presence of Functional Dependencies
I don't know if this is a bug or not, but the translation to type
families works:
class Foo a where
type FooT a :: *
instance Foo Int where
type FooT Int = Float
f :: Int - FooT Int
f = undefined
g :: Int - Float
g = undefined
h :: Int - FooT Int
h = g
You don't even need the class contexts.
Erik
On Thu, Sep 13, 2012 at 11:18 AM, satvik chauhan
mystic.sat...@gmail.com wrote:
Consider the code below :
{-# LANGUAGE
MultiParamTypeClasses,FlexibleInstances,FunctionalDependencies,UndecidableInstances,FlexibleContexts
#-}
class Foo a c | a - c
instance Foo Int Float
f :: (Foo Int a) = Int - a
f = undefined
Now when I see the inferred type of f in ghci
:t f
f :: Int - Float
Now If I add the following code
g :: Int - Float
g = undefined
h :: (Foo Int a) = Int - a
h = g
I get the error
Could not deduce (a ~ Float)
I am not able to understand what has happened here ? The restriction Foo
Int a should have restricted the type of h to Int - Float as shown in
the inferred type of f.
- Satvik
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Tue, Aug 28, 2012 at 6:09 PM, Bryan O'Sullivan b...@serpentine.com wrote: On Mon, Aug 27, 2012 at 10:52 AM, Bryan O'Sullivan b...@serpentine.com wrote: The reason you're seeing build breakage is that the .cabal files of the broken packages were edited in-place without communicating with any of the package authors. Not to flog a dead horse, but: Just yesterday we had a communication from someone on the Gentoo Linux packaging team that their checksum validation for the bloomfilter package was failing. This problem arose because of the hand-editing of the package, but confusion arose in the bug report due to misattribution of the source of the error. https://github.com/haskell/cabal/issues/1017 Hand-editing uploaded tarballs: just don't do it, kids! Not to flog a dead horse, but: All our builds broke again yesterday due to this bug. The package was iteratee-0.8.9.3, though given the vocal opposition of Bryan O'Sullivan, I won't advocate fixing it in place just now. I've built the test in the Cabal library to reject packages with conditionals in the test-suites section. I'm just not sure if we want to implement this on hackage, and for how long. I'm not quite sure how old this cabal version is that is causing the problems, but the haskell platform it comes with is 2011.2, which means the second quarter of 2011, so that is a little over a year old. It comes with Ubuntu 11.10, which is less than a year old. I was going to argue to support versions of cabal (and GHC) for at least a year. That means that if you're on Ubuntu, which has releases every 6 months, you have 6 months to upgrade. However, that year has already expired for cabal 0.10, or is about to expire if you count the Ubuntu release it came with. So what do others think? Does the haskell community want to support anything other than the bleeding edge? If so, for how long? Regards, Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Build regressions due to GHC 7.6
On Thu, Aug 30, 2012 at 7:26 AM, Bryan O'Sullivan b...@serpentine.com wrote: The FFI now requires constructors to be visible, so CInt has to be imported as CInt(..). I think there was already a warning about this one in GHC 7.4, so there was more time to fix it. Not to say I don't feel your pain, but if deprecating/warning and then removing in the next release isn't good enough, nothing can ever be changed. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Build regressions due to GHC 7.6
Note that this does not work if you want to support multiple versions
of GHC, and might not work in general, since
* The hiding of catch is needed for preludes that still have it, since
otherwise it will probably conflict with the one from
Control.Exception.
* Older versions do not have constructors for the FFI types, leading
to warnings (or errors, I'm not sure).
* Packages might not work with the new bytestring version, since the
API has breaking changes (although they're supposed to be minor).
For the first two, you need to add some CPP, for the last, you need
actual testing.
Erik
On Thu, Aug 30, 2012 at 3:03 PM, Alexander Kjeldaas
alexander.kjeld...@gmail.com wrote:
This is very unfortunate, but this is crucially a tooling issue. I am going
to wave my hands, but..
Ignore the mapreduce in the following video, but look at the use of clang to
do automatic refactoring of C++. This is *incredibly* powerful in dealing
with updates to APIs.
http://www.llvm.org/devmtg/2011-11/videos/Carruth_ClangMapReduce-desktop.mp4
But without all that fancy tech, *just* having all of Hackage source code in
one repository and using perl/regexps, fixing these types of issues is O(1)
instead of O(n).
All of the issues you mention seems to be fixable by a few lines of perl *if
we had the repository*.
[a few hours later]
Actually, I went and downloaded all of hackage, put it into a git repository
and fixed these issues:
Fix catch
perl -ni -e 'print unless /import Prelude hiding \(catch\)/' $(git grep
'import Prelude hiding (catch)')
Fix CInt constructors (lots of other stuff from Foreign.C.Types not fixed
though)
perl -p -i -e 's/^import Foreign.C.Types(.*)CInt([^(])/import
Foreign.C.Types${1}CInt(..)${1}/g' $(git grep -l '^import.*CInt')
Fix bytestring versioning
perl -p -i -e 's/bytestring( +)=([0-9. ]+)([
]*)0.10/bytestring$1=$2${3}0.11/g' $(git grep 'bytestring.* *0\.')
Patch to hackage:
http://ge.tt/6Cb5ErM/v/0
I understand that this doesn't help anyone, but if there was a way fix,
upload, and get *consensus* on a few regexps like this, then doing API
changes wouldn't be such a headache.
Alexander
On 30 August 2012 07:26, Bryan O'Sullivan b...@serpentine.com wrote:
Since the release of the GHC 7.6 RC, I've been going through my packages
and fixing up build problems so that people who upgrade to 7.6 will have a
smooth ride.
Sad to say, my experience of 7.6 is that it has felt like a particularly
rough release for backwards incompatibility. I wanted to quantify the pain,
so I did some research, and here's what I found.
I maintain 25 open source Haskell packages. Of these, the majority have
needed updates due to the GHC 7.6 release:
base16-bytestring
blaze-textual
bloomfilter
configurator
criterion
double-conversion
filemanip
HDBC-mysql
mwc-random
pcap
pool
riak-haskell-client
snappy
text
text-format
text-icu
That's 16 out of 25 packages I've had to update. I've also either reported
bugs on, or had to fix, several other people's packages along the way (maybe
four?). So let's say I've run into problems with 20 out of the combined 29
packages of mine and my upstreams.
The reasons for these problems fall into three bins:
Prelude no longer exports catch, so a lot of import Prelude hiding
(catch) had to change.
The FFI now requires constructors to be visible, so CInt has to be
imported as CInt(..).
bytestring finally got bumped to 0.10, so many upper bounds had to be
relaxed (cf my suggestion that the upper-bounds-by-default policy is
destructive).
It has been a lot of work to test 29 packages, and then modify, rebuild,
and release 20 of them. It has consumed most of my limited free time for
almost two weeks. Worse, this has felt like make-work, of no practical
benefit to anyone beyond scrambling to restore the status quo ante.
If over half of my packages needed fixing, I'm alarmed at the thought of
the effects on the rest of Hackage.
I'm torn over this. I understand and agree with the impetus to improve the
platform by tidying things up, and yet just two seemingly innocuous changes
(catch and FFI) have forced me to do a bunch of running to stand still.
I don't have any suggestions about what to do; I know that it's hard to
estimate the downstream effects of what look like small changes. And so I'm
not exactly complaining. Call this an unhappy data point.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Build regressions due to GHC 7.6
On Thu, Aug 30, 2012 at 7:24 PM, wren ng thornton w...@freegeek.org wrote: On 8/30/12 10:26 AM, Erik Hesselink wrote: * Packages might not work with the new bytestring version, since the API has breaking changes (although they're supposed to be minor). For the first two, you need to add some CPP, for the last, you need actual testing. Actually, that can be more problematic than you suspect. In particular, hsc2hs does not play nicely with Cabal's MIN_VERSION_foo(1,2,3) macros. So if you're using hsc2hs, which is common for certain FFI uses, then you have to put up with the warnings or rely on the __GLASGOW_HASKELL__ macro in lieu of the MIN_VERSION_foo(1,2,3) macros. It's doable, but it's uglier and more hackish than it ought to be. Ah yes, I found this out a while ago on one of my packages. As a tip for those doing this: the value of __GLASGOW_HASKELL__ is xyy, with x the first version component, and y the second. So, say, 7.4.2 has a value of 704, not 742. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] happstack simpleHTTP state monad
The way you wrote it, you run the state transformer once for each
request. So the state will be available within a single request, but
not between requests. If you want to persist state between requests,
you can use one the the mutable variables available (TVar or MVar are
good choices; IORefs also work, but you have to take care about
concurrency, using e.g. atomicModifyIORef). Create them before calling
simpleHTTP, then pass them in to use them in your handler. You can put
them in a ReaderT to avoid passing them, if you want.
Another choice is to use something like a database or the file system
for persistent state. Databases usually handle most concurrency
problems for you, while file systems don't.
Regards,
Erik
On Thu, Aug 30, 2012 at 7:29 PM, Corentin Dupont
corentin.dup...@gmail.com wrote:
Hi all,
I'm trying to make a web server that manages its own state. The user can
issue commands that modifies the state.
I did like below but unfortunatly the state is not keep after a command is
issued...
What is the right way to do it? Is there any example sites with an internal
state with happstack?
data Game = (the state of my game)
type NomicServer = ServerPartT (StateT Game IO)
launchWebServer :: Game - IO ()
launchWebServer initialState = do
putStrLn Starting web server...\nTo connect, drive your browser to
\http://localhost:8000/Login\;
d - getDataDir
simpleHTTP' unpackStateT nullConf $ server d
server :: FilePath - ServerPartT (StateT Game IO) Response
server d sh = mconcat [fileServe [] d, do
decodeBody (defaultBodyPolicy /tmp/
4096 4096 4096)
html - implSite http://localhost:8000/;
nomicSite
return $ toResponse html]
unpackStateT:: Game - UnWebT (StateT Game IO) Response - UnWebT IO
Response
unpackStateT g w = evalStateT w g
--handler for web routes
nomicSite :: Site PlayerCommand (NomicServer Html)
nomicSite = setDefault (Noop 0) Site {
handleSite = \f url - unRouteT (routedNomicCommands url) f
, formatPathSegments = \u - (toPathSegments u, [])
, parsePathSegments = parseSegments fromPathSegments
}
Thanks a lot,
Corentin
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] map over Bijections
If you remove the second argument (which you don't use), you have the
function `liftBij` that is in fclabels.
Erik
On Mon, Aug 27, 2012 at 3:55 PM, Sergey Mironov ier...@gmail.com wrote:
Hi. I need map equivalent for Bijection type which is defined in fclabels:
data Bijection (~) a b = Bij { fw :: a ~ b, bw :: b ~ a }
instance Category (~) = Category (Bijection (~)) where ...
I can define this function as follows:
mapBij :: Bijection (-) a c - Bijection (-) [a] [b] - Bijection (-) [a]
[c]
mapBij b1 b = (map (fw b1)) `Bij` (map (bw b1))
but do I really need to do it explicitly? Can I obtain same result
using some Category combinators or other common stuff?
Sergey
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Mon, Jul 30, 2012 at 3:33 PM, Ross Paterson r...@soi.city.ac.uk wrote: On Mon, Jul 30, 2012 at 01:46:24PM +0100, Niklas Broberg wrote: On Wed, Jul 25, 2012 at 12:22 PM, Ross Paterson r...@soi.city.ac.uk wrote: As I understand it, the plan is to modify the following packages in hackage in-situ to remove the test sections (which contain the troublesome conditionals): HUnit-1.2.5.0 bloomfilter-1.2.6.10 codemonitor-0.1 codemonitor-0.2 fixhs-0.1.4 leksah-server-0.12.0.3 leksah-server-0.12.0.4 leksah-server-0.12.0.5 pqc-0.5 pqc-0.5.1 Does anyone object? No objections, but some impatience. ;-) OK, done. I'm seeing this again, on abstract-deque-0.1.6. Ross, can you fix it again? For the future: is there any way to prevent this from happening? Perhaps a check in hackage? I'd be willing to implement this if people think this is a good idea, and I'm pointed in the right direction. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Mon, Aug 27, 2012 at 7:52 PM, Bryan O'Sullivan b...@serpentine.com wrote: On Mon, Aug 27, 2012 at 9:57 AM, Erik Hesselink hessel...@gmail.com wrote: I'm seeing this again, on abstract-deque-0.1.6. Ross, can you fix it again? Hang on a second. The reason you're seeing build breakage is that the .cabal files of the broken packages were edited in-place without communicating with any of the package authors. I understand that the collective intentions around this were good, but by fixing things without telling anyone, package maintainers have no way to know that anything has happened. Now we are seeing the problem begin to recur as people issue new releases that don't incorporate those changes. So. Let's have a little conversation about how to handle this sustainably before wasting more of Ross's time. Yes, you are right. So the question is how long to support systems with the old cabal 0.10. This is the one included with the previous haskell platform (and thus lots of linux distro's), which is less than a year old. But it's also pretty old, since there weren't any cabal releases for a while. The other question is how useful test suites in a released package are. Aren't they much more useful (and used more often) in source repositories? If we do agree that we want to prevent this problem for a while (which I'm not sure about), we should probably do it by preventing uploads for packages like this. That way, package maintainers will know what is going on, just like with the other 'package quality' issues hackage enforces. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to implement instance of MonadBaseControl IO
I'm not sure if you already have something working, but we have
several in our codebase, all following a similar pattern. For example:
newtype GeoServer a = GeoServer { unGeoServer :: ReaderT
GeoServerState (ServerPartT IO) a }
instance MonadBaseControl IO GeoServer where
newtype StM GeoServer a = StMGeoServer { unStMGeoServer :: StM
(ReaderT GeoServerState (ServerPartT IO)) a }
liftBaseWith f = GeoServer (liftBaseWith (\run - f (liftM
StMGeoServer . run . unGeoServer)))
restoreM = GeoServer . restoreM . unStMGeoServer
Erik
On Wed, Aug 22, 2012 at 9:16 AM, yi huang yi.codepla...@gmail.com wrote:
I have a `newtype Yun a = Yun { unYun :: ReaderT YunEnv (ResourceT IO) a }`
, and i need to define an instance of `MonadBaseControl IO` for it.
Newtype instance deriving don't work here. I guess the answer is simple, i
just can't figure it out, hope anybody can lightening me.
Best regards.
Yihuang.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Rigid skolem type variable escaping scope
On Wed, Aug 22, 2012 at 10:13 PM, Matthew Steele mdste...@alum.mit.edu wrote:
On Aug 22, 2012, at 3:02 PM, Lauri Alanko wrote:
Quoting Matthew Steele mdste...@alum.mit.edu:
{-# LANGUAGE Rank2Types #-}
class FooClass a where ...
foo :: (forall a. (FooClass a) = a - Int) - Bool
foo fn = ...
newtype IntFn a = IntFn (a - Int)
bar :: (forall a. (FooClass a) = IntFn a) - Bool
bar (IntFn fn) = foo fn
In case you hadn't yet discovered it, the solution here is to unpack the
IntFn a bit later in a context where the required type argument is known:
bar ifn = foo (case ifn of IntFn fn - fn)
Hope this helps.
Ah ha, thank you! Yes, this solves my problem.
However, I confess that I am still struggling to understand why unpacking
earlier, as I originally tried, is invalid here. The two implementations are:
1) bar ifn = case ifn of IntFn fn - foo fn
2) bar ifn = foo (case ifn of IntFn fn - fn)
Why is (1) invalid while (2) is valid? Is is possible to make (1) valid by
e.g. adding a type signature somewhere, or is there something fundamentally
wrong with it? (I tried a few things that I thought might work, but had no
luck.)
I can't help feeling like maybe I am missing some small but important piece
from my mental model of how rank-2 types work. (-: Maybe there's some paper
somewhere I need to read?
Look at it this way: the argument ifn has a type that says that *for
any type a you choose* it is an IntFn. But when you have unpacked it
by pattern matching, it only contains a function (a - Int) for *one
specific type a*. At that point, you've chosen your a.
The function foo wants an argument that works for *any* type a. So
passing it the function from IntFn isn't enough, since that only works
for *one specific a*. So you pass it a case expression that produces a
function for *any a*, by unpacking the IntFn only inside.
I hope that makes sense (and is correct...)
Erik
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Platform Versioning Policy: upper bounds are not our friends
I am strongly against this, especially for packages in the platform. If you fail to specify an upper bound, and I depend on your package, your dependencies can break my package! For example, say I develop executable A and I depend on library B == 1.0. Library B depends on library C = 0.5 (no upper bound). Now C 0.6 is released, which is incompatible with B. This suddenly breaks my build, even though I have not changed anything about my code or dependencies. This goes against the 'robust' aspect mentioned as one of the properties of the Haskell platform, and against the Haskell philosophy of correctness in general. This is not an imaginary problem. At my company, we've run into these problems numerous times already. Since we also have people who are not experts at Cabal and the Haskell ecosystem building our software, this can be very annoying. The fix is also not trivial: we can add a dependency on a package we don't use to all our executables or we can fork the library (B, in the example above) and add an upper bound/fix the code. Both add a lot of complexity that we don't want. Add to that the build failures and associated emails from CI systems like Jenkins. I can see the maintenance burder you have, since we have to do the same for our code. But until some Cabal feature is added to ignore upper bounds or specify soft upper bounds, please follow the PVP, also in this regard. It helps us maintain a situation where only our own actions can break our software. Erik On Wed, Aug 15, 2012 at 9:38 PM, Bryan O'Sullivan b...@serpentine.com wrote: Hi, folks - I'm sure we are all familiar with the phrase cabal dependency hell at this point, as the number of projects on Hackage that are intended to hack around the problem slowly grows. I am currently undergoing a fresh visit to that unhappy realm, as I try to rebuild some of my packages to see if they work with the GHC 7.6 release candidate. A substantial number of the difficulties I am encountering are related to packages specifying upper bounds on their dependencies. This is a recurrent problem, and its source lies in the recommendations of the PVP itself (problematic phrase highlighted in bold): When publishing a Cabal package, you should ensure that your dependencies in the build-depends field are accurate. This means specifying not only lower bounds, but also upper bounds on every dependency. I understand that the intention behind requiring tight upper bounds was good, but in practice this has worked out terribly, leading to depsolver failures that prevent a package from being installed, when everything goes smoothly with the upper bounds relaxed. The default response has been for a flurry of small updates to packages in which the upper bounds are loosened, thus guaranteeing that the problem will recur in a year or less. This is neither sensible, fun, nor sustainable. In practice, when an author bumps a version of a depended-upon package, the changes are almost always either benign, or will lead to compilation failure in the depending-upon package. A benign change will obviously have no visible effect, while a compilation failure is actually better than a depsolver failure, because it's more informative. This leaves the nasty-but-in-my-experience-rare case of runtime failures caused by semantic changes. In these instances, a downstream package should reactively add an upper bound once a problem is discovered. I propose that the sense of the recommendation around upper bounds in the PVP be reversed: upper bounds should be specified only when there is a known problem with a new version of a depended-upon package. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] fclabels 0.5
Untested, but this should be about right: osi (Bij f b) = iso (Bij b f) Erik On Mon, Aug 20, 2012 at 2:35 PM, Sergey Mironov ier...@gmail.com wrote: Hi. I'm porting old code, which uses fclabels 0.5. Old fclabels define Iso typeclass as follows: class Iso f where iso :: a :-: b - f a - f b iso (Lens a b) = osi (b - a) osi :: a :-: b - f b - f a osi (Lens a b) = iso (b - a) Newer one defines iso: class Iso (~) f where iso :: Bijection (~) a b - f a ~ f b instance Arrow (~) = Iso (~) (Lens (~) f) where iso bi = arr ((\a - lens (fw bi . _get a) (_set a . first (bw bi))) . unLens) instance Arrow (~) = Iso (~) (Bijection (~) a) where iso = arr . (.) but no osi. I'm not a guru in categories, can you help me define osi? Thanks Sergey. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Platform Versioning Policy: upper bounds are not our friends
Hub looks interesting, I'll have to try it out (though I'm not on an RPM based distro). But isn't this the goal of things like semantic versioning [0] and the PVP? To know that you can safely upgrade to a bugfix release, and relavily safely to a minor release, but on a major release, you have to take care? Haskell makes it much easier to see if you can use a new major (or minor) version of a library, since the type checker catches many (but not all!) problems for you. However, this leads to libraries breaking their API's much more easily, and that in turn causes the problems voiced in this thread. However, fixing all versions seems like a bit of a blunt instrument, as it means I'll have to do a lot of work to bring even bug fixes in. Erik [0] http://semver.org/ On Mon, Aug 20, 2012 at 3:13 PM, Chris Dornan ch...@chrisdornan.com wrote: I think we should encourage stable build environments to know precisely which package versions they have been using and to keep using them until told otherwise. Even when the types and constraints all work out there is a risk that upgraded packages will break. Everybody here wants cabal to just install the packages without problem, but if you want to insulate yourself from package upgrades surely sticking with proven combinations is the way to go. Chris -Original Message- From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-boun...@haskell.org] On Behalf Of Erik Hesselink Sent: 20 August 2012 08:33 To: Bryan O'Sullivan Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Platform Versioning Policy: upper bounds are not our friends I am strongly against this, especially for packages in the platform. If you fail to specify an upper bound, and I depend on your package, your dependencies can break my package! For example, say I develop executable A and I depend on library B == 1.0. Library B depends on library C = 0.5 (no upper bound). Now C 0.6 is released, which is incompatible with B. This suddenly breaks my build, even though I have not changed anything about my code or dependencies. This goes against the 'robust' aspect mentioned as one of the properties of the Haskell platform, and against the Haskell philosophy of correctness in general. This is not an imaginary problem. At my company, we've run into these problems numerous times already. Since we also have people who are not experts at Cabal and the Haskell ecosystem building our software, this can be very annoying. The fix is also not trivial: we can add a dependency on a package we don't use to all our executables or we can fork the library (B, in the example above) and add an upper bound/fix the code. Both add a lot of complexity that we don't want. Add to that the build failures and associated emails from CI systems like Jenkins. I can see the maintenance burder you have, since we have to do the same for our code. But until some Cabal feature is added to ignore upper bounds or specify soft upper bounds, please follow the PVP, also in this regard. It helps us maintain a situation where only our own actions can break our software. Erik On Wed, Aug 15, 2012 at 9:38 PM, Bryan O'Sullivan b...@serpentine.com wrote: Hi, folks - I'm sure we are all familiar with the phrase cabal dependency hell at this point, as the number of projects on Hackage that are intended to hack around the problem slowly grows. I am currently undergoing a fresh visit to that unhappy realm, as I try to rebuild some of my packages to see if they work with the GHC 7.6 release candidate. A substantial number of the difficulties I am encountering are related to packages specifying upper bounds on their dependencies. This is a recurrent problem, and its source lies in the recommendations of the PVP itself (problematic phrase highlighted in bold): When publishing a Cabal package, you should ensure that your dependencies in the build-depends field are accurate. This means specifying not only lower bounds, but also upper bounds on every dependency. I understand that the intention behind requiring tight upper bounds was good, but in practice this has worked out terribly, leading to depsolver failures that prevent a package from being installed, when everything goes smoothly with the upper bounds relaxed. The default response has been for a flurry of small updates to packages in which the upper bounds are loosened, thus guaranteeing that the problem will recur in a year or less. This is neither sensible, fun, nor sustainable. In practice, when an author bumps a version of a depended-upon package, the changes are almost always either benign, or will lead to compilation failure in the depending-upon package. A benign change will obviously have no visible effect, while a compilation failure is actually better than a depsolver failure, because it's more informative. This leaves the nasty-but-in-my-experience-rare case of runtime failures caused
Re: [Haskell-cafe] Flipping type constructors
The next version of GHC will have an extension for kind polymorphism. I'm not sure if it has to be enabled in the module that defines flip or in the module that uses it, but it might help. Erik On Tue, Aug 14, 2012 at 1:38 AM, Tony Morris tonymor...@gmail.com wrote: I have a data-type that is similar to EitherT, however, I have ordered the type variables like so: data EitherT (f :: * - *) (a :: *) (b :: *) = ... This allows me to declare some desirable instances: instance Functor f = Bifunctor (EitherT f) instance Foldable f = Bifoldable (EitherT f) instance Traversable f = Bitraversable (EitherT f) However, I am unable to declare a MonadTrans instance: instance MonadTrans (EitherT a) -- kind error I looked at Control.Compose.Flip to resolve this, but it does not appear to be kind-polymorphic. http://hackage.haskell.org/packages/archive/TypeCompose/0.9.1/doc/html/src/Control-Compose.html#Flip I was wondering if there are any well-developed techniques to deal with this? Of course, I could just write my own Flip with the appropriate kinds and be done with it. Maybe there is a more suitable way? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
Isn't this exactly the problem solved by all the lens packages?
Current popular ones are fclabels [0] and data-lens [1].
[0] http://hackage.haskell.org/package/fclabels
[1] http://hackage.haskell.org/package/data-lens
On Thu, Aug 2, 2012 at 7:34 AM, Jonathan Geddes
geddes.jonat...@gmail.com wrote:
Greetings,
tl;dr - What Haskell Records need are
semantic editor combinators for free.
I know this is yet another Record proposal
among many, but none of them out there
strike me as being exactly what I want in
Haskell.
Take the following types from a contrived
example.
type Salary = Integer
data Job = Job
{ title :: String
, salary :: Salary
}
data Person = Person
{ name :: String
, job :: Job
}
Since I've used record syntax, I get
getter/accessor functions (title, salary,
name, job) for free. Now suppose I want to
create an aggregate getter function: return
the salary of a given person. Piece of cake,
it's just function composition
getSalary :: Person - Salary
getSalary = salary . job
Done! Now suppose I want to write a
setter/mutator function for the same nested
field
setSalaryMessy :: Salary - Person - Person
setSalaryMessy newSalary person =
person {
job = (job person) {
salary = newSalary
}
}
Ouch! And that's not even very deeply nested.
Imagine 4 or 5 levels deep. It really makes
Haskell feel clunky next to `a.b.c.d = val`
that you see in other languages. Of course
immutability means that the semantics of
Haskell are quite different (we're creating
new values here, not updating old ones) but
it's still common to model change using these
kinds of updates.
What if along with the free getters that
the compiler generates when we use record
syntax, we also got semantic editor
combinator (SEC) functions[0] that could be
used as follows?
setSalary newSalary = job' $ salary' (const newSalary)
giveRaise amount = job' $ salary' (+amount)
givePercentRaise percent = job' $ salary' (*(1+percent))
For each field x, the compiler generates a
function x' (the tic is mnemonic for change).
These little functions aren't hard to write,
but they're classic boilerplate.
job' :: (Job - Job) - Person - Person
job' f person = person {job = f $ job person}
salary' :: (Salary - Salary) - Job - Job
salary' f job = job { salary = f $ salary job}
These type of utility functions are a dream
when working with any reference type or
State Monad.
modify $ givePercentRaise 0.25
The compiler could also generate polymorphic
SEC functions for polymorphic fields.
Further, the compiler could disallow using
old-style update syntax for fields whose SEC
update function is not in scope, giving us
fine-grained control over access and update.
On the other hand we currently have to create
new functions to achieve this (exporting the
getter means exporting the ability to update
as well, currently).
Of course this doesn't address the
namespacing issues with records, but it is
likely nicely orthogonal to other proposals
which do.
Also note that there's a package on hackage [1]
that will generate SEC functions using TH.
It's nice, but I prefer the style of field
names used above for updaters (field' vs
editField).
Let me know what you think. I'll write up an
official proposal if there's a bit of
general interest around this.
Thanks for reading,
--Jonathan
[0] - http://conal.net/blog/posts/semantic-editor-combinators
[1] -
http://hackage.haskell.org/packages/archive/sec/0.0.1/doc/html/Data-SemanticEditors.html
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
On Thu, Aug 2, 2012 at 12:30 PM, Andrew Butterfield andrew.butterfi...@scss.tcd.ie wrote: On 2 Aug 2012, at 09:25, Erik Hesselink wrote: Isn't this exactly the problem solved by all the lens packages? Current popular ones are fclabels [0] and data-lens [1]. [0] http://hackage.haskell.org/package/fclabels [1] http://hackage.haskell.org/package/data-lens Not sure what all of these do, but I have a simple solution I use in my work: They do exactly that. They create 'lenses' which are getters/setters/modifiers combined, and allow you to compose these to get/set/modify deep inside nested data types. Look at the examples in the fclabels documentation [2] for more details. [2] http://hackage.haskell.org/packages/archive/fclabels/1.1.4/doc/html/Data-Label.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC rendering of non-ASCII characters configurable?
On Wed, Aug 1, 2012 at 2:35 AM, Richard Cobbe co...@ccs.neu.edu wrote: Well, I initially went with String because I didn't want to clutter up my code with all of the calls to 'pack', especially around string literals. I'm open to being convinced that it's worth it to switch, though. For string literals, you can turn on OverloadedStrings to get rid of the calls to 'pack'. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Monday, July 23, 2012, Simon Hengel wrote: On Mon, Jul 23, 2012 at 12:51:32PM -0500, Stephen Paul Weber wrote: Currently you would have to do the upgrade manually, as `cabal-install cabal-install` won't work (or alternatively edit your local ~/.cabl/packages/hackage.haskell.org/00-index.tar). Pending a fix on hackage (hopefully) I've been using `cabal copy cabal register` to grab specific packages today. How would I go about fixing my 00-index.tar? I tride de-tarring, deleting the recent versions of HUnit, and re-tarring, and when I copy that file in cabal starts telling me no packages exist...? E.g. with vim, you can edit files in the tar file (in-place). That is what I did, and it worked fine. I use tar f 00-index.tar --delete Hunit/1.2.5.0/Hunit.cabal Works with GNU tar, not with BSD/mac. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
On Wed, Jul 18, 2012 at 10:19 AM, Ross Paterson r...@soi.city.ac.uk wrote: On Wed, Jul 18, 2012 at 06:50:31AM +0100, Andres Löh wrote: Using --avoid-reinstalls blindly or as a default flag is also unfortunately not a good idea in general. There are simply too many cases where installing older versions of packages (which is often the only thing that helps) is not really the solution you want. That's also the reason why it's not enabled by default. I need a combination of flags that I can use blindly with the greatest chance of success. The default doesn't work on packages like this one: % cabal --version cabal-install version 0.14.0 using version 1.14.0 of the Cabal library % cabal install sbv-2.2 Resolving dependencies... In order, the following would be installed: HUnit-1.2.4.3 (new package) containers-0.5.0.0 (new version) random-1.0.1.1 (new package) strict-concurrency-0.2.4.1 (new package) syb-0.3.7 (new package) template-haskell-2.7.0.0 (reinstall) changes: containers-0.4.2.1 - 0.5.0.0 QuickCheck-2.5 (new package) transformers-0.3.0.0 (new package) mtl-2.1.2 (new package) sbv-2.2 (new package) cabal: The following packages are likely to be broken by the reinstalls: ghc-7.4.1 Use --force-reinstalls if you want to install anyway. I don't think you can install this package on 7.4. As Andres said, it requires containers 0.5, but ghc 7.4's base libraries (in this case, template-haskell) use containers 0.4, and can't be reinstalled. I guess your best bet is to use sbv-2.1, which depends on containers = 0.3, or to unpack it and see if you can loosen the containers dependency and see if it still works with 0.4 So in short, no combination of flags will work in this case, I think. Failure is the best option. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
On Wed, Jul 18, 2012 at 11:29 AM, Ross Paterson r...@soi.city.ac.uk wrote: On Wed, Jul 18, 2012 at 09:35:52AM +0100, Erik Hesselink wrote: I don't think you can install this package on 7.4. As Andres said, it requires containers 0.5, but ghc 7.4's base libraries (in this case, template-haskell) use containers 0.4, and can't be reinstalled. I guess your best bet is to use sbv-2.1, which depends on containers = 0.3, or to unpack it and see if you can loosen the containers dependency and see if it still works with 0.4 I'm talking about unattended automated builds, so tweaking isn't an option. On the other hand breaking the package environment isn't so bad, because I'm throwing it away after each build. We use --force-reinstalls for automated builds as well, inside a cabal-dev sandbox. I think that flag is a fine default for such situations. So in short, no combination of flags will work in this case, I think. Failure is the best option. Actually --force-reinstalls does work in this case, and this thread began with Levent being unhappy with the failure option for his package, so I'm tempted to use that flag on all hackage builds. Does that mean that you *can* reinstall template-haskell? I didn't know that... Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Cabal install fails due to recent HUnit
Hi all, All cabal installs using cabal-install-0.10.2 are currently failing for us. This is due to the cabal file for HUnit-1.2.5.0, which was recently uploaded to hackage. The ouput I'm getting from cabal is just: Reading available packages... Resolving dependencies... cabal: Couldn't read cabal file HUnit/1.2.5.0/HUnit.cabal If I unpack HUnit-1.2.5.0 and call 'cabal configure', I get: cabal: HUnit.cabal:57: The 'type' field is required for test suites. The available test types are: exitcode-stdio-1.0 The relevant lines from the cabal file are: Test-Suite hunit-tests-optimize-0 Type: exitcode-stdio-1.0 These look fine to me. Does anyone have any idea how to go about fixing this (on hackage at least)? Could this package temporarily be removed, to avoid breaking everyone's cabal? Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
Hi Martijn, Yes, upgrading will obviously fix things (we do use 0.14 on our development machines), but we have not set up any infrastructure for building a custom cabal on production servers. We just use the one from the Ubuntu repositories, which uses Cabal 1.10.1.0 on oneiric. So until we upgrade to precise I guess we have a problem. Erik On Wed, Jul 18, 2012 at 5:24 PM, Martijn Schrage mart...@oblomov.com wrote: Hi Erik, A similar thing happened to me with the GraphViz package. As Duncan explained to me, the problem is that Cabal-1.10.0.0 (and I believe also 1.10.1.0) incorrectly reports an error when conditionals are used in test suites. Upgrading to Cabal-1.10.2.0 (or cabal-install-0.14.0 with Cabal-1.14.0) should fix the problem. Unfortunately, this means your build will not work on a fresh Haskell Platform v2012.2.0.0, until HUnit is patched in the hackage index. Cheers, Martijn Schrage -- Oblomov Systems (http://www.oblomov.com) On 18-07-12 16:26, Erik Hesselink wrote: Hi all, All cabal installs using cabal-install-0.10.2 are currently failing for us. This is due to the cabal file for HUnit-1.2.5.0, which was recently uploaded to hackage. The ouput I'm getting from cabal is just: Reading available packages... Resolving dependencies... cabal: Couldn't read cabal file HUnit/1.2.5.0/HUnit.cabal If I unpack HUnit-1.2.5.0 and call 'cabal configure', I get: cabal: HUnit.cabal:57: The 'type' field is required for test suites. The available test types are: exitcode-stdio-1.0 The relevant lines from the cabal file are: Test-Suite hunit-tests-optimize-0 Type: exitcode-stdio-1.0 These look fine to me. Does anyone have any idea how to go about fixing this (on hackage at least)? Could this package temporarily be removed, to avoid breaking everyone's cabal? Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solutions for multi-platform compilation?
We use Jenkins to build our applications. You can have Jenkins slaves for different platforms. We also use cabal-dev to sandbox the builds, separating the environments for different executables. This solution does require one server for every OS you develop for, but I guess you need that anyway, for testing. Erik On Thu, Jul 12, 2012 at 11:43 AM, Ivan Perez ivanperezdoming...@gmail.com wrote: Hi, I work developing multi-platform applications in Haskell. This poses the following problem: I cannot compile binaries for windows from linux (AFAIK). I solved this problem with the following sledgehammer: I installed windows in a VM, I installed GHC, I installed all the C/C++ headers binaries of the libraries that I use (Gtk, OpenGL, SDL, OpenCV, etc.) their Haskell counterparts, and I created several scripts that connect to the VM using SSH, push the changes to the repo, cabal clean cabal install all my packages in sequence without me having to even login into the windows machine. I did this because I was unable to get GHC to run properly in Wine at that time (over 2 years ago). This solution is still unsatisfactory because: 1) It's slow, even though Windows itself works fine (well, as well as windows can work, but it runs at a decent spped, I can play games and all). 2) When I update a library with lots of dependencies, or GHC itself, I have to rebuild almost everything. This is particularly painful with big packages like Gtk, for instance. Because I have to tell cabal where headers and libraries are located, updating a package is almost never an automatic process. I haven't always been able to make GHC just pick them up properly with pkg-config. 3) When I make a change in a library with lots of dependencies, recompiling all the packages can take several hours. I don't think it's a problem with my machine: I'm giving a fair amount of resources to windows, and I use a 3Ghz quadcore with 8GB of RAM. Another relevant fact is: I use this for commercial purposes. I have customers, each requiring a completely different program, they do not have infinite budgets and, if there's a problem in the compilation process and something requires my attention and manual intervention too often, my salary per hour can easily drop to a ridiculous amount. If I'm going to redo this, I'd rather just redo it once. Any suggestions? How do you solve this kind of problem in your work environment? Cheers, Ivan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] how to check thunk
There is also the 'isevaluated' package (which depends on vacuum, but seems to do something more involved than your code). Erik On Mon, Jul 2, 2012 at 7:40 AM, Chaddaï Fouché chaddai.fou...@gmail.com wrote: On Mon, Jul 2, 2012 at 5:29 AM, Kazu Yamamoto k...@iij.ad.jp wrote: Hello, Are there any ways to see if a value is a thunk or memorized? I would like to have a function like: isThunk :: a - IO Bool vacuum allow that and much more though I don't know if it still works correctly on GHC 7.4. Anyway your isThunk is isThunk a = fmap GHC.Vacuum.ClosureType.isThunk GHC.Vacuum.closureType (Feel free to arrange your imports to make that a bit more readable ;) http://hackage.haskell.org/package/vacuum -- Jedaï ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Using promoted lists
Hi Yves,
The type level numbers have kind Nat, not Int (and so also can't be
negative). They have to be imported from GHC.TypeLits (I'm not sure if
this will change). So the following code works for me in HEAD:
{-# LANGUAGE TypeFamilies, DataKinds #-}
import GHC.TypeLits
type family Something a :: Nat
type instance Something String = 42
Regards,
Erik
On Fri, Jun 8, 2012 at 8:45 AM, Yves Parès yves.pa...@gmail.com wrote:
Thanks for your answers, Anthony and Erik.
I'll try with fundeps. I know about HList, but back at the time when I
looked at it I found quite complex.
Anthony, the link you gave me [1] tends to show that actually Bool type is
promoted.
type family Member x (l :: [*]) :: Bool
type instance Member x (x ': xs) = True
works.
So if I understand well, unlike what I thought when I saw the compilation
fail, the two x's type variables are actually unified, but then the second
instance
type instance Member x (y ': xs) = True
encompasses the first, so GHC refuses to handle it (as it would at the value
level with regular case expressions).
So yes, [1] is exactly what I was trying to do.
Out of curiosity, I tried with Int, and it works too, I can express:
type family Something a :: Int
But then, the following doesn't compile
type instance Something String = 42
( The wild guess '42 does not either ) So I guess we don't have type-level
integers for now. How are promoted Ints usable then?
[1] http://hackage.haskell.org/trac/ghc/wiki/NewAxioms
2012/6/8 AntC anthony_clay...@clear.net.nz
Yves Parès yves.pares at gmail.com writes:
The doc page
http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/kind-
polymorphism-and-promotion.html#promotion show that lists are now usable
as
types.So I'm trying to make a type level function to test if a type list
contains a type. Unless I'm wrong, that calls to the use of a type family.
{-# LANGUAGE DataKinds, TypeOperators, KindSignatures, TypeFamilies #-}
data HBool = HTrue | HFalse
-- Mandatory as Bool type is not currently promoted to a kind
type family Member x (l :: [*]) :: HBool
type instance Member x (x ': xs) = HTrue
type instance Member x (y ': xs) = Member x xs
type instance Member x (y ': '[]) = HFalse
But the compiler complains about my instance conflicting.
Hi Yves, always when you're asking a question like this, give the error
message in full -- usually it will explain what's wrong.
In this case I can guess: you have overlapping instances (the first
overlaps
the second, the third overlaps the second), which are not allowed for type
functions (currently -- unless the equations are confluent).
There's some early work on introducing overlaps to type functions (in a
controlled way). http://hackage.haskell.org/trac/ghc/wiki/NewAxioms
And as it happens, several threads are going on in the lists re options
and
implications.
Is what I'm trying to do feasible?
Promoted lists are really just the same as HLists, but using the standard
Haskell syntax.
A membership test is feasible with FunDeps (because they do allow
overlaps),
but I guess you know the HList stuff, judging from your HBool. It's
feasible
using type equality constraints to achieve the same as HList (so ~ is
equivalent to HList's TypeCast), also with overlaps.
Second question: how can type level tuples (also mentioned in the doc
page)
be exploited? Aren't they redundant with type-level lists?
Type-level tuples are fixed length, and provide a flat structure (any
element
is equally accessible), whereas lists are expandable, with a nested
structure
that means you have to scan down the structure to get to the element you
want.
AntC
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ByteString.getContents fails for files 2GB on OS X
Do you have a 32bit or 64bit GHC build? That might have something to do with it, if you're nearing 2^32 (or 2^31) bytes. Erik On Fri, Jun 8, 2012 at 2:25 AM, Shaun Jackman sjack...@gmail.com wrote: Hi, Data.ByteString.Char8.getContents fails for files 2GB on OS X. Is there a fix for this? $ cat getContents.hs main = getContents $ ./getContents smallFile $ ./getContents bigFile getContents: stdin: hGetBuf: invalid argument (Invalid argument) $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 Mac OS X 10.7.4 64-bit As a workaround, I used ByteString.Lazy instead of the strict ByteString, which worked, but found it was ~4 times slower for my program, so I'd like to get the strict ByteString working with large files. Cheers, Shaun ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Using promoted lists
If you want to get rid of the overlap in your type families, you have
to add an extra argument indicating if the two types are equal. For
this, you need a type family to indicate equality of types. Sadly, the
naive implementation (TEQ x x = True, TEQ x y = False) overlaps and
isn't allowed. I'm not sure how to work around this, I guess you do
need FunDeps, and then you are pulled into HList land. See also my
attempt at extensible records [1].
Regards,
Erik
[1] https://gist.github.com/2492939
On Thu, Jun 7, 2012 at 9:52 PM, Yves Parès yves.pa...@gmail.com wrote:
The doc page
http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/kind-polymorphism-and-promotion.html#promotion
show that lists are now usable as types.
So I'm trying to make a type level function to test if a type list contains
a type. Unless I'm wrong, that calls to the use of a type family.
{-# LANGUAGE DataKinds, TypeOperators, KindSignatures, TypeFamilies #-}
data HBool = HTrue | HFalse -- Mandatory as Bool type is not currently
promoted to a kind
type family Member x (l :: [*]) :: HBool
type instance Member x (x ': xs) = HTrue
type instance Member x (y ': xs) = Member x xs
type instance Member x (y ': '[]) = HFalse
But the compiler complains about my instance conflicting. Is what I'm trying
to do feasible?
Second question: how can type level tuples (also mentioned in the doc page)
be exploited? Aren't they redundant with type-level lists?
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Embed Haskell
Hi Rosario,
lhs2tex [1] has the '\eval' command. See section 12 of the manual.
Erik
[1] http://www.andres-loeh.de/lhs2tex/
On Sun, Apr 22, 2012 at 17:59, Rosario Borda rosario.bo...@sinervis.com wrote:
Hi All,
Is there a simple method for embed haskell in other languages, like latex?
An example in quasiquotation notation:
\documentclass{article}
\title{Hello World}
\begin{document}
\maketitle
Hello world! and reversed: [haskell: putStrLn reverse Hello, World!]
\end{document}
Many thanks, :)
Rosario
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] strange GHCi type inference behavior involving map and partially applied functions
GHCi is defaulting the 'a' in 'Show a' to unit because of the extended
defaulting feature [1] in GHCi. If you turn on
NoMonomorphismRestriction in GHCi, you get the same behavior as in
GHC. If you turn on ExtendedDefaulting in GHC, you get the same
behavior as in GHCi.
Erik
[1]
http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-evaluation.html#extended-default-rules
On Sun, Apr 15, 2012 at 22:31, Ting Lei tin...@hotmail.com wrote:
Hi All,
I found a really strange case where GHC and GHCi behave differently in
inferring types. It took me hours to figure this out.
The following program
{-# LANGUAGE NoMonomorphismRestriction #-}
g x i = x ++ show i
[a,b] = map g [X,Y]
will not load without NoMonomorphismRestriction. With that option, it
will load and return the correct types (as expected):
*Main :t [a,b]
[a,b] :: Show a = [a - [Char]]
*Main a 1
X1
However, if I do the same thing GHCi, the type inferencing seems to have
been messed up:
*Main let g x i = x ++ show i
*Main let [a,b] = map g [X,Y]
*Main :t [a,b]
[a,b] :: [() - [Char]]
*Main :t map g [X,Y]
map g [X,Y] :: Show a = [a - [Char]]
Note how in the last two instances the terms on the left and right-hand
sides of the definition return different types. Also, the correct return
type should be a list of unary functions taking an (Show a) as
the parameter. Now the result is unusable:
*Main a 1
interactive:52:3:
No instance for (Num ())
arising from the literal `1'
Possible fix: add an instance declaration for (Num ())
In the first argument of `a', namely `1'
In the expression: a 1
In an equation for `it': it = a 1
I am using GHCi 7.4.1 under windows. I also tried this under GHC 7.0x
Is this a GHCi bug or could anyone please explain why this can of strange
behavior happens?
If this is a bug, could anyone with an account help file a bug for this?
Thanks in advance,
Ting
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generalizing (++) for monoids instead of using ()
See the relevant trac ticket [1] and the linked mailing list thread. Erik [1] http://hackage.haskell.org/trac/ghc/ticket/3339 On Sun, Apr 1, 2012 at 22:58, aditya bhargava bluemangrou...@gmail.com wrote: After asking this question: http://stackoverflow.com/questions/9963050/standard-way-of-joining-two-data-texts-without-mappend I found out that the new infix operator for `mappend` is (). I'm wondering why ghc 7.4 didn't generalize (++) to work on monoids instead. To me, (++) is much more clear. () means not equal to for me. Can anyone shed light on this decision? Adit -- adit.io ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is there a generic way to detect mzero?
On Mon, Mar 26, 2012 at 21:24, Antoine Latter aslat...@gmail.com wrote: On Mon, Mar 26, 2012 at 1:33 PM, Ting Lei tin...@hotmail.com wrote: Hi, I was writing a code trying to use MonadPlus to detect some error cases (representing missing values etc. in pure code). With the Maybe monad, I can do this: can0 :: (a - Maybe b) - a - Bool can0 f x = case f x of Nothing - False Just x - True And I got the expected result: *Main can0 (\x - Just x) 1 True But, when I try to generalize this using MonadPlus, as follows: can :: (MonadPlus m) = (a - m b) - a - Bool can f x = case f x of mzero - False _ - True I got a warning: __testError.hs:31:11: Warning: Pattern match(es) are overlapped In a case alternative: _ - ... Ok, modules loaded: Main. Well, you can sort of do it with only MonadPlus - but it really depends on your choice of Monad whether or not it does anything like what you want: can :: (MonadPlus m) = (a - m ()) - a - m Bool can f x = (f x return True) | return false For 'Maybe' this works great, but for something like 'List' I couldn't even tell you what it would do without reasoning through it. So you might be better off with the suggestion from Tobias using Eq Well, if you accept the following MonadPlus laws: mzero = f == mzero mzero `mplus` m == m Then you can say that for a well-behaving MonadPlus, mzero will return (only) False in that function. However, I don't think it's guaranteed that a non-mzero value will give (only) True. In fact, the list monad will always return the final False. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Haskell Platform Versions Comparison Chart
On Sun, Mar 18, 2012 at 20:28, Simon Hengel s...@typeful.net wrote: ** Not really suer about Mac OS X, but I think it requires Xcode, can someone confirm this. Yes, the platform on Mac OS X requires Xcode, which includes gcc. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is there a better way to subtyping?
However, be aware that aFields, bFields and cFields are now partial
functions that will crash when applied to the wrong constructor. Not
a-okay in my book.
Erik
On Wed, Mar 14, 2012 at 02:24, John Meacham j...@repetae.net wrote:
Why not
data Super
= SuperA {
commonFields :: ()
aFields :: ()
}
| SuperB {
commonFields :: ()
bFields :: ()
}
| SuperC {
commonFields :: ()
cFields :: ()
}
reusing the common field names between constructors like this is a-okay.
John
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Failing to find a function
Not a single name, but I believe liftA2 mplus is the same function, and much shorter (and more general). It uses the Applicative instance for (a -). Of course, it also works with liftM2. Erik On Sun, Feb 19, 2012 at 12:50, Jon Fairbairn jon.fairba...@cl.cam.ac.uk wrote: This is probably a failure of my search fu or some other mental lacuna, but is there already a definition of this function somewhere: \a b - runKleisli $ (Kleisli a) + Kleisli b ? Hoogling for its type MonadPlus m = (a - m b) - (a - m b) - a - m b doesn’t net me anything useful. -- Jón Fairbairn jon.fairba...@cl.cam.ac.uk ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Concurrency strategy for 2 threads and rare events
You could use throwTo to raise an exception in the thread you want to stop. Otherwise, having some variable (IORef, TVar, MVar) that the long running thread occasionally checks seems like a good solution. Erik On Wed, Feb 8, 2012 at 17:04, JP Moresmau jpmores...@gmail.com wrote: Hello, I'm wondering what's the best strategy to use in the following scenario: - 2 threads - One perform some work that will take time, possibly go on forever - Another waits for user input (like commands from the keyboard) that affects thread 1 (causing it to stop, in the simplest case) I've read a bit on MVar and channels, but they seem to be a lot for cases where the reading thread block for input. In my case, I expect to have something that thread 2 updates when an event occur, and thread 1 checks it regularly. So thread 1 should not block, but should check is there something and there is, act on it, otherwise continue doing what it was currently doing. I suppose I could just tryTakeMVar on a MVar, but is there something more adapted to my needs? Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal-1.10.1.0 and bytestring-0.9.2.1 hackage problem.
Same workaround as last time works I.e: tar -f ~/.cabal/packages/hackage.haskell.org/00-index.tar --delete bytestring/0.9.2.1 This will only work until the next 'cabal update', right? Does anyone have a better workaround? This is the cabal-install shipped with Ubuntu 12.04 (i.e. the unreleased beta, which will become the new LTS in April), so buggy or not, it ought not to be broken if we can avoid it. I've filed a bug: It's also the cabal-install shipped with Ubuntu 11.10, i.e. the current version that everyone uses. So it would be nice if the offending version could also be removed from hackage or something similar to prevent this problem for everyone. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] zlib 0.5.3.2 broken?
I recently ran into this as well. I found this stack overflow question [1], where Daniel Fischer notes that a proper solution has been found, and it shouldn't be long until it reaches hackage. That was one and a half weeks ago. Erik [1] http://stackoverflow.com/questions/8961413/zlib-build-error-with-ghc On Thu, Feb 2, 2012 at 06:59, Michael Snoyman mich...@snoyman.com wrote: Hi all, I've received a number of reports of a broken zlib 0.5.3.2 (and experienced it myself). Is this a generally known issue? As a temporary workaround, I've released a new version of zlib-bindings that has an upper bound on zlib to avoid 0.5.3.2 (and later). Once a new patched version is released, I'll release an updated version of zlib-bindings to remove that overly restrictive upper bound. Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data newtype differences. Today: strictness
An interesting use case for this is that while data Void = Void Void has infinitely many values (undefined, Void undefined, Void (Void undefined) etc), the newtype version newtype Void = Void Void has only one, bottom. This is a way to define the empty datatype without extensions. Erik On Tue, Jan 24, 2012 at 11:25, Ketil Malde ke...@malde.org wrote: Yves Parès yves.pa...@gmail.com writes: I had for long thought that data and newtype were equivalent, but then I spotted some differences when it comes to strictness. data Test = Test Int newtype TestN = TestN Int Interesting. I'd thought that data Test = Test !Int and newtype Test = Test Int would be equivalent, but here you (well, I had to add the ! myself) show a situation where they're not. I guess pm (Test _) = 12 is the same as pm = \x - case x of Test _ - 12 which perhaps makes it clearer why it breaks on undefined... -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Spurious pattern match warnings with GADTs
Type classes are open, so nothing prevents someone from adding an instance 'C0 T1' and calling 't' with constructor 'B', causing a crash due to the missing pattern. Erik On Thu, Jan 12, 2012 at 21:40, Tom Hawkins tomahawk...@gmail.com wrote: Let's say I have: data T0 data T1 data T a where A :: T T0 B :: T T1 Then I can write the following without getting any non-exhaustive pattern match warnings: t :: T T0 - String t a = case a of A - A However, if I use type classes to constrain the constructors, instead of using the phantom types directly: class C0 a instance C0 T0 class C1 a instance C1 T1 data T a where A :: C0 a = T a B :: C1 a = T a Then I get a non-exhaustive pattern match warning on 't'. How come? (I'm using GHC 7.0.4) It appears that the intelligent pattern analysis of the first example is a relatively recent addition [1]. -Tom [1] http://hackage.haskell.org/trac/ghc/ticket/3476 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] If you'd design a Haskell-like language, what would you do different?
I would have compose (probably not called '.') read the same way we read this sentence (and unix pipes) ie left to right. You can use from Control.Arrow for that if you want. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: partial-lens 0.0.1
How does this relate to the Maybe lenses in fclabels [1]? Erik [1] http://hackage.haskell.org/packages/archive/fclabels/1.0.4/doc/html/Data-Label-Maybe.html On Wed, Dec 21, 2011 at 04:54, rocon...@theorem.ca wrote: Do you miss null references from your old imperative programming days? Wish that the worlds best imperative language had null references? Now your wishes have come true with the new partial-lens package! partial-lens augment edwardk's data-lens package with partial lens. Partial lenses are like regular lenses but have the possibility of not referencing anything. In other words, null references are possible. One notable different with null references from this package is that you can set them without getting a run-time error. Instead setting a null reference is a no-op; however it is possible to determine if setting failed from the return value of the assignment operation. Actually I don't have any applications for partial lenses myself, so if you find this library useful, please let me know. I wrote this mostly because we know what partial lenses are in theory (they are the coalgebras of the (Identity :+: Store b) comonad) but I wanted to see what a real library would look like. -- Russell O'Connor http://r6.ca/ ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.'' ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Alternative versus Monoid
On Wed, Dec 21, 2011 at 14:10, Bas van Dijk v.dijk@gmail.com wrote:
On 16 December 2011 16:26, Yves Parès limestr...@gmail.com wrote:
1) What about the First type? Do we {-# DEPRECATE #-} it?
Personnaly, I'm in favor of following the same logic than Int:
Int itself is not a monoid. You have to be specific: it's either Sum or
Mult.
It should be the same for Maybe: we remove its instance of Monoid, and we
only use First and Last.
The reason you need to be specific with Int is that it's not clear
which semantics (sum or product) you want. The semantics of Maybe are
clear: it's failure-and-prioritized-choice.
Are you sure? There are (at least) four Monoid instances for Maybe
[1]. With a direct instance for Maybe and its Dual you have only
covered two.
Erik
[1] https://byorgey.wordpress.com/2011/04/18/monoids-for-maybe/
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: partial-lens 0.0.1
On Wed, Dec 21, 2011 at 16:39, rocon...@theorem.ca wrote: On Wed, 21 Dec 2011, Erik Hesselink wrote: How does this relate to the Maybe lenses in fclabels [1]? Erik [1] http://hackage.haskell.org/packages/archive/fclabels/1.0.4/doc/html/Data-Label-Maybe.html It appears to be somewhere between similar and the same. *** Comparison of API Data.Label.Maybe.get corresponds to Data.Lens.Partial.getPL Data.Label.Maybe.set roughly corresponds to Data.Lens.Partial.trySetPL except that trySetPL will bail out early if the reference is null. We can match the signature of set more precisely by: Data.Label.Maybe.set l v r ~ Data.Lens.Partial.trySetPL l r * pure v Data.Label.Maybe.modify would correspond to Data.Lens.Partial.tryModPL if I had implemented it ... which maybe I ought to. Data.Label.Maybe.embed corresponds to a composition of totalLens and maybeLens. More specifically Data.Label.Maybe.embed l ~ Data.Lens.Partial.maybeLens . Data.Lens.Partial.totalLens l Data.Label.MaybeM.gets roughly corresponds to Data.Lens.Partial.Lazy.accessPlus except that accessPlus is particular to StateT because partial-lens is a Haskell 98 compliant package. I need to write partial-lens-fd which will contain a function precisely corresponding to Data.Label.MaybeM.gets I don't have Data.Label.MaybeM.asks, because there was no corresponding functionality in data-lens. We should probably add a version of this. *** Comparison of representation The usual differences between data-lens and fclabels applies to partial-lens as well. The representation for data-lens and partial-lens allows modify to be done with one case analysis on a record since the getter and setters are combined into one coalgebra whereas in fclabels two case analysis must be done: one for the getter and one for the setter. When chains of lenses are composed, I'm told the differences become more apparent. In partial-lens, the combination of getter and setter into a single coalgebraic operations means that the getter and setter are statically forced to return Nothing on the same record; but this is not enforced with the fclabels representation. That said, perhaps the MaybeLens from fclabels is trying to do something different. I don't know what laws you expect to hold for the getter and setters of a maybe lens since it isn't documented (actually I appear to have also forgotten to document the coalgebra laws for a comonad in my package) so perhaps MaybeLens are intended to be more general than partial lenses. For example maybe a user wants to make it illegal to set the birth date to be greater than the death date in a record. In this case getting the birth date will succeed, but setting will fail if the provided birth date out of bounds. This is possible to write using MaybeLens, but is impossible with partial lenses since with partial-lenses either the reference is null, meaning getting and setting both fail, or it is not null which means that getting and setting both succeed. Thanks for the detailed explanation! It seems they are indeed (almost) the same, apart from the differences in representation. The original motivation for the Maybe lenses in fclabels was accessing record fields with Maybe types and composing these lenses (even in the presence of multiple Maybes). It does not come from a category theoretical starting point, hence no laws (yet). Your final example is interesting, I'd never considered doing something like that. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How hard is it to start a web startup using Haskell?
At Silk [1] we use Haskell for the backend of our web application. The frontend is Javascript with some functional aspects, and we have a shallow ruby layer as a website (but not for the actual application). Erik [1] http://www.silkapp.com On Mon, Dec 19, 2011 at 11:04, Ivan Perez ivanperezdoming...@gmail.com wrote: I'm actually trying to make a list of companies and people using Haskell for for-profit real world software development. I'd like to know the names of those startups, if possible. -- Ivan On 18 December 2011 18:42, Michael Snoyman mich...@snoyman.com wrote: On Sun, Dec 18, 2011 at 6:57 PM, Gracjan Polak gracjanpo...@gmail.com wrote: Hi all, The question 'How hard is it to start a technical startup with Haskell?' happened a couple of times on this list. Sometimes it was in the form 'How hard is to find Haskell programmers?' or 'Are there any Haskell jobs?'. I'd like to provide one data point as an answer: http://www.reddit.com/r/haskell/comments/ngbbp/haskell_only_esigning_startup_closes_second_angel/ Full disclosure: I'm one of two that founded this startup. How are others doing businesses using Haskell doing these days? I don't run a startup myself, but I know of at least three startups using Haskell for web development (through Yesod), and my company is basing its new web products on Yesod as well. I think there are plenty of highly qualified Haskell programmers out there, especially if you're willing to let someone work remotely. Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generating Code
On Fri, Dec 9, 2011 at 20:45, L Corbijn aspergesoe...@gmail.com wrote: So I'm interested if there are other libraries that are more suitable to the task of generating haskell code for library use, and thus generate 'human readable' exported code (so no TH). I'm also interested in how other projects generate code for their packages. Since you ask how other packages solve this problem, and since most packages use template haskell, I have to ask: why can't you use template haskell for this? Another option (also not code generation, but very useful in reducing boilerplate) is generic programming, for example using the 'regular' package, or the new generics in GHC 7.2. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting string to System.Time.ClockTime
I'm not sure if you really need ClockTime (from old-time), but if you don't, the types from the 'time' package are all parseable with `parseTime` [1]. Erik [1] http://hackage.haskell.org/packages/archive/time/latest/doc/html/Data-Time-Format.html#v:parseTime On Thu, Dec 8, 2011 at 14:16, dokondr doko...@gmail.com wrote: Hi, What would be the simplest way to convert strings like Wed, 07 Dec 2011 10:09:21 + to System.Time.ClockTime ? Thanks! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to get a file path to the program invoked?
The operator / is an alias for `combine`, which the documentation says:
Combine two paths, if the second path isAbsolute, then it returns the second.
In this case, / is absolute, so it is returned.
If you wish to add a trailing path separator, use `addTrailingPathSeparator`.
Erik
On Mon, Dec 5, 2011 at 15:53, dokondr doko...@gmail.com wrote:
Balazs, thanks for your comments!
The first comment works just fine.
With / operator I get this:
Main System.Environment.Executable System.FilePath /abc / /
/
Instead of getting /abc/ I get /. What am I doing wrong?
On Mon, Dec 5, 2011 at 6:03 PM, Balazs Komuves bkomu...@gmail.com wrote:
Two small comments:
1) This should work on Windows too, if you just leave out the word Posix
from the source:
import System.FilePath (splitFileName)
2) In general when dealing with paths, use the / operator (from
System.FilePath)
instead of ++ / ++
Balazs
On Mon, Dec 5, 2011 at 1:44 PM, dokondr doko...@gmail.com wrote:
This is how I finally solved this problem for POSIX complaint system:
--
-- TestRun
--
module Main where
import System.Cmd (rawSystem)
import System.Directory (getCurrentDirectory)
import System.Environment.Executable (ScriptPath(..), getScriptPath)
import System.FilePath.Posix (splitFileName)
main = do
path - getMyPath
putStrLn $ myPath = ++ path
let cmdLine = path ++ args.sh
rawSystem cmdLine [iphone, test-twitts.txt]
{--
data ScriptPath Source
Constructors:
Executable FilePath it was (probably) a proper compiled executable
RunGHC FilePath it was a script run by runghc/runhaskell
Interactive we are in GHCi
--}
getMyPath = do
curDir - getCurrentDirectory -- from System.Directory
scriptPath - getScriptPath -- from System.Environment.Executable
let path = getMyPath' scriptPath curDir
return path
getMyPath' (Executable path) _ = fst (splitFileName path)
getMyPath' (RunGHC path) _ = fst (splitFileName path)
getMyPath' Interactive curDir = curDir++/
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] writeFile: commitBuffer: invalid argument (Illegal byte sequence)
What is the value of your LANG environment variable? Does it still give the error if you set it to e.g. en_US.UTF-8? Erik On Sun, Dec 4, 2011 at 13:12, dokondr doko...@gmail.com wrote: Correct url of a bad string: http://twitter.com/#!/search/Hoping%20You%20Have%20A%20iPhone%20When%20I%20Do%20This%20lang%3Aen On Sun, Dec 4, 2011 at 3:08 PM, dokondr doko...@gmail.com wrote: Hi, In GHC 7.0.3 / Mac OS X when trying to: writeFile someFile (Hoping You Have A iPhone When I Do This) Lol Sleep Is When You Close These ---gt; \55357\56384 I get: commitBuffer: invalid argument (Illegal byte sequence) The string I am trying to write can also be seen here: http://twitter.com/#!/search/Hoping%20You%20Have%20A%20iPhone%20When%20I%20Do%20This%20lang%3Aen It looks like 'writeFile' can not write unicode characters. Any workarounds? Thanks! Dmitri ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] writeFile: commitBuffer: invalid argument (Illegal byte sequence)
Yes, you can set the text encoding on the handle you're reading this text from [1]. The default text encoding is determined by the environment, which is why I asked about LANG. If you're entering literal strings, see Albert Lai's answer. Erik [1] http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-IO.html#g:23 On Sun, Dec 4, 2011 at 19:13, dokondr doko...@gmail.com wrote: Is there any other way to solve this problem without changing LANG environment variable? On Sun, Dec 4, 2011 at 8:27 PM, Erik Hesselink hessel...@gmail.com wrote: What is the value of your LANG environment variable? Does it still give the error if you set it to e.g. en_US.UTF-8? Erik On Sun, Dec 4, 2011 at 13:12, dokondr doko...@gmail.com wrote: Correct url of a bad string: http://twitter.com/#!/search/Hoping%20You%20Have%20A%20iPhone%20When%20I%20Do%20This%20lang%3Aen On Sun, Dec 4, 2011 at 3:08 PM, dokondr doko...@gmail.com wrote: Hi, In GHC 7.0.3 / Mac OS X when trying to: writeFile someFile (Hoping You Have A iPhone When I Do This) Lol Sleep Is When You Close These ---gt; \55357\56384 I get: commitBuffer: invalid argument (Illegal byte sequence) The string I am trying to write can also be seen here: http://twitter.com/#!/search/Hoping%20You%20Have%20A%20iPhone%20When%20I%20Do%20This%20lang%3Aen It looks like 'writeFile' can not write unicode characters. Any workarounds? Thanks! Dmitri ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (no subject)
On Mon, Nov 28, 2011 at 23:55, Willem O dub...@hotmail.com wrote: And I added this function: createPoint :: Int - Point createPoint x = Point x When I loaded the file containing all this into ghci and executed 'Vector $ map createPoint [1..5]' the result was '(1, 2, 3, 4, 5)' (without the quotes). Note that you do not need this function. You can just use the 'Point' constructor: map Point [1..5] Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Proposed change to mime-mail
Hi all, I've found and solved a problem with mime-mail, and Michael Snoyman asked me to send a request for feedback to -cafe, so here goes. In short, the issue is with address headers containing 'special' (non-ascii) characters. In mime-mail, these are automatically encoded according to RFC 2047. However, the email address part of an address header is not allowed to be encoded. My patch adds 'from', 'to', 'cc' and 'bcc' fields to the Mail data type, and creates correct headers from these fields. This does make the Mail type a bit larger, so I've also added a smart constructor with initializes these fields with empty lists (only 'from' is required). For more details, see my initial bug report [1] and my patches in the pull request [2]. If you have any comments on this change, please let me know. Regards, Erik [1] https://github.com/snoyberg/mime-mail/issues/5 [2] https://github.com/snoyberg/mime-mail/pull/6 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Dutch National FP Day 2012
On Wed, Nov 16, 2011 at 16:19, Henning Thielemann lemm...@henning-thielemann.de wrote: On Wed, 16 Nov 2011, Sean Leather wrote: (Sent on behalf of Doaitse Swierstra) Despite some last minute changes to the planning we are happy to announce that the next Dutch functional programming day will take place on January 6, 2012, at the university campus De Uithof of Utrecht University. In case you want to give a presentation please send title, speaker and abstract to doai...@swierstra.net before Dec 1. What is the language of the talks and the participants? English or Dutch? In past years the language of the talks has always been English. Also, most Dutch people speak English pretty well, I think. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal install: Could not find module `Text.XML.HXT.Arrow'
This is because hSimpleDB doesn't specify version ranges on its dependencies, when it should. Since hxt changed its module structure going from 9.0 to 9.1, hSimpleDB doesn't build against 9.0. You can try to build it by adding '--constraint=hxt==9.0.\*' after your cabal-install command. You can also ask the author to add version ranges to the package. Erik On Tue, Nov 8, 2011 at 11:58, dokondr doko...@gmail.com wrote: Hi, On Mac OSX, ghc-6.12.3, I have successfully installed the 'hxt' package: http://hackage.haskell.org/package/hxt-8.5.2 Registering hxt-9.1.4... Installing library in /Users/user/.cabal/lib/hxt-9.1.4/ghc-6.12.3 Now when I try to install hSimpleDB (http://hackage.haskell.org/package/hSimpleDB) I get the following error: cabal install hSimpleDB ... Registering HTTP-4000.0.9... Installing library in /Users/user/.cabal/lib/HTTP-4000.0.9/ghc-6.12.3 Registering HTTP-4000.0.9... Configuring hSimpleDB-0.3... Preprocessing library hSimpleDB-0.3... Building hSimpleDB-0.3... src/Network/AWS/Authentication.hs:47:7: Could not find module `Text.XML.HXT.Arrow': Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: hSimpleDB-0.3 failed during the building phase. The exception was: ExitFailure 1 Any ideas how to solve this? Thanks! Dmitri ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal install: Could not find module `Text.XML.HXT.Arrow'
On Tue, Nov 8, 2011 at 12:16, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: On 8 November 2011 22:10, Erik Hesselink hessel...@gmail.com wrote: This is because hSimpleDB doesn't specify version ranges on its dependencies, when it should. Since hxt changed its module structure going from 9.0 to 9.1, hSimpleDB doesn't build against 9.0. You can try to build it by adding '--constraint=hxt==9.0.\*' after your cabal-install command. You can also ask the author to add version ranges to the package. Is the escape needed if you're using single quotes? I don't know. I always escape *s in shell commands to be sure the shell doesn't expand them, but in most of the cases, it probably works without them. It will likely even work without quotes and without the escape, unless you have files matching the pattern. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] SMP parallelism increasing GC time dramatically
On Mon, Oct 10, 2011 at 16:44, Tom Thorne thomas.thorn...@gmail.com wrote: thanks! I just tried setting -A32M and this seems to fix the parallel GC problems, I now get a speedup with parallel GC on and performance is the same as passing -qg. I had tried -H before and it only made things worse, but -A seems to do the trick. You might be able to use ghc-gc-tune [1] to find the right settings for -A and -H. Erik [1] http://hackage.haskell.org/package/ghc-gc-tune ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] DSL for data definition (e.g. compiling Haskell type defs into Google's protocol buffers type defs)
On Tue, Oct 4, 2011 at 20:33, Karel Gardas karel.gar...@centrum.cz wrote:
data PersonType = Person {
id :: Int
, name :: String
, email :: Maybe String
}
deriving (Show, Data, Typeable)
so I have `PersonType' as type constructor and Person as value constructor
(or data constructor) -- speaking using terms from Real World Haskell,
Chapter 3[1]. And now I see that none of typeOf/dataTypeOf/toContr is
applicable to *type constructor* but all are applicable to *value/data
constructor*. Ditto happen when testing Color versus RED, GREEN, BLUE. At
least GHCi complains this way:
*Main typeOf Color
interactive:0:8: Not in scope: data constructor `Color'
*Main typeOf PersonType
interactive:0:8: Not in scope: data constructor `PersonType'
But, I'd like to start processing of data definition from the *type
constructor*. So:
emit_proto PersonType 1
emit_proto Color 1
Is that possible at all? I mean in the scope/context of GHC's
Data/Data.Data/Data.Typeable etc. modules. (w/o considering TH now).
A definition of 'typeOf' is not supposed to use its argument, since
the normal way to call it is to pass undefined. The documentation
says:
The value of the argument should be ignored by any instance of
Typeable, so that it is safe to pass undefined as the argument.
So you should call it like:
typeOf (undefined :: PersonType).
Erik
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] which tags program should I use?
I use hothasktags [1] which works very well. The only problems are sometimes with obscure extensions, where haskell-src-exts (which hothasktags uses) can't parse the file, but that happens very rarely. Regarding speed: it takes 2-3 s on about 250 source files totaling about 25000 lines on my laptop. Regards, Erik [1] https://github.com/luqui/hothasktags/ On Sun, Sep 25, 2011 at 15:41, Henry Laxen nadine.and.he...@pobox.com wrote: Dear Group, I have a simple question, that as far as I can tell, has never really been well answered. I would like to generate TAGS files for haskell source. Reading the http://www.haskell.org/haskellwiki/Tags page suggests using :etags in GHCI or hasktags, or gasbag. Of the three, hasktags comes closest to working but it has (for me) a major inconvenience, namely it finds both function definitions and type signatures, resulting in two TAGS entries such as: ./Main.hs,63 module Main where 6,7 main :: 24,25 main = 25,26 Now when I do an emacs find-tag (I use icicles) I will always have to choose which tag I want to visit, and the completion buffer contains something like: main :: hs/Main.hs main = hs/Main.hs Granted, this is a minor (and very specialized) complaint, but if hasktags were to select only ONE of either the type signature (my first choice) or the function definition, (if no type signature) this annoyance would disappear. I also tried using etags, which I think would work, but it seems to have one killer bug (feature), namely that it dies if it finds an uninterpreted import: when (not is_interpreted) $ let mName = GHC.moduleNameString (GHC.moduleName m) in ghcError (CmdLineError (module ' ++ mName ++ ' is not interpreted)) I think it would work much better if it just warned you, instead of dying. This makes it unusable any time you import something precompiled. Now some looking at the README of hasktags leads me to: In the past this tool was distributed with ghc. I forked and added some features. hasktags itself was moved out of the ghc repository. Then I only verified that my fork finds at least as much tags as the one forked by Igloo. That makes me feel a little queasy. A google search for hasktags igloo turns up http://hackage.haskell.org/trac/ghc/ticket/1508 whose title is hasktags program needs replacement which makes me feel even more queasy. So I guess my question is, what are us disciples of the one true editor to do? Thanks in advance for you sage advice. Best wishes, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: cabal-ghci 0.1
On Fri, Sep 9, 2011 at 22:17, Jason Dagit dag...@gmail.com wrote: On Thu, Sep 8, 2011 at 11:06 PM, Etienne Laurin etie...@atnnn.com wrote: Hello fellow hackers. Here is a helpful package I wrote to ease the development of projects using cabal. It includes a :cabalset ghci command to set ghci options for your project, and a cabal-ghci executable to launch ghci with those options. Is :cabalset custom per project or could I put the same things in my ~/.ghci? Did you know you can also put a .ghci file in your project dir, and if you start ghci from that dir, it will also load that file? I think that allows you to replicate some of the functionality of this tool, since it allows per project ghci options. Erik ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] compiler pragma's
On Fri, Sep 9, 2011 at 23:41, Vasili I. Galchin vigalc...@gmail.com wrote:
Is there a list of compiler pragmas? Specifically I am looking at how to
specify more than one type variable in a class definition. Also I have
forgotten the meta syntax for specifying a pragma ... some kind of Haskell
comment?
If you're using GHC, there is a list of language extensions in the
user's guide chapter 7 [1]. Specifically, 7.13 specifies the language
pragma's. To turn on a language extension, you use the LANGUAGE pragma
with the specific extension(s) you want. In this case, you want
MultiParamTypeClasses, section 7.6.1.1. So the exact syntax will be:
{-# LANGUAGE MultiParamTypeClasses #-}
Put this at the top of your source file.
Note that the link is to the documentation for the latest version of
GHC (7.2.1 as of this writing). There are also links to specific
version, that might be more appropriate depending on the version of
GHC you have.
Erik
[1]
http://haskell.org/ghc/docs/latest/html/users_guide/ghc-language-features.html
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
