Re: [Haskell-cafe] How to understand the 'forall' ?
On Wed, Sep 16, 2009 at 11:58 AM, Cristiano Paris fr...@theshire.orgwrote: On Wed, Sep 16, 2009 at 7:12 PM, Ryan Ingram ryani.s...@gmail.com wrote: Here's the difference between these two types: test1 :: forall a. a - Int -- The caller of test1 determines the type for test1 test2 :: (forall a. a) - Int -- The internals of test2 determines what type, or types, to instantiate the argument at I can easily understand your explanation for test2: the type var a is closed under existential (?) quantification. I can't do the same for test1, even if it seems that a is closed under universal (?) quantification as well. Both are universally quantified, but at a different level. To look at it in System F-land, you have two levels of types that can get passed in lambdas. Explicitly: Haskell: test1 :: forall a. a - Int test1 _ = 1 test2 :: (forall a. a) - Int test2 x = x explicitly in System F: test1 = /\a \(x :: a). 1 test2 = \(x :: forall a. a). x @Int /\ is type-level lambda, and @ is type-level application. In test1, the caller specifies a and then passes in an object of that type. In test2, the caller must pass in an object which is of all types, and test2 asks for that object to be instantiated at the type Int Or, to put it another way, since there are no non-bottom objects of type (forall a. a): Why? Informally, because you can't create something that can be any type. For example, what could the result of test3 :: (forall a. a) - Int test3 x = length (x `asTypeOf` [()]) be? How could you call it? test1 converts *anything* to an Int. Is the only possible implementation of test1 the one ignoring its argument (apart from bottom of course)? There's one way that doesn't entirely ignore its argument. test4 x = seq x 1 But I don't like to talk about that one :) -- ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Can't install Haskell Platform (Ubuntu 9.02)
Yes that worked. --- On Wed, 9/16/09, Paulo Tanimoto tanim...@arizona.edu wrote: From: Paulo Tanimoto tanim...@arizona.edu Subject: Re: [Haskell-cafe] Can't install Haskell Platform (Ubuntu 9.02) To: Gregory Propf gregorypr...@yahoo.com Cc: Haskell-Cafe haskell-cafe@haskell.org Date: Wednesday, September 16, 2009, 7:24 PM Hi Gregory, On Wed, Sep 16, 2009 at 6:51 PM, Gregory Propf gregorypr...@yahoo.com wrote: I'm trying to install the Haskell Platform. I'm using Ubuntu 9.02 and GHC 6.10.4 on a 64 bit AMD and keep getting this crap when I do 'make install'. The stuff builds OK and the script in question does indeed exist. Anybody know what this is. I've looked online and none of the other people with this issue seem to know the fix. Error: The mtl-1.1.0.2/Setup script does not exist or cannot be run Can you try this: http://trac.haskell.org/haskell-platform/ticket/84 Let us know if you need help applying the patch. Paulo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trouble installing leksah
I now have the Haskell platform install problem solved but I'm now trying to get the leksah IDE installed and I'm getting this. runhaskell Setup configure Configuring leksah-0.6.1... Setup: At least the following dependencies are missing: glib =0.10, gtk =0.10, gtksourceview2 =0.10.0 I am aware that these are actually C development libraries, not Haskell modules. The thing is that they are all installed, using the Ubuntu synaptic tool. This is Ubuntu 9.04. Is there something I need to tell cabal about where these libraries are? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble installing leksah
On Thu, Sep 17, 2009 at 9:01 AM, Gregory Propf gregorypr...@yahoo.com wrote: I now have the Haskell platform install problem solved but I'm now trying to get the leksah IDE installed and I'm getting this. runhaskell Setup configure Configuring leksah-0.6.1... Setup: At least the following dependencies are missing: glib =0.10, gtk =0.10, gtksourceview2 =0.10.0 I am aware that these are actually C development libraries, not Haskell modules. The thing is that they are all installed, using the Ubuntu synaptic tool. This is Ubuntu 9.04. Is there something I need to tell cabal about where these libraries are? You need to install gtk2hs, a haskell binding to gtk libraries. When you build it, make sure to build the gtksourceview component as well. On (K)ubuntu you will need to download the gtk2hs tarball, install gtk dev libraries, and the usual configure, make and make install. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Where can I find a non-fee-based version of Hudak's paper, Conception, evolution, and application of functional programming languages?
Does anybody know where I can find a non-fee-based version of Paul Hudak's paper, Conception, evolution, and application of functional programming languages [1]? There used to be a version that did not require an ACM account available at http://www.cs.berkeley.edu/~jcondit/pl-prelim/hudak89functional.pdf , but when I try to download that file, the following error message appears: The server has encountered a problem because access is restricted. Your request was : http://www.cs.berkeley.edu/~jcondit/pl-prelim/hudak89functional.pdf -- Benjamin L. Russell [1] Hudak, Paul. Conception, Evolution and Application of Functional Programming Languages. New York, NY: _ACM Computing Surveys (CSUR)_ 21(3) (September 1989): 359-411. http://portal.acm.org/citation.cfm?id=72551.72554coll=ACMdl=ACMidx=J204part=journalWantType=Journalstitle=ACM%20Computing%20Surveys%20%28CSUR%29CFID=52128875CFTOKEN=48215788. Also available at http://www.cs.berkeley.edu/~jcondit/pl-prelim/hudak89functional.pdf. -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 Furuike ya, kawazu tobikomu mizu no oto. -- Matsuo Basho^ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Suggested additions to System.FilePath.Posix/Windows
Hello Neil I used System.FilePath.Posix quite extensively recently, and I thank you for the package filepath. There were however two words that I needed which I could not construct from those in System.FilePath.Posix. They are maybe of interest to you and others. I submit these two words to you for consideration for inclusion in System.FilePath.Posix. Please change the names as you see fit. I do not know if they make sense for System.FilePath.Windows. If the do not make sense, then please feel free to drop them so as to preserve the interface. As requested, I Cc'ed the haskell-cafe, but I am not at the moment following these threads, so if anyone else responds, please Cc me if you wish. Thanks again and cheers, - Marcus P.S. Here they are. Although I use ksh(1) as an example, this is a feature of POSIX shells. -- | 'reduceFilePath' returns a pathname that is reduced to canonical -- form equivalent to that of ksh(1), that is, symbolic link names are -- treated literally when finding the directory name. See @cd -L@ of -- ksh(1). Specifically, extraneous separators @(\/\)@, dot -- @(\.\)@, and double-dot @(\..\)@ directories are removed. reduceFilePath :: FilePath - FilePath reduceFilePath = joinPath . filePathComponents This is in turn built on filePathComponents that does all the work. filePathComponents :: FilePath - [FilePath] filePathComponents = [] filePathComponents (c:cs) = reverse $ snd $ foldl accumulate (if c == pathSeparator then ([],[/]) else ([c],[])) (cs++[pathSeparator]) where accumulate :: (String,[String]) - Char - (String,[String]) accumulate (cs, css) c = if c == pathSeparator then ([],(if null cs then id else cons cs) css) else (cs++[c],css) cons :: String - [String] - [String] cons cs css | cs == . = css | cs /= .. || null css = cs : css | otherwise = let hd = head css tl = tail css in if hd == [pathSeparator] then css else if hd == .. then cs : css else if null tl then [.] else tl // -- Marcus D. Gabriel, Ph.D. Saint Louis, FRANCE http://www.marcus.gabriel.namemailto:mar...@gabriel.name Tel: +33.3.89.69.05.06 Portable: +33.6.34.56.07.75 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Question about haskell.cs.yale.edu/
Today I received the request below. At first the URL confused me, but apparently www.haskell.org is known under two names :-) The request should probably be handled by someone involved in ICFP. /M On Wed, Sep 16, 2009 at 11:53 PM, Peter Green peter.gr...@frixo.com wrote: Hi, I was wondering whether you would be interested in linking to my website http://www.frixo.com from your page? http://haskell.cs.yale.edu/haskellwiki/icfp_2009_local_arrangements Frixo is a road traffic reporting site and think it may be a useful resource for your readers. It provides users with live traffic information and gets updated every 3 minutes using various road sensors. Thank you for your consideration. Kind Regards, Peter http://www.frixo.com -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANNOUNCE: graphviz-2999.5.1.0
I'm pleased to announce version 2999.5.1.0 [1] of the graphviz library, which provides bindings to the GraphViz [2] suite of tools for drawing graphs. [1] http://hackage.haskell.org/package/graphviz-2999.5.1.0 [2] http://www.graphviz.org/ This is mainly a bug-fix release; as such, there is no API change (though if you use the graphvizWithHandle function in Data.GraphViz.Commands, you should ensure that your Handle - IO a function closes the Handle when done). Changes in this release are: * Potentially fixed the graphvizWithHandle bug where warnings would be emitted about Handles not being closed correctly or too early; correct approach spotted by Nikolas Mayr. * Fixed up Parsing of various Attribute sub-values, especially Point and Spline (and hence Pos, which uses them). * Pre-process out comments and join together multi-line strings before parsing. * Properly parse Doubles like .2. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble installing leksah
There's no such named package in Hackage though. That was the first thing I looked for. All Hackage has with the string gtk2hs is this stuff gtk2hs-cast-glade library: A type class for cast functions of Gtk2hs: glade packagegtk2hs-cast-glib library: A type class for cast functions of Gtk2hs: glib packagegtk2hs-cast-gnomevfs library: A type class for cast functions of Gtk2hs: gnomevfs packagegtk2hs-cast-gtk library: A type class for cast functions of Gtk2hs: gtk packagegtk2hs-cast-gtkglext library: A type class for cast functions of Gtk2hs: gtkglext packagegtk2hs-cast-gtksourceview2 library: A type class for cast functions of Gtk2hs: gtksourceview2 packagegtk2hs-cast-th library: A type class for cast functions of Gtk2hs: TH packagegtk2hs-rpn library: Adds a module to gtk2hs allowing layouts to be defined using reverse polish notation. --- On Thu, 9/17/09, david48 dav.vire+hask...@gmail.com wrote: From: david48 dav.vire+hask...@gmail.com Subject: Re: [Haskell-cafe] Trouble installing leksah To: Gregory Propf gregorypr...@yahoo.com Cc: Haskell-Cafe haskell-cafe@haskell.org Date: Thursday, September 17, 2009, 12:39 AM On Thu, Sep 17, 2009 at 9:01 AM, Gregory Propf gregorypr...@yahoo.com wrote: I now have the Haskell platform install problem solved but I'm now trying to get the leksah IDE installed and I'm getting this. runhaskell Setup configure Configuring leksah-0.6.1... Setup: At least the following dependencies are missing: glib =0.10, gtk =0.10, gtksourceview2 =0.10.0 I am aware that these are actually C development libraries, not Haskell modules. The thing is that they are all installed, using the Ubuntu synaptic tool. This is Ubuntu 9.04. Is there something I need to tell cabal about where these libraries are? You need to install gtk2hs, a haskell binding to gtk libraries. When you build it, make sure to build the gtksourceview component as well. On (K)ubuntu you will need to download the gtk2hs tarball, install gtk dev libraries, and the usual configure, make and make install. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble installing leksah
Am Donnerstag 17 September 2009 12:35:19 schrieb Gregory Propf: There's no such named package in Hackage though. That was the first thing I looked for. All Hackage has with the string gtk2hs is this stuff AFAIK, gtk2hs is not yet cabalized and not on Hackage, look at http://haskell.org/gtk2hs/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Hi all. This email is in literate Haskell; you should be able to load
it into ghci and verify what I'm saying (nb: it won't compile without
alteration: see below).
I'm trying to do something which may anyway be stupid / not the best
approach to what I'm trying to achieve; however, it's not working and
I can't see why not. So I'm asking for help on two fronts:
1) Why is this failing?
2) Maybe more usefully, how should I actually be doing this? It seems
an ugly approach; a voice in my head is saying scrap your
boilerplate, but I've no idea yet if that's actually applicable
here; should I look at it?
On with the show...
I need these for subclass stuff later on...
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE UndecidableInstances #-}
module Ambig where
I wish to define a number of algebraic data types with the ability to
turn Int values into instances of those types. So I define a
typeclass saying this is possible. I use Maybe so I can encode the
existence of out-of-range Int values, which will vary from target type
to target type.
class Target a where
convert :: Int - Maybe a
E.g. here's a type Foo which only wants values between 1 and 10:
data Foo = Foo Int deriving (Show)
instance Target Foo where
convert n | n `elem` [1..10] = Just $ Foo n
| otherwise = Nothing
(That's a simple example; some are rather more complex. How to do
this isn't what I'm asking about, really.)
So we have, for example:
*Ambig (convert 1) :: Maybe Foo
Just (Foo 1)
*Ambig (convert 11) :: Maybe Foo
Nothing
Now, some of those algebraic data type types happen to be
enumerations; in this case, my idea is to list the constructors, with
the rule that each constructor's position in the list is the Int which
gets converted into that constructor.
class Enumerated a where
constructors :: [a]
E.g. here's a type Bar with three constructors:
data Bar = X | Y | Z deriving (Show)
instance Enumerated Bar where
constructors = [X, Y, Z]
(This is certainly ugly. Any suggestions?)
Now we get to the crux. If a type is an instance of Enumerated, it
should also be a Target, because we should be able to convert from Int
just by list lookup. But we include a bounds check, naturally...
instance (Enumerated a) = Target a where
convert n | n `elem` [0..len-1] = Just $ constructors !! n
| otherwise = Nothing
where len = length constructors
So I would _hope_ that then, e.g., we'd have:
*Ambig (convert 0) :: Maybe Bar
Just X
*Ambig (convert 1) :: Maybe Bar
Just Y
*Ambig (convert 3) :: Maybe Bar
Nothing
Sadly, this function doesn't compile, dying with an Ambiguous type
variable error:
Ambig.lhs:75:29:
Ambiguous type variable `a' in the constraint:
`Enumerated a'
arising from a use of `constructors' at Ambig.lhs:74:29-40
Probable fix: add a type signature that fixes these type
variable(s)
If we replace length constructors with 3 (say), it compiles (but
is useless). Adding a type signature doesn't help: it's misplaced
in that context. If I break it out of the instance declaration so I
can add one, I still get the same problem:
convert' :: (Enumerated a, Target a) = Int - Maybe a
convert' n | n `elem` [0..len-1] = Just $ constructors !! n
| otherwise = Nothing
where len = length constructors
I guess I see roughly what's going on; the question is which
constructors instance is meant?, right? In the Just part it's OK,
because it can be inferred from the function's return type (right?).
But in the guard we don't have that help, so it could be any
Enumerated instance?
Any advice appreciated! Particularly if this is just a dumb approach.
For context, this is related to deserialisation of binary data (they'll
actually be Word8's, not Int's) into a variety of data structures.
Hmmm, maybe I should just be using Data.Binary...
Many thanks,
-Andy
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Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Hey Andy,
On Thu, Sep 17, 2009 at 15:40, Andy Gimblett hask...@gimbo.org.uk wrote:
Now, some of those algebraic data type types happen to be
enumerations; in this case, my idea is to list the constructors, with
the rule that each constructor's position in the list is the Int which
gets converted into that constructor.
class Enumerated a where
constructors :: [a]
E.g. here's a type Bar with three constructors:
data Bar = X | Y | Z deriving (Show)
instance Enumerated Bar where
constructors = [X, Y, Z]
(This is certainly ugly. Any suggestions?)
|constructors| is expressible in SYB:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE FlexibleContexts #-}
module Test where
import Data.Data
import Data.Generics.Aliases (extB)
-- | Construct the empty value for a datatype. For algebraic datatypes, the
-- leftmost constructor is chosen.
empty :: forall a. Data a = a
empty = general
`extB` char
`extB` int
`extB` integer
`extB` float
`extB` double where
-- Generic case
general :: Data a = a
general = fromConstrB empty (indexConstr (dataTypeOf general) 1)
-- Base cases
char= '\NUL'
int = 0 :: Int
integer = 0 :: Integer
float = 0.0:: Float
double = 0.0:: Double
-- | Return a list of values of a datatype. Each value is one of the
possible
-- constructors of the datatype, populated with 'empty' values.
constrs :: forall a. Data a = [a]
constrs = general
`extB` char
`extB` int
`extB` integer
`extB` float
`extB` double where
-- Generic case
general :: Data a = [a]
general = map (fromConstrB empty)
(dataTypeConstrs (dataTypeOf (unList general))) where
unList :: Data a = [a] - a
unList = undefined
-- Base cases
char= \NUL
int = [0 :: Int]
integer = [0 :: Integer]
float = [0.0 :: Float]
double = [0.0 :: Double]
|constrs| is similar to your |constructors|, but in this way you get it for
free for any datatype with a |Data| instance. Then I guess your |convert|
is:
convert :: forall a. Data a = Int - Maybe a
convert n = let cs :: [a]
cs = constrs
in if (length cs n) then (Just (cs !! n)) else Nothing
Note that ScopedTypeVariables are essential to typecheck this code.
Cheers,
Pedro
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Re: [Haskell-cafe] How to understand the 'forall' ?
On Thu, Sep 17, 2009 at 8:36 AM, Ryan Ingram ryani.s...@gmail.com wrote: ... Explicitly: Haskell: test1 :: forall a. a - Int test1 _ = 1 test2 :: (forall a. a) - Int test2 x = x explicitly in System F: test1 = /\a \(x :: a). 1 test2 = \(x :: forall a. a). x @Int /\ is type-level lambda, and @ is type-level application. Ok. But let me be pedantic: where is the universal quantification in test1? It seems to me the a is a free variable in test1 while being closed under universal quantification in test2. In test1, the caller specifies a and then passes in an object of that type. The witness? In test2, the caller must pass in an object which is of all types, and test2 asks for that object to be instantiated at the type Int of all types means a value which belongs to all the sets of all the types, i.e. bottom? Or, to put it another way, since there are no non-bottom objects of type (forall a. a): Why? Informally, because you can't create something that can be any type. Yes, I misread the initial statement. I missed the non-bottom part :) There's one way that doesn't entirely ignore its argument. test4 x = seq x 1 But I don't like to talk about that one :) :) Thank you Ryan, you were very explicative. I may say that the use of the System-F's notation, making everything explicit, clarifies this a bit. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Am Donnerstag 17 September 2009 15:56:03 schrieb José Pedro Magalhães: Hey Andy, On Thu, Sep 17, 2009 at 15:40, Andy Gimblett hask...@gimbo.org.uk wrote: Now, some of those algebraic data type types happen to be enumerations; in this case, my idea is to list the constructors, with the rule that each constructor's position in the list is the Int which gets converted into that constructor. class Enumerated a where constructors :: [a] E.g. here's a type Bar with three constructors: data Bar = X | Y | Z deriving (Show) instance Enumerated Bar where constructors = [X, Y, Z] (This is certainly ugly. Any suggestions?) |constructors| is expressible in SYB: Wow. What about data Bar = X | Y | Z deriving (Show, Eq, Ord, Enum, Bounded) instance Enumerated Bar where constructors = [minBound .. maxBound] ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Hello, On Thu, Sep 17, 2009 at 16:05, Daniel Fischer daniel.is.fisc...@web.dewrote: Am Donnerstag 17 September 2009 15:56:03 schrieb José Pedro Magalhães: Hey Andy, On Thu, Sep 17, 2009 at 15:40, Andy Gimblett hask...@gimbo.org.uk wrote: Now, some of those algebraic data type types happen to be enumerations; in this case, my idea is to list the constructors, with the rule that each constructor's position in the list is the Int which gets converted into that constructor. class Enumerated a where constructors :: [a] E.g. here's a type Bar with three constructors: data Bar = X | Y | Z deriving (Show) instance Enumerated Bar where constructors = [X, Y, Z] (This is certainly ugly. Any suggestions?) |constructors| is expressible in SYB: Wow. What about data Bar = X | Y | Z deriving (Show, Eq, Ord, Enum, Bounded) instance Enumerated Bar where constructors = [minBound .. maxBound] ? Oh yes, that will certainly work for this very simple datatype. However, one cannot automatically derive instances of |Bounded| for datatypes with non-nullary constructors. Cheers, Pedro ___ 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] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
On 17 Sep 2009, at 15:21, José Pedro Magalhães wrote: E.g. here's a type Bar with three constructors: data Bar = X | Y | Z deriving (Show) instance Enumerated Bar where constructors = [X, Y, Z] (This is certainly ugly. Any suggestions?) |constructors| is expressible in SYB: Wow. What about data Bar = X | Y | Z deriving (Show, Eq, Ord, Enum, Bounded) instance Enumerated Bar where constructors = [minBound .. maxBound] ? Oh yes, that will certainly work for this very simple datatype. However, one cannot automatically derive instances of |Bounded| for datatypes with non-nullary constructors. That would be OK in this instance, I think; I'm already dealing with some of those cases by hand, but there were enough purely nullary ones that this seemed worth doing. I don't know if that will work any better with Foo/convert, though... :-) Thanks though Daniel - it's good to meet Enum and Bounded. -Andy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Am Donnerstag 17 September 2009 16:30:14 schrieb Andy Gimblett: On 17 Sep 2009, at 15:21, José Pedro Magalhães wrote: E.g. here's a type Bar with three constructors: data Bar = X | Y | Z deriving (Show) instance Enumerated Bar where constructors = [X, Y, Z] (This is certainly ugly. Any suggestions?) |constructors| is expressible in SYB: Wow. What about data Bar = X | Y | Z deriving (Show, Eq, Ord, Enum, Bounded) instance Enumerated Bar where constructors = [minBound .. maxBound] ? Oh yes, that will certainly work for this very simple datatype. However, one cannot automatically derive instances of |Bounded| for datatypes with non-nullary constructors. Andy's original message hasn't found its way into my inbox yet (neither has yours which Andy here quotes), so I don't know what Andy wants to do. From the part you quoted, I drew the conclusion that one thing Andy wanted was a more elegant way for the case of nullary constructors. For that, SYB is certainly overkill. I haven't looked at your code, I suppose it also does something reasonable in the presence of non-nullary constructors, in which case the separate treatment of only nullary constructors would of course be unnecessary. That would be OK in this instance, I think; I'm already dealing with some of those cases by hand, but there were enough purely nullary ones that this seemed worth doing. I don't know if that will work any better with Foo/convert, though... :-) I'll probably understand that when your original message arrives :-) Thanks though Daniel - it's good to meet Enum and Bounded. -Andy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On Thu, 9/17/09, Tom Doris tomdo...@gmail.com wrote: From: Tom Doris tomdo...@gmail.com Subject: Re: [Haskell-beginners] map question To: Joost Kremers joostkrem...@fastmail.fm Cc: beginn...@haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners -Inline Attachment Follows- ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Where can I find a non-fee-based version of Hudak's paper, Conception, evolution, and application of functional programming languages?
Does anybody know where I can find a non-fee-based version of Paul Hudak's paper, Conception, evolution, and application of functional programming languages [1]? There used to be a version that did not seems you can get a djvu copy here http://lib.org.by/info/Cs_Computer science/CsPl_Programming languages/Hudak P. Conception, evolution, and application of functional programming languages (ACM comp.surveys 21, 1989)(T)(53s).djvu the site is in Russian and you wait 30s before a link to the file appears. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Am Donnerstag 17 September 2009 15:40:10 schrieb Andy Gimblett: instance (Enumerated a) = Target a where convert n | n `elem` [0..len-1] = Just $ constructors !! n | otherwise = Nothing where len = length constructors Yes, the second appearance of 'constructors' is at an unspecified type. instance (Enumerated a) = Target a where convert n | n 0 = Nothing | otherwise = case drop n constructors of (x:_) - Just x _ - Nothing would make it compile. But there'd be a risk that Target is unusable, depending on how instance resolution is done. I guess I see roughly what's going on; the question is which constructors instance is meant?, right? In the Just part it's OK, because it can be inferred from the function's return type (right?). But in the guard we don't have that help, so it could be any Enumerated instance? Exactly. Any advice appreciated! Particularly if this is just a dumb approach. For context, this is related to deserialisation of binary data (they'll actually be Word8's, not Int's) into a variety of data structures. Hmmm, maybe I should just be using Data.Binary... Many thanks, -Andy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Where can I find a non-fee-based version of Hudak's paper, Conception, evolution, and application of functional programming languages?
On 2009-09-17, at 1:41 AM, Benjamin L.Russell wrote: Does anybody know where I can find a non-fee-based version of Paul Hudak's paper, Conception, evolution, and application of functional programming languages [1]? When in doubt, check citeseer. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.83.6505 -joh ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
On 17 Sep 2009, at 16:50, Daniel Fischer wrote: Yes, the second appearance of 'constructors' is at an unspecified type. instance (Enumerated a) = Target a where convert n | n 0 = Nothing | otherwise = case drop n constructors of (x:_) - Just x _ - Nothing would make it compile. Neat trick. It works: thanks! But there'd be a risk that Target is unusable, depending on how instance resolution is done. Unusable? How so? Sorry, but I don't follow... -Andy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
(-) happens to be the only prefix operator in haskell, it also an infix operator. so: 4 - 2 2 -3 -3 ((-) 5) 3 -- note that in this case (-) is treated like any regular function so 5 is the first parameter 2 (5 - ) 3 2 (-5 ) -5 (flip (-) 5) 3 -2 It's a little wart brought about by the ambiguity in common mathematical syntax. If you play around in ghci you should get the hang of it pretty quick. - Job On Thu, Sep 17, 2009 at 11:08 AM, Gregory Propf gregorypr...@yahoo.comwrote: Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On *Thu, 9/17/09, Tom Doris tomdo...@gmail.com* wrote: From: Tom Doris tomdo...@gmail.com Subject: Re: [Haskell-beginners] map question To: Joost Kremers joostkrem...@fastmail.fm Cc: beginn...@haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers joostkrem...@fastmail.fmhttp://mc/compose?to=joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany ___ Beginners mailing list beginn...@haskell.org http://mc/compose?to=beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners -Inline Attachment Follows- ___ Beginners mailing list beginn...@haskell.org http://mc/compose?to=beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners ___ 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] Re: [Haskell-beginners] map question
2009/9/17 Joost Kremers joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost The reason that map (-1) [1,2,3,4] doesn't work as you'd expect it to is that - is ambiguous in Haskell (some may disagree). -1 means -1 in Haskell, i.e. negative 1, not the function that subtracts 1 from its argument. (-) 1 is the function that subtracts its argument from 1, which is not what you were looking for either! You're looking for the function that subtracts 1 from its argument, which is `subtract 1'. Prelude map (subtract 1) [1..4] [0,1,2,3] Note that `subtract' is just another name for `flip (-)', i.e. subtraction with its argument in reverse order. -- Deniz Dogan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to understand the 'forall' ?
On Thu, Sep 17, 2009 at 6:59 AM, Cristiano Paris fr...@theshire.org wrote:
On Thu, Sep 17, 2009 at 8:36 AM, Ryan Ingram ryani.s...@gmail.com wrote:
...
Explicitly:
Haskell:
test1 :: forall a. a - Int
test1 _ = 1
test2 :: (forall a. a) - Int
test2 x = x
explicitly in System F:
test1 = /\a \(x :: a). 1
test2 = \(x :: forall a. a). x @Int
/\ is type-level lambda, and @ is type-level application.
Ok. But let me be pedantic: where is the universal quantification in
test1? It seems to me the a is a free variable in test1 while being
closed under universal quantification in test2.
The universal quantification is right in the extra lambda: it works for all
types a.
Just like this works on all lists [a]:
length = /\a. \(xs :: [a]). case xs of { [] - 0 ; (x:ys) - 1 + length @a
ys }
Here are some uses of test1:
v1 = test1 @Int 0
v2 = test1 @[Char] hello
v3 = test1 @() ()
Here's a use of test2:
v4 = test2 (/\a. error @a broken)
given error :: forall a. String - a
-- ryan
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Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Am Donnerstag 17 September 2009 18:01:36 schrieb Andy Gimblett: On 17 Sep 2009, at 16:50, Daniel Fischer wrote: Yes, the second appearance of 'constructors' is at an unspecified type. instance (Enumerated a) = Target a where convert n | n 0 = Nothing | otherwise = case drop n constructors of (x:_) - Just x _ - Nothing would make it compile. Neat trick. It works: thanks! But there'd be a risk that Target is unusable, depending on how instance resolution is done. Unusable? How so? Sorry, but I don't follow... Cf. Section 7.6.3.3 of the user's guide: When matching, GHC takes no account of the context of the instance declaration (context1 etc). GHC's default behaviour is that exactly one instance must match the constraint it is trying to resolve. It is fine for there to be a potential of overlap (by including both declarations (A) and (B), say); an error is only reported if a particular constraint matches more than one. The -XOverlappingInstances flag instructs GHC to allow more than one instance to match, provided there is a most specific one. For example, the constraint C Int [Int] matches instances (A), (C) and (D), but the last is more specific, and hence is chosen. If there is no most-specific match, the program is rejected. So for the matching, you have now instance Target a where ... which matches everything. Add an instance declaration of the form instance (SomeClass b) = Target b where ... and you're hosed. Though I think that wouldn't compile, at least not without IncoherentInstances. Actually, I think now that with one-parameter type classes, if it compiles, it will most likely work, at least I don't see the problems one can create with multi-parameter type classes now. -Andy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)
Here's a way that works more closely to your original version: instance Enumerated a = Target a where convert n | n = 0 n numConstrs = Just (constrs !! n) | otherwise = Nothing where constrs = constructors numConstrs = length constrs Alternatively: instance Enumerated a = Target a where convert n | n = 0 n numConstrs = Just result | otherwise = Nothing where numConstrs = length (constructors `asTypeOf` [result]) result = constructors !! n However let me warn you that you aren't going to be happy with this instance when it comes time to use this. Instead, you probably want one of the following: defaultConvert :: Enumerated a = Int - a defaultConvert n | n = 0 n numConstrs = Just (WithEnumerated (constrs !! n)) | otherwise = Nothing where constrs = constructors numConstrs = length constrs (a) instance Target SomeEnumeratedType where convert = defaultConvert (b) newtype WithEnumerated a = WithEnumerated a instance Enumerated a = Target (WithEnumerated a) where convert n = WithEnumerated (defaultConvert n) OverlappingInstances basically never does what you want in the long run. -- ryan On Thu, Sep 17, 2009 at 9:01 AM, Andy Gimblett hask...@gimbo.org.uk wrote: On 17 Sep 2009, at 16:50, Daniel Fischer wrote: Yes, the second appearance of 'constructors' is at an unspecified type. instance (Enumerated a) = Target a where convert n | n 0 = Nothing | otherwise = case drop n constructors of (x:_) - Just x _ - Nothing would make it compile. Neat trick. It works: thanks! But there'd be a risk that Target is unusable, depending on how instance resolution is done. Unusable? How so? Sorry, but I don't follow... -Andy ___ 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] weak pointers and memoization (was Re: memoization)
What are you trying to use this for? It seems to me that for memo tables you almost never have references to they keys outside the lookup table since the keys are usually computed right at the last minute, and then discarded (otherwise it might be easier to just cache stuff outside the function). For example with a naive fibs, the values you are passing in are computed, and probably don't exist before you do the recursive call, and then are discarded shortly afterward. It seems like putting a cap on the cache size, and then just overwriting old entries would be better. Am I missing something? - Job On Wed, Sep 16, 2009 at 4:48 PM, Rodney Price rodpr...@raytheon.com wrote: How does garbage collection work in an example like the one below? You memoize a function with some sort of lookup table, which stores function arguments as keys and function results as values. As long as the function remains in scope, the keys in the lookup table remain in memory, which means that the keys themselves always remain reachable and they cannot be garbage collected. Right? So what do you do in the case where you know that, after some period of time, some entries in the lookup table will never be accessed? That is, there are no references to the keys for some entries remaining, except for the references in the lookup table itself. You'd like to allow the memory occupied by the keys to be garbage collected. Otherwise, if the function stays around for a long time, the size of the lookup table always grows. How do you avoid the space leak? I notice that there is a function in Data.IORef, mkWeakIORef :: IORef a - IO () - IO (Weak (IORef a)) which looks promising. In the code below, however, there's only one IORef, so either the entire table gets garbage collected or none of it does. I've been reading the paper Stretching the storage manager: weak pointers and stable names in Haskell, which seems to answer my question. When I attempt to run the memoization code in the paper on the simple fib example, I find that -- apparently due to lazy evaluation -- no new entries are entered into the lookup table, and therefore no lookups are ever successful! So apparently there is some interaction between lazy evaluation and garbage collection that I don't understand. My head hurts. Is it necessary to make the table lookup operation strict? Or is it something entirely different that I am missing? -Rod On Thu, 10 Sep 2009 18:33:47 -0700 Ryan Ingram ryani.s...@gmail.com wrote: memoIO :: Ord a = (a - b) - IO (a - IO b) memoIO f = do cache - newIORef M.empty return $ \x - do m - readIORef cache case M.lookup x m of Just y - return y Nothing - do let res = f x writeIORef cache $ M.insert x res m return res memo :: Ord a = (a - b) - (a - b) memo f = unsafePerformIO $ do fmemo - memoIO f return (unsafePerformIO . fmemo) I don't think there is any valid transformation that breaks this, since the compiler can't lift anything through unsafePerformIO. Am I mistaken? -- ryan ___ 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] Peano axioms
Hi,
Below are two attempts to define Peano arithmetic in Haskell.
The first attempt, Peano1, consists of just a signature in the class
with the axioms in the instance. In the second attempt, Peano2, I am
trying to move the axioms into the class. The reason is, I want to put
as much specification as possible into the class. Then I would like to
include properties in the class such as commutativity something like:
infixl 5 `com`
com :: Int - Int - Int
x `com` y = (x + y)
commutative com a b = (a `com` b) == (b `com` a)
I seem to be able to include just one default equation the Peano2 attempt.
Any ideas?
I have looked at
http://www.haskell.org/haskellwiki/Peano_numbers
Regards,
Pat
-- Attempt 1
-- In this attempt the axioms are in the instance and things seem OK
module Peano1 where
infixl 6 `eq`
infixl 5 `plus`
class Peano1 n where
suc :: n - n
eq :: n - n - Bool
plus :: n - n - n
data Nat = One | Suc Nat deriving Show
instance Peano1 Nat where
suc = Suc
One `eq` One = True
(Suc m) `eq` (Suc n) = m `eq` n
_`eq`_ = False
m `plus` One = Suc m
m `plus` (Suc n) = Suc (m `plus` n)
-- Evaluation *Peano1 Suc(One) `plus` ( Suc (One))
-- Attempt 2
-- In this attempt the axioms are in the class and things are not OK.
module Peano2 where
infixl 6 `eq`
infixl 5 `plus`
class Peano2 n where
one :: n
eq :: n - n - Bool
plus :: n - n - n
suc :: n - n
suc a = a `plus` one
{-
I cannot add the remaining default axioms
one `eq` one = True
(suc m) `eq` (suc n) = m `eq` n
(suc a) `eq` (suc b) = a `eq` b
_`eq`_ = False
-}
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Re: [Haskell-cafe] Is it safe to use unsafePerformIO here?
On Tue, Sep 15, 2009 at 11:31 PM, Daniel Fischer daniel.is.fisc...@web.de wrote: ... Yeah, you do *not* want the whole file to be read here, except above for testing purposes. That's not true. Sometimes I want to, sometimes don't. But I want to use the same code for reading files and exploit laziness to avoid reading the body. Still, ByteStrings are probably the better choice (if you want the body and that can be large). That's not a problem by now. To avoid reading the body without unsafePerformIO: readBit fn = Control.Exception.bracket (openFile fn ReadMode) hClose (\h - do l - hGetLine h let i = read l bdy - hGetContents h return $ Bit i bdy) Same problem with the withFile-version: nothing gets printed if I try to print out the body: that's way I used seq. I'm starting to think that the only way to do this without using unsafePerformIO is to have the body being an IO action: simply, under Haskell assumption, that's not possible to write, because Haskell enforce safety above all. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Peano axioms
The problem is that you are using 'suc' as if it is a constructor: ((suc m)
`eq` (suc n) = m `eq` n)
You'll have to change it to something else, and it will probably require
adding an unpacking function to your class and it will probably be messy.
I'd suggest you make use of the Eq typeclass and defined the Eq instances
separately:
class (Eq n) = Peano2 n where
one :: n
plus :: n - n - n
suc :: n - n
suc a = a `plus` one
- Job
On Thu, Sep 17, 2009 at 2:36 PM, pat browne patrick.bro...@comp.dit.iewrote:
Hi,
Below are two attempts to define Peano arithmetic in Haskell.
The first attempt, Peano1, consists of just a signature in the class
with the axioms in the instance. In the second attempt, Peano2, I am
trying to move the axioms into the class. The reason is, I want to put
as much specification as possible into the class. Then I would like to
include properties in the class such as commutativity something like:
infixl 5 `com`
com :: Int - Int - Int
x `com` y = (x + y)
commutative com a b = (a `com` b) == (b `com` a)
I seem to be able to include just one default equation the Peano2 attempt.
Any ideas?
I have looked at
http://www.haskell.org/haskellwiki/Peano_numbers
Regards,
Pat
-- Attempt 1
-- In this attempt the axioms are in the instance and things seem OK
module Peano1 where
infixl 6 `eq`
infixl 5 `plus`
class Peano1 n where
suc :: n - n
eq :: n - n - Bool
plus :: n - n - n
data Nat = One | Suc Nat deriving Show
instance Peano1 Nat where
suc = Suc
One `eq` One = True
(Suc m) `eq` (Suc n) = m `eq` n
_`eq`_ = False
m `plus` One = Suc m
m `plus` (Suc n) = Suc (m `plus` n)
-- Evaluation *Peano1 Suc(One) `plus` ( Suc (One))
-- Attempt 2
-- In this attempt the axioms are in the class and things are not OK.
module Peano2 where
infixl 6 `eq`
infixl 5 `plus`
class Peano2 n where
one :: n
eq :: n - n - Bool
plus :: n - n - n
suc :: n - n
suc a = a `plus` one
{-
I cannot add the remaining default axioms
one `eq` one = True
(suc m) `eq` (suc n) = m `eq` n
(suc a) `eq` (suc b) = a `eq` b
_`eq`_ = False
-}
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[Haskell-cafe] Composition, delegation and interfaces -- a 20K ft critique of Noop
Dear Programmers, Someone just asked me to give my opinion on Noop's composition proposalhttp://code.google.com/p/noop/wiki/ProposalForComposition. It reminds me a little bit of Selfhttp://en.wikipedia.org/wiki/Self_%28programming_language%29which found its way into JavaScript. It also reminds me a little of Haskell's type classes http://en.wikipedia.org/wiki/Type_class. In general, movement away from inheritance is good. The proposal, however, feels a bit like looking for the lost quarter where the light is good, rather than where you lost it. Before considering delegation machinery, let's consider the *value*of an interface. How many interfaces are there? One way to see that is just to consider all the sub interfaces of a single interface with n methods on it. Hmmm... that's 2^n interfaces. That's a lot. Does that give us any confidence that any one way of carving up functionality via interfaces is going to be sane? Further, in practice, do we see random distribution through this very large space? What we see over and over again in practice is that the answer to these questions is 'no!'. That means that there is *something* that binds a collection of methods together. What might that something be? One place to look is mathematics. Which maths should we look at? The maths of category has been very fruitful both in explaining existing functional programming techniques and -- perhaps more importantly -- suggesting ways to improve them as well as wholly new techniques. What we find in category theory is that it is natural to collect maps (read functions) together. A good example of such a beast is a monad. A monad -- viewed categorically -- is - a map, T, taking types to new types and functions on those types to new functions. Let's call the universe of types and functions expressible in our model of computation (as proscribed by our programming language), C. Then T : C - C. - a higher order map, unit. Just like T takes C to C, we can understand a noop like map that takes C to C, call it Id. Then unit : Id - T. We intuitively think about it as putting basic types inside the container T, but it's really a higher order map. - another higher order map, mult : T^2 - T. We talk about it as a kind of flattening (and in Scala it's called flatMap), but it's a higher order map. Now, one is not finished spelling out a monad when giving this collection of maps. One must also show that they satisfy certain constraints. - T is functorial, meaning T g f = T(g) T(f) - unit and mult are natural transformations, look up the meaning because unpacking it here would take to long - mult( mult T ) = mult( T mult ) - mult( unit T ) = mult( T unit ) This set of constraints must go with the monad. This example provides a little more detail in terms of what binds a group of maps together, and hence of what *might* replace the notion of interface and *explain* what we see in practice. Good programmers invariably pick out just a few factorizations of possible interfaces -- from the giant sea of factorizations (read different ways to carve up the functionality). The reason -- i submit -- is because in their minds there are some constraints they know or at least intuit must hold -- but they have no good way at the language level to express those constraints. A really practical language should help the programmer by providing a way express and check the constraints that hold amongst the maps in an interface. i submit that this idea is not the same as design by contract. i am not proposing an Eiffel-like mechanism. Again, taking a functional approach to computation via category theory leads one towards modeling interfaces as categorical situations like monads, comonads, distribution laws, etc. This means that a large number of the constraints come down to - functoriality - naturality - coherence Language support for this approach might include *keywords for these kinds of assertions*. It is a gnarly beast to offer automatic and/or compiler support for checking general constraints. Even this limited family of constraints that i'm proposing can generate some very difficult computations, with very bad complexity. However, for those situations where a general purpose solution to check assertions of functoriality, naturality and coherence are infeasible, one can use these hints to generate tests to probe for possible failures. This idea follows the in the same spirit of replacing proof with proof-checking. Of course, this is not the only way to go. i've yet to be convinced that category theory offers a good account of concurrency. Specifically, categorical composition does not line up well with concurrent composition. So, interfaces organized around types for concurrency is also something to consider. In this case, one might find a natural beginning in interfaces in which -- roughly speaking -- the methods constitute the tokens of a formal language the constructors of which are
Re: [Haskell-cafe] Is it safe to use unsafePerformIO here?
Am Donnerstag 17 September 2009 21:07:28 schrieb Cristiano Paris: On Tue, Sep 15, 2009 at 11:31 PM, Daniel Fischer daniel.is.fisc...@web.de wrote: ... Yeah, you do *not* want the whole file to be read here, except above for testing purposes. That's not true. Sometimes I want to, sometimes don't. The for the case of sorting by metadata was tacitly assumed :) But I want to use the same code for reading files and exploit laziness to avoid reading the body. Still, ByteStrings are probably the better choice (if you want the body and that can be large). That's not a problem by now. To avoid reading the body without unsafePerformIO: readBit fn = Control.Exception.bracket (openFile fn ReadMode) hClose (\h - do l - hGetLine h let i = read l bdy - hGetContents h return $ Bit i bdy) Same problem with the withFile-version: nothing gets printed if I try to print out the body: that's way I used seq. Ah, yes. The file is closed too soon. I'm starting to think that the only way to do this without using unsafePerformIO is to have the body being an IO action: simply, under Haskell assumption, that's not possible to write, because Haskell enforce safety above all. Well, what about readBit fn = do txt - readFile fn let (l,_:bdy) = span (/= '\n') txt return $ Bit (read l) bdy ? With main = do args - getArgs let n = case args of (a:_) - read a _ - 1000 bl - mapM readBit [file1.txt,file2.txt] mapM_ (putStrLn . show . index) $ sortBy (comparing index) bl mapM_ (putStrLn . take 20 . drop n . body) bl ./cparis3 30 +RTS -sstderr 2 3 CCGGGCGCGGTGGCTCACGC CCGGGCGCGGTGGCTCACGC 408,320 bytes allocated in the heap 1,220 bytes copied during GC 34,440 bytes maximum residency (1 sample(s)) 31,096 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) ./cparis3 2 +RTS -sstderr 2 3 TTAGCCGGGCGTGGTG TTAGCCGGGCGTGGTG 1,069,168 bytes allocated in the heap 105,700 bytes copied during GC 137,356 bytes maximum residency (1 sample(s)) 27,344 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) ./cparis3 200 +RTS -sstderr 2 3 CCTGGCCAACATGGTGAAAC CCTGGCCAACATGGTGAAAC 80,939,296 bytes allocated in the heap 8,925,240 bytes copied during GC 137,056 bytes maximum residency (2 sample(s)) 45,528 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) %GC time 38.5% (27.0% elapsed) Alloc rate1,264,577,704 bytes per MUT second Productivity 61.5% of total user, 38.8% of total elapsed ./cparis3 2000 +RTS -sstderr 2 3 CAGAGCGAGACTCCGTCTCA CAGAGCGAGACTCCGTCTCA 806,034,756 bytes allocated in the heap 76,775,944 bytes copied during GC 136,876 bytes maximum residency (2 sample(s)) 43,324 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1536 collections, 0 parallel, 0.35s, 0.35s elapsed Generation 1: 2 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time0.00s ( 0.00s elapsed) MUT time0.53s ( 0.67s elapsed) GCtime0.35s ( 0.36s elapsed) EXIT time0.00s ( 0.00s elapsed) Total time0.88s ( 1.02s elapsed) %GC time 40.0% (34.9% elapsed) Alloc rate1,526,482,681 bytes per MUT second Productivity 60.0% of total user, 51.7% of total elapsed Seems to work as desired. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is it safe to use unsafePerformIO here?
On Thu, Sep 17, 2009 at 10:01 PM, Daniel Fischer daniel.is.fisc...@web.de wrote: ... readBit fn = do txt - readFile fn let (l,_:bdy) = span (/= '\n') txt return $ Bit (read l) bdy ? With main = do args - getArgs let n = case args of (a:_) - read a _ - 1000 bl - mapM readBit [file1.txt,file2.txt] mapM_ (putStrLn . show . index) $ sortBy (comparing index) bl mapM_ (putStrLn . take 20 . drop n . body) bl Yes, it *seems* to work but... the files don't get closed (readFile is unfinished until body is read) so I think I'm going to have problems when the number of files to read is higher than the maximum number of open handles a process can have. That's a possibility I considered even if not directly using readFile. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] algebra/grammar/language for expressing time intervals
Iain Alexander wrote: You might want to take a look at RFC 2445 Internet Calendaring and Scheduling Core Object Specification Section 4.8.5.4 Recurrence Rule Another source of inspiration might be the syntax used in remind[1]. /M [1]: http://www.roaringpenguin.com/products/remind -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Peano axioms
I don't understand your goal. Isn't Peano arithmetic summarized in Haskell as: data Peano = Zero | Succ Peano deriving Eq This corresponds to a first-order logic over a signature that has equality, a constant symbol 0, and a one-place successor function symbol S. Function symbols such as and + can be introduced as defined function symbols that do not add substantive information to the theory. The only axioms you want are the ones for equality. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: Unification in a Commutative Monoid (cmu 1.1) and a new release of Abelian group unification and matching (agum 2.2)
Package cmu 1.1 provides unification in a commutative monoid, also know as ACU-unification. The core computation finds the minimal non-zero solutions to homogeneous linear Diaphantine equations. The linear equation solver has been place in a separate module so it can be used for other applications Package agum 2.2 provides unification and matching in an Abelian group, also know as AG-unification and matching. The core computation finds the integer solutions to inhomogeneous linear equations. The linear equation solver has been place in a separate module so it can be used for other applications John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is it safe to use unsafePerformIO here?
Am Donnerstag 17 September 2009 22:20:55 schrieb Cristiano Paris: On Thu, Sep 17, 2009 at 10:01 PM, Daniel Fischer daniel.is.fisc...@web.de wrote: ... readBit fn = do txt - readFile fn let (l,_:bdy) = span (/= '\n') txt return $ Bit (read l) bdy ? With main = do args - getArgs let n = case args of (a:_) - read a _ - 1000 bl - mapM readBit [file1.txt,file2.txt] mapM_ (putStrLn . show . index) $ sortBy (comparing index) bl mapM_ (putStrLn . take 20 . drop n . body) bl Yes, it *seems* to work but... the files don't get closed (readFile is unfinished until body is read) so I think I'm going to have problems when the number of files to read is higher than the maximum number of open handles a process can have. Indeed. If the number of files is large, reading lazily with readFile is not so good. Eat the cake and have it. If you have a lot of files, want to read the metadata of all, select a (much) smaller number of files by some criterion on the set of metadata and then read the body of the selected files, it's hairy. Reading all bodies immediately is probably out due to memory restrictions. The clean approach would be to separate the reading of metadata and body. The drawback is that then you have a second entry into IO. Using unsafePerformIO, you can pretend that you don't reenter IO. Whether that is safe in your situation, I don't know. Probably not (rule of thumb: all nontrivial actions wrapped in unsafePerformIO aren't safe, though chances aren't bad that it works most of the time). That's a possibility I considered even if not directly using readFile. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] help with FFI
Hello
Lets say I have a library in C with a header like this:
#include stdio.h
/*really big structure*/
typedef struct {
int *a;
int *b;
/*lots of stuff
...
*/
int *z;
} foo;
/*this function allocate memory and fill the structure, reading from a
file*/
int create_foo(foo *f,FILE *file,int x,int y);
/*some functions that use the structure*/
int use_foo(foo *f,int w);
/*a funtion that releases the memory*/
int destroy_foo(foo *f);
And I want to use it in haskell using FFI. I can create a .hsc file like
this:
{-# LANGUAGE CPP, ForeignFunctionInterface #-}
import Foreign
import Foreign.C.Types
#include foo.h
newtype Foo = Foo ()
foreign import ccall static foo.h create_foo
c_create_foo :: Ptr (Foo) - Ptr (CFile) - CInt - CInt - IO CInt
foreign import ccall static foo.h use_foo
c_use_foo :: Ptr (Foo) - CInt - IO CInt
foreign import ccall static foo.h destroy_foo
c_destroy_foo :: Ptr (Foo) - IO CInt
It compiles but I have no idea how to call c_create_foo and get back Foo
Any suggestions?
thanks
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Re: [Haskell-cafe] weak pointers and memoization (was Re: memoization)
In my case, the results of each computation are used to generate a node in a graph structure (dag). The key, oddly, is a hash of a two-tuple that gets stored in the data structure after the computation of the node finishes. If I don't memoize the function to build a node, the cost of generating the tree is exponential; if I do, it's somewhere between linear and quadratic. Another process prunes parts of this graph structure as time goes on. The entire data structure is intended to be persistent, lasting for days at a time in a server-like application. If the parts pruned aren't garbage collected, the space leak will eventually be catastrophic. Either the memo table or the graph structure itself will outgrow available memory. -Rod On Thu, 17 Sep 2009 13:32:13 -0400 Job Vranish jvran...@gmail.com wrote: What are you trying to use this for? It seems to me that for memo tables you almost never have references to they keys outside the lookup table since the keys are usually computed right at the last minute, and then discarded (otherwise it might be easier to just cache stuff outside the function). For example with a naive fibs, the values you are passing in are computed, and probably don't exist before you do the recursive call, and then are discarded shortly afterward. It seems like putting a cap on the cache size, and then just overwriting old entries would be better. Am I missing something? - Job On Wed, Sep 16, 2009 at 4:48 PM, Rodney Price rodpr...@raytheon.com wrote: How does garbage collection work in an example like the one below? You memoize a function with some sort of lookup table, which stores function arguments as keys and function results as values. As long as the function remains in scope, the keys in the lookup table remain in memory, which means that the keys themselves always remain reachable and they cannot be garbage collected. Right? So what do you do in the case where you know that, after some period of time, some entries in the lookup table will never be accessed? That is, there are no references to the keys for some entries remaining, except for the references in the lookup table itself. You'd like to allow the memory occupied by the keys to be garbage collected. Otherwise, if the function stays around for a long time, the size of the lookup table always grows. How do you avoid the space leak? I notice that there is a function in Data.IORef, mkWeakIORef :: IORef a - IO () - IO (Weak (IORef a)) which looks promising. In the code below, however, there's only one IORef, so either the entire table gets garbage collected or none of it does. I've been reading the paper Stretching the storage manager: weak pointers and stable names in Haskell, which seems to answer my question. When I attempt to run the memoization code in the paper on the simple fib example, I find that -- apparently due to lazy evaluation -- no new entries are entered into the lookup table, and therefore no lookups are ever successful! So apparently there is some interaction between lazy evaluation and garbage collection that I don't understand. My head hurts. Is it necessary to make the table lookup operation strict? Or is it something entirely different that I am missing? -Rod On Thu, 10 Sep 2009 18:33:47 -0700 Ryan Ingram ryani.s...@gmail.com wrote: memoIO :: Ord a = (a - b) - IO (a - IO b) memoIO f = do cache - newIORef M.empty return $ \x - do m - readIORef cache case M.lookup x m of Just y - return y Nothing - do let res = f x writeIORef cache $ M.insert x res m return res memo :: Ord a = (a - b) - (a - b) memo f = unsafePerformIO $ do fmemo - memoIO f return (unsafePerformIO . fmemo) I don't think there is any valid transformation that breaks this, since the compiler can't lift anything through unsafePerformIO. Am I mistaken? -- ryan ___ 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] Re: help with FFI
typedef struct {
int *a;
int *b;
/*lots of stuff
...
*/
int *z;
} foo;
int create_foo(foo *f,FILE *file,int x,int y);
int use_foo(foo *f,int w);
int destroy_foo(foo *f);
newtype Foo = Foo ()
foreign import ccall static foo.h create_foo
c_create_foo :: Ptr (Foo) - Ptr (CFile) - CInt - CInt - IO CInt
foreign import ccall static foo.h use_foo
c_use_foo :: Ptr (Foo) - CInt - IO CInt
foreign import ccall static foo.h destroy_foo
c_destroy_foo :: Ptr (Foo) - IO CInt
Your 'create_foo' is only an initialization function, i.e., it
initializes data in a Foo but do not allocate memory for the data
such pointer points to. So, what you would like to do is something
like this:
import Foreign.Marshall.Alloc
(...)
ptrFoo - malloc
c_create_foo ptrFoo ptrCFile (...)
(...)
free ptrFoo
Looking at malloc you see:
malloc :: Storable a = IO (Ptr a)
and this gives you the hint: you need to make an
instance of Storable class with your Foo type.
Storable method 'sizeOf' should provide the proper
size to be used in such kind of memory allocation.
(After that, you would probably want to learn
about ForeignPtr, which would call, say, free and
c_destroy_foo when your pointer is not beeing used
anymore.)
Best,
Maurício
P.S.: (Warning: spam promoting my own work.)
If you use my package bindings-common, available in hackage, you
can get that type and instance in a .hsc file by doing this:
#bindings_starttype struct foo
#bindings_stoptype _
But this package is not very popular yet, so
I can't guarantee it's bug free.
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[Haskell-cafe] About the parse error (possibly incorrect indentation)
Hi, I am trying to get the function showMinProp to return String, but I always get an error of parse error (possibly incorrect indentation) Who can help with this? any idea? Thank u in advance! data Prop = Var String | Negation Prop | BinOp Op Prop Prop data Op = And | Or | Implies | Equiv derived Eq showOp :: Op - String showOp And = showOp Or = | showOp Implies = = showOp Equiv= = instance Show Op where show = showOp precList = [(And,4),(Or,3),(Implies,2),(Equiv,1)] showProp :: Prop - String showProp (Var s) = s showProp (Negation p) = ~ ++ showProp p showProp (BinOp op p q) = paren (showProp a p ++ space (showOp op) ++ showProp a q) showMinProp :: Int - Prop - String showMinProp preNo (BinOp op p q) = case op of And - let a = 4 Or - let a = 3 Implies - let a = 2 Equiv - let a = 1 if (a preNo) then (showMinProp a p ++ space (showOp op) ++ showMinProp a q) else paren (showMinProp a p ++ space (showOp op) ++ showMinProp a q)) space s = ++ s ++ paren s = ( ++ s ++ ) instance Show Prop where show = showProp ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] About the parse error (possibly incorrect indentation)
Am Freitag 18 September 2009 02:17:22 schrieb xu zhang: showMinProp :: Int - Prop - String showMinProp preNo (BinOp op p q) = case op of And - let a = 4 Or - let a = 3 Implies - let a = 2 Equiv - let a = 1 if (a preNo) then (showMinProp a p ++ space (showOp op) ++ showMinProp a q) else paren (showMinProp a p ++ space (showOp op) ++ showMinProp a q)) showMinProp preNo (BinOp op p q) = let a = case op of And - 4 ... in if a preNo then (...) else (...) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to understand the 'forall' ?
Cristiano Paris wrote: On Wed, Sep 16, 2009 at 7:12 PM, Ryan Ingram ryani.s...@gmail.com wrote: Here's the difference between these two types: test1 :: forall a. a - Int -- The caller of test1 determines the type for test1 test2 :: (forall a. a) - Int -- The internals of test2 determines what type, or types, to instantiate the argument at I can easily understand your explanation for test2: the type var a is closed under existential (?) quantification. I can't do the same for test1, even if it seems that a is closed under universal (?) quantification as well. It's not existential, it's rank-2 universal. The first function is saying that for every type, a, there is a function, te...@a, taking a into Int. This is very different than the second function which says there is a function, test2, taking values which belong to all types into Int. The test2 function is not polymorphic, instead it takes arguments which are polymorphic. Since _|_ is the only inhabitant of all types, it may be helpful to consider these functions instead: f :: forall a. (a - a) - Int g :: (forall a. a - a) - Int The function f takes a monomorphic function of type (a-a) for some predefined a. Which a? Well it can be any of them, but it can only be one of them at a time. That is, the invocation of f will make the a concrete. The function g takes a polymorphic function which, being polymorphic, will work for every a. That is, the invocation of g does not make the type concrete; only the invocation of the argument within the body of g will make the type concrete. Moreover, the argument must be able to be made concrete for every a, it can't pick and choose. This is different from existential quantification which means the argument works for some a, but it won't tell us which one. To put it another way, this is a valid definition of g: (\ r - let _ = r Nothing ; _ r hello in 42) Since r is polymorphic, we can use it at two different types. Whereas if we gave this definition for f it wouldn't type check since we can't unify (Maybe a) and String. Or, to put it another way, since there are no non-bottom objects of type (forall a. a): Why? By definition of Haskell semantics. The only value belonging to all types is _|_ (whether undefined or various exception bottoms). Perhaps it'd make more sense to look at another type. What values inhabit (forall a. Maybe a)? Well _|_ inhabits all types, so it's there. And Nothing doesn't say anything about a, so it's there too since it works for all a. And (Just _|_) is there since Just doesn't say anything about the type a and _|_ belongs to all types so it doesn't say anything about a either. And that's it. Any other value must say something about the type a, thus restricting it, and then it would no longer be universally quantified. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ∀ lexing in ghc and ghci
Hi, I'm getting different behavior in ghci and ghc with the identifier ∀. In ghc I need to wrap it with parens, as in (∀) :: Var - Base - Formula - Formula (∀) = All In ghci, I get an error this way Formula.lhs:112:2: Invalid type signature In ghci I can do ∀ :: Var - Base - Formula - Formula ∀ = All fine. But then ghc complains. What's going on here? Thanks! Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ∀ lexing in ghc and ghci
Am Freitag 18 September 2009 03:31:13 schrieb Sean McLaughlin: Hi, I'm getting different behavior in ghci and ghc with the identifier ∀. In ghc I need to wrap it with parens, as in (∀) :: Var - Base - Formula - Formula (∀) = All In ghci, I get an error this way Formula.lhs:112:2: Invalid type signature In ghci I can do ∀ :: Var - Base - Formula - Formula ∀ = All fine. But then ghc complains. What's going on here? Very odd: GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude let ∀ :: Int - Int - Int; ∀ x y = x*(y-x) interactive:1:4: parse error on input `∀' Prelude let (∀) :: Int - Int - Int; x ∀ y = x*(y-x) Prelude 3 ∀ 5 6 Maybe your encodings aren't UTF8? Thanks! Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ∀ lexing in ghc and ghci
Hi Daniel, Would you try putting that in a file and loading it in ghci? Your example also works for me. Prelude let (∀) = 5 Prelude (∀) 5 Sean On Thu, Sep 17, 2009 at 9:41 PM, Daniel Fischer daniel.is.fisc...@web.dewrote: Am Freitag 18 September 2009 03:31:13 schrieb Sean McLaughlin: Hi, I'm getting different behavior in ghci and ghc with the identifier ∀. In ghc I need to wrap it with parens, as in (∀) :: Var - Base - Formula - Formula (∀) = All In ghci, I get an error this way Formula.lhs:112:2: Invalid type signature In ghci I can do ∀ :: Var - Base - Formula - Formula ∀ = All fine. But then ghc complains. What's going on here? Very odd: GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude let ∀ :: Int - Int - Int; ∀ x y = x*(y-x) interactive:1:4: parse error on input `∀' Prelude let (∀) :: Int - Int - Int; x ∀ y = x*(y-x) Prelude 3 ∀ 5 6 Maybe your encodings aren't UTF8? Thanks! Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ∀ lexing in ghc and ghci
Am Freitag 18 September 2009 03:51:40 schrieb Sean McLaughlin: Hi Daniel, Would you try putting that in a file and loading it in ghci? Your example also works for me. Prelude let (∀) = 5 Prelude (∀) 5 Sean Sure: da...@linux-mkk1:~/Haskell/CafeTesting cat Forall.hs module Forall where (∀) :: Int - Int - Int x ∀ y = x*(y-x) da...@linux-mkk1:~/Haskell/CafeTesting ghci Forall GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Forall ( Forall.hs, interpreted ) Ok, modules loaded: Forall. *Forall 7 ∀ 4 -21 *Forall Works here. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is it safe to use unsafePerformIO here?
I am confused about why this thread is talking about unsafePerformIO at all. It seems like everything you all want to do can be accomplished with the much less evil unsafeInterleaveIO instead. (Which is still a bit evil; but it's the difference between stealing cookies from the cookie jar and committing genocide) I wrote this function recently for a quick'n'dirty script: readFiles :: [FilePath] - String readFiles [] = return readFiles (f:fs) = do f_data - readFile f rest - unsafeInterleaveIO (readFiles fs) return (f_data ++ rest) It lazily reads from many files and concatenates all the input. But I probably wouldn't use it in a serious application. -- ryan On Thu, Sep 17, 2009 at 3:41 PM, Daniel Fischer daniel.is.fisc...@web.dewrote: Am Donnerstag 17 September 2009 22:20:55 schrieb Cristiano Paris: On Thu, Sep 17, 2009 at 10:01 PM, Daniel Fischer daniel.is.fisc...@web.de wrote: ... readBit fn = do txt - readFile fn let (l,_:bdy) = span (/= '\n') txt return $ Bit (read l) bdy ? With main = do args - getArgs let n = case args of (a:_) - read a _ - 1000 bl - mapM readBit [file1.txt,file2.txt] mapM_ (putStrLn . show . index) $ sortBy (comparing index) bl mapM_ (putStrLn . take 20 . drop n . body) bl Yes, it *seems* to work but... the files don't get closed (readFile is unfinished until body is read) so I think I'm going to have problems when the number of files to read is higher than the maximum number of open handles a process can have. Indeed. If the number of files is large, reading lazily with readFile is not so good. Eat the cake and have it. If you have a lot of files, want to read the metadata of all, select a (much) smaller number of files by some criterion on the set of metadata and then read the body of the selected files, it's hairy. Reading all bodies immediately is probably out due to memory restrictions. The clean approach would be to separate the reading of metadata and body. The drawback is that then you have a second entry into IO. Using unsafePerformIO, you can pretend that you don't reenter IO. Whether that is safe in your situation, I don't know. Probably not (rule of thumb: all nontrivial actions wrapped in unsafePerformIO aren't safe, though chances aren't bad that it works most of the time). That's a possibility I considered even if not directly using readFile. Cristiano ___ 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] Re: [Haskell-beginners] map question
Heh, perhaps we should petition to have a new computer key and symbol added to the world's way of writing maths, something like maybe a downward angled slash to mean prefix (-) :) --- On Thu, 9/17/09, Job Vranish jvran...@gmail.com wrote: From: Job Vranish jvran...@gmail.com Subject: Re: [Haskell-cafe] Re: [Haskell-beginners] map question To: Gregory Propf gregorypr...@yahoo.com Cc: Tom Doris tomdo...@gmail.com, Haskell-Cafe haskell-cafe@haskell.org, joostkrem...@fastmail.fm Date: Thursday, September 17, 2009, 9:04 AM (-) happens to be the only prefix operator in haskell, it also an infix operator. so: 4 - 2 2 -3 -3 ((-) 5) 3 -- note that in this case (-) is treated like any regular function so 5 is the first parameter 2 (5 - ) 3 2 (-5 ) -5 (flip (-) 5) 3 -2 It's a little wart brought about by the ambiguity in common mathematical syntax. If you play around in ghci you should get the hang of it pretty quick. - Job On Thu, Sep 17, 2009 at 11:08 AM, Gregory Propf gregorypr...@yahoo.com wrote: Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On Thu, 9/17/09, Tom Doris tomdo...@gmail.com wrote: From: Tom Doris tomdo...@gmail.com Subject: Re: [Haskell-beginners] map question To: Joost Kremers joostkrem...@fastmail.fm Cc: beginn...@haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners -Inline Attachment Follows- ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners ___ 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] Re: Where can I find a non-fee-based version of Hudak's paper, Conception, evolution, and application of functional programming languages?
On Thu, 17 Sep 2009 08:55:19 -0700, John Melesky l...@phaedrusdeinus.org wrote: On 2009-09-17, at 1:41 AM, Benjamin L.Russell wrote: Does anybody know where I can find a non-fee-based version of Paul Hudak's paper, Conception, evolution, and application of functional programming languages [1]? When in doubt, check citeseer. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.83.6505 Thank you; although attempting a download from the referenced DOWNLOAD site at http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=4185A586D773743C182087024049A81E?doi=10.1.1.83.6505rep=rep1type=urli=0 resulted in the same error as before, I was still able to download a cached copy from the referenced CACHED site at http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=4185A586D773743C182087024049A81E?doi=10.1.1.83.6505rep=rep1type=pdf; that is a very useful service. -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 Furuike ya, kawazu tobikomu mizu no oto. -- Matsuo Basho^ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ∀ lexing in ghc and ghci
Weird. OK, thanks a lot! I'm switching to ¥ until I get this figured out. Sean On Thu, Sep 17, 2009 at 10:00 PM, Daniel Fischer daniel.is.fisc...@web.dewrote: Am Freitag 18 September 2009 03:51:40 schrieb Sean McLaughlin: Hi Daniel, Would you try putting that in a file and loading it in ghci? Your example also works for me. Prelude let (∀) = 5 Prelude (∀) 5 Sean Sure: da...@linux-mkk1:~/Haskell/CafeTesting cat Forall.hs module Forall where (∀) :: Int - Int - Int x ∀ y = x*(y-x) da...@linux-mkk1:~/Haskell/CafeTesting ghci Forall GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Forall ( Forall.hs, interpreted ) Ok, modules loaded: Forall. *Forall 7 ∀ 4 -21 *Forall Works here. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] code-build-test cycle
I'm working on a GUI application in qtHaskell, and I have a bit of a bind. Using ghci, it launches quickly but runs slowly. On the other hand, compiling (mainly linking) takes a while---several minutes. The truth is that I can compile it much faster if I selectively import the needed modules, so figure the actual compilation/link time is more like 15 to 30 seconds. (This is Windows on a very old laptop.) I'm used to working in Python, so I'm used to a nearly instant code-build-test cycle, and GUI applications in PyQt run briskly, faster than ghci/qtHaskell. Now I'm wondering if Hugs is a faster interpreter. So during development I don't want to give up the quick cycle you get with an interpreter, but the application may be much too slow to use in any meaningful way without compilation. Any advice welcome. Maybe there is a way to speed up the interpretation. -Mike ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ∀ lexing in ghc and ghci
Am Friday 18 September 2009 04:41:13 schrieben Sie: Weird. OK, thanks a lot! I'm switching to ¥ until I get this figured out. Sean What does your ghci say for Data.Char.isSymbol (toEnum 8704) ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is it safe to use unsafePerformIO here?
Am Freitag 18 September 2009 04:06:11 schrieb Ryan Ingram: I am confused about why this thread is talking about unsafePerformIO at all. It seems like everything you all want to do can be accomplished with the much less evil unsafeInterleaveIO instead. (Which is still a bit evil; but it's the difference between stealing cookies from the cookie jar and committing genocide) I find that remark in rather bad taste. I wrote this function recently for a quick'n'dirty script: readFiles :: [FilePath] - String readFiles [] = return readFiles (f:fs) = do f_data - readFile f rest - unsafeInterleaveIO (readFiles fs) return (f_data ++ rest) It lazily reads from many files and concatenates all the input. But I probably wouldn't use it in a serious application. -- ryan But that does something completely different from what Cristiano wants to do. He wants to read many files files quasi-parallel. As far as I can tell, he needs to read a small chunk from the beginning of every file, then, depending on what he got from that, he needs to read the rest of some files. If he reads all the files lazily, he (maybe) runs into the open file limit (a semi-closed handle is still open from the OS' point of view, isn't it?). So he has to close the first files before he opens the Nth. But what if later he finds out that he has to read the body of a previously closed file? I would separate the reading of headers and bodies, reopening the files whose body is needed, for some (maybe compelling) reason he wants to do it differently. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] code-build-test cycle
Am Freitag 18 September 2009 04:42:32 schrieb Michael Mossey: I'm working on a GUI application in qtHaskell, and I have a bit of a bind. Using ghci, it launches quickly but runs slowly. On the other hand, compiling (mainly linking) takes a while---several minutes. The truth is Is the library you're using built with split-objs? If not, that would explain the long link time. that I can compile it much faster if I selectively import the needed modules, so figure the actual compilation/link time is more like 15 to 30 seconds. (This is Windows on a very old laptop.) I'm used to working in Python, so I'm used to a nearly instant code-build-test cycle, and GUI applications in PyQt run briskly, faster than ghci/qtHaskell. Now I'm wondering if Hugs is a faster interpreter. Usually it isn't. It's faster loading the code than ghci, but slower running it. So during development I don't want to give up the quick cycle you get with an interpreter, but the application may be much too slow to use in any meaningful way without compilation. Any advice welcome. Maybe there is a way to speed up the interpretation. Smaller modules, so that only the hopefully few modules that were changed or depend on a changed module need be recompiled? (Not sure that would help with linking, though) -Mike ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] About the parse error (possibly incorrect indentation)
On Sep 17, 2009, at 20:17 , xu zhang wrote:
case op of
And - let a = 4
Or - let a = 3
Implies - let a = 2
Equiv - let a = 1
let isn't an assignment command, it's a scoping command. That is,
let a = 3 in ...
is equivalent to something like
{ int a = 3; ... } // except the type can be inferred
Note that the in is required unless part of a do-block, where block
structure is implied.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon universityKF8NH
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Re: [Haskell-cafe] code-build-test cycle
Hello Michael, Friday, September 18, 2009, 6:42:32 AM, you wrote: Now I'm wondering if Hugs is a faster interpreter. 2x slower, and incompatib;e with qtHaskell meaningful way without compilation. Any advice welcome. Maybe there is a way to speed up the interpretation. if compilation is fast and only linking is slow, you may recompile haskell modules every time but use ghci to omit linking. just execute ghc compilation command inside ghci before running your app -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
