[Haskell-cafe] Re: the real world of Haskell books (Re: Online Real World Haskell, problem with Sqlite3 chapters)
Bryan == Bryan O'Sullivan [EMAIL PROTECTED] writes: Bryan Other tech books face the same problem, which, if they sell Bryan successfully and the authors haven't moved into caves afterwards Bryan to recover, they address with subsequent editions. If readers Bryan find that specific pieces of information have bitrotted, I'm sure Bryan we'll hear about it. In that case, we'll create a wiki page with Bryan errata, and link to it from the book site. I think we should be happy that RWH book is done and considering it is for 'real-world' users I'm sure they will find a way to cope for upgrading libs, non-working packages etc. A blind uncle is better than no uncle :-D Congrats to RWH's 'Gang od Three' ;) Sincerely, Gour -- Gour | Zagreb, Croatia | GPG key: C6E7162D pgpXrLgWpDaNO.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] darcs hacking sprint, venues confirmed! (25-26 October)
Dear darcs users and Haskell hackers, Earlier I wrote an announcement for the darcs hacking sprint on 25-26 October. Tonight, I am delighted to announce that we have two venues confirmed for the sprint and one serious offer. Venues -- We plan to host the sprint across three sites: * CONFIRMED: Brighton, UK (University of Brighton) * CONFIRMED: Portland, USA(Galois Connections) * likely:Paris,France (Université Paris Diderot) So if you were waiting to book tickets, this is the time! For more details, please see http://wiki.darcs.net/index.html/Sprints Agenda -- During this first sprint, we shall be focusing our attention on the day to day performance issues that darcs users commonly face. This is what we are reaching for: 1. Fast network operations. We want to make it very pleasant for users to darcs get a repository and pull some patches to it over http and ssh. Git does this very well, and we plan to learn from them. 2. Cutting memory consumption. We want to profile the heck out of operations like darcs record, darcs convert and darcs whatsnew. What's eating up all the memory? And how can we can cut it down to size? 3. Responsiveness. Sometimes basic darcs commands can take long enough for programmers to lose their train of thought. We want to track down these lost seconds and kill that dreaded context switch. Of course, if you are interested in other areas, then you can work on those instead. Note that if you are new to the darcs code or to Haskell, there will also be a lot interesting jobs for you to get started with. Everyone will have something to hack on, so come join us! Thanks very much to the University of Brighton, Galois connections and University of Paris VII for their generous offers. Hope to see you there, everyone! :-) -- Eric Kow http://www.nltg.brighton.ac.uk/home/Eric.Kow PGP Key ID: 08AC04F9 pgpA6LUf9gHEz.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Top Level -
Ganesh Sittampalam wrote: But it's limited to the initialisers. An IORef holding an Integer isn't much memory, and it only ever gets leaked once. It happens every time you load and unload, surely? No. An initialiser is only ever run once per run of the RTS. Also I thought this was a general discussion with Data.Unique as a concrete example; something else might leak substantially more memory. Your witnesses stuff would leak one Integer per module, wouldn't it? It would leak one Integer per IOWitness initialiser for the run of the RTS. Finally, any memory leak at all can be unacceptable in some contexts. It's certainly not something we should just dismiss as oh, it's only small. Since it's of the order of the number of uniquely identified initialisers, it's arguably not a memory leak so much as a static overhead. The only way to get a continuous leak is to load and unload an endless stream of _different_ modules, each with their own initialisers. -- Ashley Yakeley ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Top Level -
On Sat, 6 Sep 2008, Ashley Yakeley wrote: Ganesh Sittampalam wrote: But it's limited to the initialisers. An IORef holding an Integer isn't much memory, and it only ever gets leaked once. It happens every time you load and unload, surely? No. An initialiser is only ever run once per run of the RTS. Oh, I see. Yes, sorry. Since it's of the order of the number of uniquely identified initialisers, it's arguably not a memory leak so much as a static overhead. The only way to get a continuous leak is to load and unload an endless stream of _different_ modules, each with their own initialisers. I would call it a leak if something that is no longer being used cannot be reclaimed. The endless stream of different modules is possible in long-running systems where the code being run evolves or changes over time (e.g. something like lambdabot, which runs user-provided code). Cheers, Ganesh ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Top Level -
Ganesh Sittampalam wrote: I would call it a leak if something that is no longer being used cannot be reclaimed. The endless stream of different modules is possible in long-running systems where the code being run evolves or changes over time (e.g. something like lambdabot, which runs user-provided code). This might be fixable with an option to the dynamic load function. Let us say a module M has a number of top-level - of the form val - exp The set of ACIO expressions exp is the static initialisers of M. The RTS must note when each static initialiser is run, and cache its result val. Let's call this cache of vals the static results cache of M. When M is loaded, and a static results cache for M already exists, then it will be used for the vals of M. It is the static results cache that might leak. Let us have an flag to the dynamic load function, to mark the static results cache of M as freeable. If the static results cache is freeable, then it will be deleted when M is unloaded (and M is not part of the main program). If you pass True for this flag, your code is unsafe if all of the following: * M has static initialisers * M will be loaded again after unloading * Values from M will be stored elsewhere in the program. If you pass False for this flag, your code will continuously leak memory if you continuously load modules * that are all different * that contain static initialisers There may also have to be some way to specify how to apply the flag to dependencies as well. In general I'm way beyond my knowledge of the RTS, so I may have something Very Wrong here. I don't think hs-plugins implements unloading at all currently. -- Ashley Yakeley ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [xmonad] ANNOUNCE: xmonad 0.8 released!
nomeata: Hi, Am Freitag, den 05.09.2008, 18:38 -0700 schrieb Don Stewart: xmonad packages are available in the package systems of at least: Debian, Gentoo, Arch, Ubuntu, OpenBSD, NetBSD, FreeBSD, Gobo, NixOS, Source Mage, Slackware and 0.8 packages will appear in coming days (some are already available). Debian xmonad and xmonad-contrib package uploaded (although xmonad-contrib is in the two-day delayed queue, to give autobuilders a chance to build xmonad first). Great work, Joachim! Thanks for being so responsive on this. Go Debian! -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] What's the best way to give up?
Hi, all. Say I have a function solve which is a constraint solver. It reconfigures its input to be a solution. If there is no solution, it returns the input. solve :: a - Either a a solve input output = maybe (Left input) Right $ solve' input If there is a solution, it finds it in a few seconds. If there is no solution, it goes away for days proving that. So, I'd like to give up on it if it doesn't return in a few seconds. I can think of several ways of doing that. I could keep a tally of the number of variable assignments and give up when it reaches an impossibly huge number. I could change the type to a - IO (Either a a ) and use getCPUTime. Is there a standard way to do this? Can you think of another way to do it? Cheers, David ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GADTs and instances
Hello haskellers,
Anyone know the trick for making a Binary instance of a GADT.
See sample code below followed by the type error reported by ghc version
6.8.3
Thanks,
Tony
{-# LANGUAGE GADTs #-}
module GADTTest where
import Data.Binary
import Control.Monad (liftM)
data Query a where
Lookup :: String - Query (Maybe Int)
Fetch :: [String] - Query [Int]
instance (Binary a) = Binary (Query a) where
put (Lookup x) = putWord8 0 put x
put (Fetch x) = putWord8 1 put x
get = getWord8 = \tag - case tag of
0 - liftM Lookup get
1 - liftM Fetch get
-
GADTTest.hs:12:0:
Couldn't match expected type `Maybe Int'
against inferred type `[Int]'
When trying to generalise the type inferred for `get'
Signature type: forall a. (Binary a) = Get (Query a)
Type to generalise: Get (Query a)
In the instance declaration for `Binary (Query a)'
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Re: [Haskell-cafe] What's the best way to give up?
Hi there, On Sat, Sep 6, 2008 at 13:30, David F. Place [EMAIL PROTECTED] wrote: If there is a solution, it finds it in a few seconds. If there is no solution, it goes away for days proving that. So, I'd like to give up on it if it doesn't return in a few seconds. I can think of several ways of doing that. I could keep a tally of the number of variable assignments and give up when it reaches an impossibly huge number. I could change the type to a - IO (Either a a ) and use getCPUTime. Is there a standard way to do this? Can you think of another way to do it? I don't know, but this seems relevant: http://www.haskell.org/pipermail/haskell-cafe/2005-October/011946.html I'd make a generic (i.e. higher-order) function that handles the timeout for any computation. i.e. timeout :: (a - b) - Int - a - IO (Maybe b) or similar. Arnar ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What's the best way to give up?
a - IO (Either a a ) and use getCPUTime. Is there a standard way to do this? Can you think of another way to do it? I don't know, but this seems relevant: http://www.haskell.org/pipermail/haskell-cafe/2005-October/011946.html I'd make a generic (i.e. higher-order) function that handles the timeout for any computation. i.e. timeout :: (a - b) - Int - a - IO (Maybe b) http://haskell.org/ghc/docs/latest/html/libraries/base/System-Timeout Perhaps this is for not so much for pure computations as hanging when connecting to nonresponsive servers, although I also use it in a case where buggy long-running code could frustrate the responsiveness of unrelated processes.. --Lane ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Top Level -
On 2008 Sep 6, at 6:10, Ashley Yakeley wrote: Ganesh Sittampalam wrote: I would call it a leak if something that is no longer being used cannot be reclaimed. The endless stream of different modules is possible in long-running systems where the code being run evolves or changes over time (e.g. something like lambdabot, which runs user-provided code). This might be fixable with an option to the dynamic load function. Let us say a module M has a number of top-level - of the form val - exp The set of ACIO expressions exp is the static initialisers of M. The RTS must note when each static initialiser is run, and cache its result val. Let's call this cache of vals the static results cache of M. When M is loaded, and a static results cache for M already exists, then it will be used for the vals of M. This sounds reachable to me, and therefore static overhead and not a leak. Your proposed freeable flag is still useful, but I think this is not a problem. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What's the best way to give up?
On 2008 Sep 6, at 7:30, David F. Place wrote: Say I have a function solve which is a constraint solver. It reconfigures its input to be a solution. If there is no solution, it returns the input. solve :: a - Either a a solve input output = maybe (Left input) Right $ solve' input If there is a solution, it finds it in a few seconds. If there is no solution, it goes away for days proving that. So, I'd like to give up on it if it doesn't return in a few seconds. http://www.haskell.org/haskellwiki/Timing_out_computations -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Top Level -
On Sat, 6 Sep 2008, Brandon S. Allbery KF8NH wrote: On 2008 Sep 6, at 6:10, Ashley Yakeley wrote: The set of ACIO expressions exp is the static initialisers of M. The RTS must note when each static initialiser is run, and cache its result val. Let's call this cache of vals the static results cache of M. When M is loaded, and a static results cache for M already exists, then it will be used for the vals of M. This sounds reachable to me, and therefore static overhead and not a leak. You can call it what you like, but it's still unacceptable behaviour, particularly since clients of M will have no way of telling from its API that it will happen. Ganesh ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Top Level -
On 2008 Sep 6, at 11:22, Ganesh Sittampalam wrote: On Sat, 6 Sep 2008, Brandon S. Allbery KF8NH wrote: On 2008 Sep 6, at 6:10, Ashley Yakeley wrote: The set of ACIO expressions exp is the static initialisers of M. The RTS must note when each static initialiser is run, and cache its result val. Let's call this cache of vals the static results cache of M. When M is loaded, and a static results cache for M already exists, then it will be used for the vals of M. This sounds reachable to me, and therefore static overhead and not a leak. You can call it what you like, but it's still unacceptable behaviour, particularly since clients of M will have no way of telling from its API that it will happen. You want run-once behavior without giving the runtime the ability to tell that it's already been run? -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Top Level -
On Sat, 6 Sep 2008, Brandon S. Allbery KF8NH wrote: On 2008 Sep 6, at 11:22, Ganesh Sittampalam wrote: On Sat, 6 Sep 2008, Brandon S. Allbery KF8NH wrote: On 2008 Sep 6, at 6:10, Ashley Yakeley wrote: The set of ACIO expressions exp is the static initialisers of M. The RTS must note when each static initialiser is run, and cache its result val. Let's call this cache of vals the static results cache of M. When M is loaded, and a static results cache for M already exists, then it will be used for the vals of M. This sounds reachable to me, and therefore static overhead and not a leak. You can call it what you like, but it's still unacceptable behaviour, particularly since clients of M will have no way of telling from its API that it will happen. You want run-once behavior without giving the runtime the ability to tell that it's already been run? I don't want run-once behaviour, other people do. I'm just trying to pin down what it would mean. Ganesh ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What's the best way to give up?
On Sat, 2008-09-06 at 11:10 -0400, Brandon S. Allbery KF8NH wrote:
On 2008 Sep 6, at 7:30, David F. Place wrote:
Say I have a function solve which is a constraint solver. It
reconfigures its input to be a solution. If there is no solution,
it returns the input.
solve :: a - Either a a
solve input output = maybe (Left input) Right $ solve' input
If there is a solution, it finds it in a few seconds. If there is
no solution, it goes away for days proving that. So, I'd like to
give up on it if it doesn't return in a few seconds.
http://www.haskell.org/haskellwiki/Timing_out_computations
Thanks, Arnar, Christopher and Brandon. I looked into all your
suggestions.
I came up with this:
import Control.Exception
import System.Timeout
apt :: Int - (a - a) - a - IO (Either a a)
apt time f x =
do { ans - timeout time $ evaluate (f x)
; return $ maybe (Left x) Right ans
}
test x = sum [1..x]
test2 x = length $ repeat x
Which seems to do what I want for test:
*Main apt 100 test 100
Right 5050
*Main apt 100 test 1000
Right 500500
*Main apt 100 test 1
Right 50005000
*Main apt 100 test 10
Left 10
*Main apt 100 test 100
Left 100
*Main apt 100 test 1000
Left 1000
But, the following sets the cpu spinning and can't be interrupted.
*Main apt 100 test2 100
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Re: [Haskell-cafe] What's the best way to give up?
On Sat, 2008-09-06 at 12:17 -0400, David F. Place wrote:
On Sat, 2008-09-06 at 11:10 -0400, Brandon S. Allbery KF8NH wrote:
On 2008 Sep 6, at 7:30, David F. Place wrote:
Say I have a function solve which is a constraint solver. It
reconfigures its input to be a solution. If there is no solution,
it returns the input.
solve :: a - Either a a
solve input output = maybe (Left input) Right $ solve' input
If there is a solution, it finds it in a few seconds. If there is
no solution, it goes away for days proving that. So, I'd like to
give up on it if it doesn't return in a few seconds.
http://www.haskell.org/haskellwiki/Timing_out_computations
Thanks, Arnar, Christopher and Brandon. I looked into all your
suggestions.
I came up with this:
import Control.Exception
import System.Timeout
apt :: Int - (a - a) - a - IO (Either a a)
apt time f x =
do { ans - timeout time $ evaluate (f x)
; return $ maybe (Left x) Right ans
}
test x = sum [1..x]
test2 x = length $ repeat x
Which seems to do what I want for test:
*Main apt 100 test 100
Right 5050
*Main apt 100 test 1000
Right 500500
*Main apt 100 test 1
Right 50005000
*Main apt 100 test 10
Left 10
*Main apt 100 test 100
Left 100
*Main apt 100 test 1000
Left 1000
But, the following sets the cpu spinning and can't be interrupted.
*Main apt 100 test2 100
length (repeat x) doesn't require allocating any memory. GHC only
performs a context switch at heap checks. If there are no allocations,
there are no heap checks and therefore no context switches.
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Re: [Haskell-cafe] Re: Functional references
Tim Newsham's approach with invertible functions is interesting, though it feels like it's another layer wrapped on top of the primitive idea of functional references. Not all of the refs are using invertible functions.. however, the ones that are invertible are more flexible.. you can use them to make functional references that are mapped over lists, for example. I've reformulated my code a few times since initially posting it.. In the latest incarnation, FRefGen (generic FRefs with arbitrary get/modify) and Inv (invertible functions) are both instances of an FRef class. In earlier versions I had a function for converting an invertible into an FRef. ~wren Tim Newsham http://www.thenewsh.com/~newsham/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] experimental static blog engine in Haskell (file based, markdown syntax)
Hia, It's called Panda. It's pretty young, no theme, no tags, no comments, around 360 lines of code and uses Kibro to bootstrap. hosted on GitHub: http://github.com/nfjinjing/panda/ a quick demo at: http://jinjing.blog.easymic.com/ cheers, - jinjing ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] experimental static blog engine in Haskell (file based, markdown syntax)
nfjinjing: Hia, It's called Panda. It's pretty young, no theme, no tags, no comments, around 360 lines of code and uses Kibro to bootstrap. hosted on GitHub: http://github.com/nfjinjing/panda/ a quick demo at: http://jinjing.blog.easymic.com/ Awesome! Kibro seems to be taking off. Will you release it on hackage.haskell.org? -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell Weekly News: Issue 84 - September 6, 2008
--- Haskell Weekly News http://sequence.complete.org/hwn/20080906 Issue 84 - September 06, 2008 --- Welcome to issue 84 of HWN, a newsletter covering developments in the [1]Haskell community. This is the This issue is not late since the HWN will henceforth be published on Saturday now that I have real work to do edition. Featured this week: darcs hacking sprint plans solidify, xmonad 0.8 released, typed sprintf and sscanf, and tons of discussion about everything from functional references to splitting up the base library to the future direction of Haskell. Announcements unicode-properties 3.2.0.0, unicode-names 3.2.0.0. Ashley Yakeley [2]announced the release of the [3]unicode-properties 3.2.0.0 and [4]unicode-names 3.2.0.0 packages, which are representations in Haskell of various data in the Unicode 3.2.0 Character Database. experimental static blog engine in Haskell. jinjing [5]announced the initial release of [6]Panda, an experimental static blog engine written in Haskell. darcs hacking sprint, venues confirmed! (25-26 October). Eric Y. Kow [7]announced that two venues (Brighton, UK and Portland, Oregon, USA) have been confirmed for the [8]darcs hacking sprint on 25-26 October. darcs weekly news #2. Eric Y. Kow The second weekly issue of the [9]darcs weekly news has been published. xmonad 0.8 released!. Don Stewart [10]announced the release of [11]xmonad 0.8, featuring a general purpose gaps replacement, locale support, the ability to create your own configuration parsers, and various other [12]enhancements and fixes. POPL logo design contest. Don Stewart [13]forwarded a message announcing a logo design contest for POPL 2009. Dust off your magic markers/photoshop skills and get designing! ICFP09 Announcement. Matthew Fluet [14]announced [15]ICFP 2009, to be held 31st August to 2nd September 2009 in Edinburgh. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Fast parallel binary-trees for the shootout: Control.Parallel.Strategies FTW!. Don Stewart [16]announced that the Computer Language Shootout recently got a quad core 64 bit machine, and outlined a plan and some initial results for porting the Haskell entries to take advantage of the available parallelism. The initial view on typed sprintf and sscanf. oleg [17]announced an implementation of typed sprintf and sscanf functions sharing the same formatting specifications, which also led to some interesting discussion and alternative proposals. Discussion Generalize groupBy in a useful way?. Bart Massey [18]proposed changing the implementation of groupBy to extend its usefulness for predicates which are not equivalence relations. Splitting SYB from the base package in GHC 6.10. José Pedro Magalhães [19]initiated a discussion regarding splitting the SYB libraries out of the base package for GHC 6.10. The base library and GHC 6.10. Ian Lynagh initiated a [20]discussion on further splitting up the base package for GHC 6.10. Functional references. Tim Newsham began a [21]discussion on functional references and the possibility of merging the existing four or five implementations into something more standard. Types and Trees. Matt Morrow [22]wrote something about types and type representations, involving some commutative diagrams and some code. I haven't read it yet but it looks neat! Research language vs. professional language. Ryan Ingram started an interesting [23]discussion on the future direction(s) of the Haskell language. language proposal: ad-hoc overloading. Ryan Ingram [24]proposed adding ad-hoc name overloading to Haskell, prompting quite a bit of discussion. Top Level -. Ashley Yakeley originally [25]asked whether there is any interest in implementing a top level - to run monadic code. This set off a cascade of discussion the likes of which have been rarely seen on the Cafe. I would tell you what the discussion has been about but I must confess that I haven't read it. Blog noise [26]Haskell news from the [27]blogosphere. * Eric Kow (kowey): [28]darcs hacking sprint (25-26 October 2008). * Magnus Therning: [29]Confusion with HaskellDB. * Xmonad: [30]xmonad 0.8 released!. * Real-World Haskell: [31]Speaking in Silicon Valley next week. * Luke Palmer: [32]Composable Input for Fruit. * Luke Palmer: [33]Slicing open the belly of the IO monad in an alternate universe. * FP Lunch: [34]Modular Monad Transformers. * Douglas M. Auclair (geophf): [35]Fuzzy unification parser in Haskell. * Bjoern Edstroem: [36]Speeding up Haskell
Re: [Haskell-cafe] GADTs and instances
The problem is that the caller of get is allowed to say what type of
Query they want with this instance, for example:
] get :: Get (Query Int)
because Int is an instance of Binary, and you claim
] instance Binary a = Binary (Query a)
In fact, since the type of each query is unique, the tagging in put
doesn't help you.
So there is no way to write a generic Binary instance for Query a.
But don't give up! Here's a solution in literate haskell...
{-# LANGUAGE ExistentialQuantification, FlexibleInstances, GADTs #-}
module Query where
import Data.Binary
import Control.Monad (liftM)
data Query a where
Lookup :: String - Query (Maybe Int)
Fetch :: [String] - Query [Int]
One obvious strategy is to use existential types:
data SomeQuery = forall a. SomeQuery (Query a)
You can then make SomeQuery an instance of Binary using very similar
code to your implementation. Now, the code that calls get needs to
be able to deal with any Query type it gets back inside the SomeQuery.
Another possibility is to drop the tagging altogether and use a helper class:
class BinaryQuery a where
putQ :: Query a - Put
getQ :: Get (Query a)
instance BinaryQuery a = Binary (Query a) where
put = putQ
get = getQ
instance BinaryQuery (Maybe Int) where
putQ (Lookup x) = put x
getQ = liftM Lookup get
instance BinaryQuery [Int] where
putQ (Fetch xs) = put xs
getQ = liftM Fetch get
You can also combine the strategies and use SomeQuery when the stored
value needs a tag:
instance Binary SomeQuery where
put (SomeQuery x@(Lookup _)) = putWord8 0 put x
put (SomeQuery x@(Fetch _)) = putWord8 1 put x
get = getWord8 = \tag - case tag of
0 - liftM SomeQuery (get :: Get (Query (Maybe Int)))
1 - liftM SomeQuery (get :: Get (Query [Int]))
The pattern matching in put specializes the type of x, allowing the
query-level put to find the correct implementation. Similarily, in
get we choose the correct get based on the input type.
-- ryan
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Re: [Haskell-cafe] Re: [xmonad] ANNOUNCE: xmonad 0.8 released!
Don Stewart wrote: nomeata: Hi, Am Freitag, den 05.09.2008, 18:38 -0700 schrieb Don Stewart: xmonad packages are available in the package systems of at least: Debian, Gentoo, Arch, Ubuntu, OpenBSD, NetBSD, FreeBSD, Gobo, NixOS, Source Mage, Slackware and 0.8 packages will appear in coming days (some are already available). Debian xmonad and xmonad-contrib package uploaded (although xmonad-contrib is in the two-day delayed queue, to give autobuilders a chance to build xmonad first). Great work, Joachim! Thanks for being so responsive on this. Go Debian! Committed to Gentoo Linux earlier this evening. http://packages.gentoo.org/package/x11-wm/xmonad http://packages.gentoo.org/package/x11-wm/xmonad-contrib Cheers, Lennart K ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [xmonad] ANNOUNCE: xmonad 0.8 released!
kolmodin: Don Stewart wrote: nomeata: Hi, Am Freitag, den 05.09.2008, 18:38 -0700 schrieb Don Stewart: xmonad packages are available in the package systems of at least: Debian, Gentoo, Arch, Ubuntu, OpenBSD, NetBSD, FreeBSD, Gobo, NixOS, Source Mage, Slackware and 0.8 packages will appear in coming days (some are already available). Debian xmonad and xmonad-contrib package uploaded (although xmonad-contrib is in the two-day delayed queue, to give autobuilders a chance to build xmonad first). Great work, Joachim! Thanks for being so responsive on this. Go Debian! Committed to Gentoo Linux earlier this evening. http://packages.gentoo.org/package/x11-wm/xmonad http://packages.gentoo.org/package/x11-wm/xmonad-contrib Gentoo steps up! Excellent work guys. And if I read those graphs correctly, that's xmonad on hppa and sparc too? -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [xmonad] ANNOUNCE: xmonad 0.8 released!
Don Stewart wrote: kolmodin: Don Stewart wrote: nomeata: Hi, Debian xmonad and xmonad-contrib package uploaded (although xmonad-contrib is in the two-day delayed queue, to give autobuilders a chance to build xmonad first). Great work, Joachim! Thanks for being so responsive on this. Go Debian! Committed to Gentoo Linux earlier this evening. http://packages.gentoo.org/package/x11-wm/xmonad http://packages.gentoo.org/package/x11-wm/xmonad-contrib Gentoo steps up! Excellent work guys. And if I read those graphs correctly, that's xmonad on hppa and sparc too? Yep, that's right. Our arch teams has tested an earlier version, thus it's available for this version too since it doesn't introduce any major changes in dependencies or build procedure. Although probably no one has yet build it using our ebuild for this version, we're confident it'll work ;) I'm pleased with upgrading and switching to avoidStruts without even having to log out, well done :) Cheers, Lennart K ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Top Level -
Ganesh Sittampalam wrote: The set of ACIO expressions exp is the static initialisers of M. The RTS must note when each static initialiser is run, and cache its result val. Let's call this cache of vals the static results cache of M. When M is loaded, and a static results cache for M already exists, then it will be used for the vals of M. This sounds reachable to me, and therefore static overhead and not a leak. You can call it what you like, but it's still unacceptable behaviour, particularly since clients of M will have no way of telling from its API that it will happen. That what will happen? -- Ashley Yakeley ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Top Level -
On 2008 Sep 6, at 18:06, Ashley Yakeley wrote: Ganesh Sittampalam wrote: The set of ACIO expressions exp is the static initialisers of M. The RTS must note when each static initialiser is run, and cache its result val. Let's call this cache of vals the static results cache of M. When M is loaded, and a static results cache for M already exists, then it will be used for the vals of M. This sounds reachable to me, and therefore static overhead and not a leak. You can call it what you like, but it's still unacceptable behaviour, particularly since clients of M will have no way of telling from its API that it will happen. That what will happen? I have no idea what Ganesh is complaining about, but if the point of ACIO is one-time initialization, it is pretty much required that something will be allocated to record the fact that a given item has been initialized (otherwise it can't guarantee the initializer runs exactly once, after all), and therefore that is part of the base ACIO API. So what exactly is the issue here? -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Top Level -
On 2008 Sep 6, at 18:25, Ashley Yakeley wrote: 1. Results from initialisers cannot be GC'd even if they become otherwise unreachable, because the dynamic loader might re-load the module (and then we'd need those original results). 2. If the dynamic loader loads an endless stream of different modules containing initialisers, memory will thus leak. I think if the issue is this vs. not being able to guarantee any once- only semantics, i consider the former necessary overhead for proper program behavior. And that, given that there exists extra-program global state that one might want to access, once-only initialization is a necessity. (Whoever it was who jumped off this point to say Haskell should exit the real world misses the point: it's not even all that useful from a theoretical standpoint if it's not allowed to interact with anything outside itself, and stdin/stdout tend to require once-only initialization. You can't really hide from it.) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] (no subject)
Hello. I'm trying to apply the nested regions (as in Lightweight Monadic
Regions by Oleg Kiselyov and Chung-chieh Shan) design pattern, if that's the
proper term. I was hoping to gain a bit more type safety in this little library
I'm working on -- Streaming Component Combinators, available on Hackage. I
guess the problem is that I'm getting too much type safety now, because I can't
get the thing to compile. Most of the existing code works, the only exceptions
seem to be various higher-order functions. I've reduced the problem to several
lines of Literate Haskell code below, can anybody think of a solution or a
reason there can't be one?
{-# LANGUAGE MultiParamTypeClasses, EmptyDataDecls, Rank2Types #-}
{-# LANGUAGE FunctionalDependencies, FlexibleInstances, IncoherentInstances
#-}
module Main where
main = undefined
I'll call the main type, originally a monad transformer, simply Region. I'm
leaving out the Monad and MonadTransformer instances, because they don't
contribute to the problem. The parameter r is the phantom region type.
newtype Region r a = Region a
The Ancestor class is used to denote relationship between two regions where one
is nested in another.
data Child r
class Ancestor r r'
instance Ancestor r (Child r)
instance Ancestor r1 r2 = Ancestor r1 (Child r2)
Handle is a simple wrapper around a value. It carries information about the
region that originates the value.
data Handle r x = Handle x
A typical calculation in the Region monad will take a bunch of Handles
inherited from an Ancestor region and do something with them. The Ancestor
constraint is there to ensure that the handles are not fake but genuinely
inherited.
type SingleHandler x y = forall r1s rs. Ancestor r1s rs =
Handle r1s x - Region rs y
type DoubleHandler x y z = forall r1d r2d rd. (Ancestor r1d rd, Ancestor r2d
rd) =
Handle r1d x - Handle r2d y - Region rd z
And now I'm getting to the problem. The following higher-order function doesn't
type-check:
mapD :: (SingleHandler x z - SingleHandler y z)
- DoubleHandler x w z - DoubleHandler y w z
mapD f d = \y w- f (\x- d x w) y
I get the same error from GHC 6.8.2 and 6.8.2:
Test.lhs:36:28:
Could not deduce (Ancestor r2d rs)
from the context (Ancestor r1s rs)
arising from a use of `d' at Test.lhs:36:28-32
Possible fix:
add (Ancestor r2d rs) to the context of
the polymorphic type
`forall r1s rs. (Ancestor r1s rs) = Handle r1s x - Region rs z'
In the expression: d x w
In the first argument of `f', namely `(\ x - d x w)'
In the expression: f (\ x - d x w) y
The same code compiles just fine if all the Ancestor constraints are removed. I
don't see any place to add the extra (Ancestor r2d rs) constraint, as GHC
recommends. I think it ought to be able to figure things out based on the
exisisting constraints, but I may be wrong: perhaps higher-order functions pose
an insurmountable problem for type-level programming in Haskell. Can anybody
shed any light on this?
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[Haskell-cafe] A problem with nested regions and higher-order functions
I forgot the subject, sorry for reposting...
I'm trying to apply the nested regions (as in Lightweight Monadic Regions by
Oleg Kiselyov and Chung-chieh Shan) design pattern, if that's the proper term,
in hope to gain a bit more type safety in this little library I'm working on
(Streaming Component Combinators, available on Hackage). I guess the problem is
that I'm getting too much type safety now, because I can't get the thing to
compile. Most of the existing code works, the only exceptions seem to be
various higher-order functions. I've reduced the problem to several lines of
Literate Haskell code below, can anybody find a solution or confirm there isn't
one?
{-# LANGUAGE EmptyDataDecls, Rank2Types #-}
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances, IncoherentInstances #-}
module Main where
main = undefined
I'll call the main type, originally a monad transformer, simply Region. I'm
leaving out the Monad and MonadTransformer instances, because they don't
contribute to the problem. The parameter r is the phantom region type.
newtype Region r a = Region a
The Ancestor class is used to denote relationship between two regions where one
is nested in another.
data Child r
class Ancestor r r'
instance Ancestor r (Child r)
instance Ancestor r1 r2 = Ancestor r1 (Child r2)
Handle is a simple wrapper around a value. It carries information about the
region that originates the value.
data Handle r x = Handle x
A typical calculation in the Region monad will take a bunch of Handles
inherited from an Ancestor region and do something with them. The Ancestor
constraint is there to ensure that the handles are not fake but genuinely
inherited.
type SingleHandler x y = forall r1s rs. Ancestor r1s rs =
Handle r1s x - Region rs y
type DoubleHandler x y z = forall r1d r2d rd. (Ancestor r1d rd, Ancestor r2d
rd) =
Handle r1d x - Handle r2d y - Region rd z
And now I get to the problem. The following higher-order function doesn't
type-check:
mapD :: (SingleHandler x z - SingleHandler y z)
- DoubleHandler x w z - DoubleHandler y w z
mapD f d = \y w- f (\x- d x w) y
I get the same error from GHC 6.8.2 and 6.8.2:
Test.lhs:36:28:
Could not deduce (Ancestor r2d rs)
from the context (Ancestor r1s rs)
arising from a use of `d' at Test.lhs:36:28-32
Possible fix:
add (Ancestor r2d rs) to the context of
the polymorphic type
`forall r1s rs. (Ancestor r1s rs) = Handle r1s x - Region rs z'
In the expression: d x w
In the first argument of `f', namely `(\ x - d x w)'
In the expression: f (\ x - d x w) y
The same code compiles just fine if all the Ancestor constraints are removed. I
don't see any place to add the extra (Ancestor r2d rs) constraint, as GHC
recommends. I think it ought to be able to figure things out based on the
exisisting constraints, but I may be wrong: perhaps higher-order functions pose
a fundamental problem for type-level programming. Can anybody shed any light on
this?
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