April 2008 - Haskell-Cafe

library version problem
by Galchin, Vasili 12 Apr '08

12 Apr '08

Hello, I doing work using Linux. The wrong version (for me) of the unix package seems to be visible. I see possibilities to use ghc-pkg to suppress the unix package that I don't want(2.3.0.0) but that seems dangerious. Details are below . What should I do? Regards, vasili When I do: > ghci :m System.Posix I am getting the wrong version of the Unix package. I know this to be true because I did ghc-pkg latest unix and got unix-2.3.0.0 I want unix.2.2.0.0 because this version has changes that I made to the unix package. libHSunix-2.2.0.0.a is installed under /usr/local/lib/unix-2.2.0.0/ghc-6.8.2 I did a "nm -a libHSunix-2.2.0.0.a" and found symbols that I added.

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do construct in wxhaskell
by Jodi-Ann Prince 12 Apr '08

12 Apr '08

hi, im working ona project, and im having problem loading some code in wxhaskell: onOpen :: Frame a -> TextCtrl b -> MenuItem c -> StatusField -> IO () onOpen f sw mclose status = do mbfname <- fileOpenDialog f False True "Open image" fileTypes "" "" case (mbfname) of (Nothing) -> return () (Just (fname)) -> do fileContent <- readFile fname set sw [text := fileContent] set mclose [enabled := True] set status [text := fname] return () i keep getting the error : "the last statement in a 'do' construct must be an expression." ive tried rearranging it many times, but i still get the same error. any help would be greatly appreciated. _________________________________________________________________ Connect to the next generation of MSN Messenger http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=…

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Longest increasing subsequence
by Matt Amos 11 Apr '08

11 Apr '08

I'm trying to break out of my imperative mind-set by learning Haskell in small snippets. After a few successes I've hit a bit of a roadblock with one of the classic dynamic programming problems, the longest increasing subsequence: http://en.wikipedia.org/wiki/Longest_increasing_subsequence The most efficient algorithm relies on destructive updates, so a simple translation doesn't seem possible. I've been trying to think of a way of removing the need for destructive updates while keeping the efficiency, but so far without much luck. Ideally, I'd like to avoid threading the whole thing with a state monad, since then it would just be an imperative algorithm in disguise. Any suggestions would be very gratefully appreciated. Cheers, Matt

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Control.Parallel.Strategies
by Sebastian Sylvan 11 Apr '08

11 Apr '08

Hi, I was toying with the Control.Parallel.Strategies library, but can't seem to get it to actually do anything in parallel! Here's the code: import System.Random import Control.Parallel.Strategies fib :: Int -> Int fib 0 = 1 fib 1 = 1 fib n = fib (n-1) + fib (n-2) main = print $ parMap rnf fib $ take 80 $ randomRs (30,35) (mkStdGen 123) I compile with "-threaded", and run with +RTS -N2 -RTS, but yet it stays at 50% on my dual core machine! I should point out that any manual threading with forkIO does indeed use 100% of the CPU, and this problem happens on my work computer too (Vista on my home computer, XP on the work one). Anything I'm missing here? Thanks, -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862

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Haskell IDE: Someone else interested in enhancing Shim ? / How to contact author Benedikt Schmidt ?
by Marc Weber 11 Apr '08

11 Apr '08

Hi, I've added some features to shim. You can get my patches here: http://mawercer.de/~publicrepos/shim/ The Vim interface now supports :GrepScope regex Which greps the whole scope of all used packages by regex. Very useful to find all functions taking/ returning some type etc :ModulesExporting identifier returns all modules exporting identifiers. Can be used to add imports more automatically. Completion: camelCaseMatching Use pSL<c-x><c-o> to get putStrLn If you want to join and add features contact me via irc or email. I'd like to talk to the author about my patches. I've send him two mails but I haven't received any response yet. Sincerly Marc Weber

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Re: [Haskell-cafe] library version problem
by Galchin, Vasili 11 Apr '08

11 Apr '08

Hi Philip, Before I got your email, I deregistered unix-2.3.0.0 which made my unix-2.2.0.0 namespace changes visible. However, deregistering seems to made other things worse, e.g. runhaskell Setup.hs configure gives an error message "unknown parameter package: unix-2.3.0.0". Sigh .. how do I get back to where i was in order to do a "hide"? Kind regards, Vasili On Thu, Apr 10, 2008 at 8:22 PM, Philip Weaver <philip.weaver(a)gmail.com> wrote: > 2008/4/10 Galchin, Vasili <vigalchin(a)gmail.com>: > > Hello, > > > > I doing work using Linux. The wrong version (for me) of the unix > > package seems to be visible. I see possibilities to use ghc-pkg to > suppress > > the unix package that I don't want(2.3.0.0) but that seems dangerious. > > Details are below . What should I do? > > If you don't want to use it, then it's safe to hide it: > > ghc-pkg hide unix-2.3.0.0 > > You can always unhide it later. > > You can also tell ghc to use a specific version of a package: > > ghc -package unix-2.2.0.0 > > Of course, you'll need to make sure unix-2.2.0.0 is registered with > ghc-pkg. > > > > > Regards, vasili > > > > > > > > When I do: > > > > > ghci > > :m System.Posix > > > > I am getting the wrong version of the Unix package. I know this to be > true > > because I did > > ghc-pkg latest unix > > > > and got unix-2.3.0.0 > > > > I want unix.2.2.0.0 because this version has changes that I made to the > unix > > package. libHSunix-2.2.0.0.a is installed under > > /usr/local/lib/unix-2.2.0.0/ghc-6.8.2 I did a "nm -a > libHSunix-2.2.0.0.a" > > and found symbols that I added. > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > >

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type families and type signatures
by Ganesh Sittampalam 11 Apr '08

11 Apr '08

Hi, The following program doesn't compile in latest GHC HEAD, although it does if I remove the signature on foo'. Is this expected? Cheers, Ganesh {-# LANGUAGE TypeFamilies #-} module Test7 where type family Id a type instance Id Int = Int foo :: Id a -> Id a foo = id foo' :: Id a -> Id a foo' = foo

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RE: [Haskell-cafe] type families and type signatures
by Sittampalam, Ganesh 11 Apr '08

11 Apr '08

Manuel Chakravarty wrote: >Ganesh Sittampalam: >> On Mon, 7 Apr 2008, Manuel M T Chakravarty wrote: >> >>> Ganesh Sittampalam: >>>> The following program doesn't compile in latest GHC HEAD, although >>>> it does if I remove the signature on foo'. Is this expected? >>> >>> Yes, unfortunately, this is expected, although it is very >>> unintuitive. This is for the following reason. >>> >>> Let's alpha-rename the signatures and use explicit foralls for >>> clarity: >>> >>> foo :: forall a. Id a -> Id a >>> foo' :: forall b. Id b -> Id b >>> >>> GHC will try to match (Id a) against (Id b). As Id is a type synonym >>> family, it would *not* be valid to derive (a ~ b) from this. After >>> all, Id could have the same result for different argument types. >>> (That's not the case for your one instance, but maybe in another >>> module, there are additional instances for Id, where that is the >>> case.) >> >> Can't it derive (Id a ~ Id b), though? > That's what it does derive as a proof obligation and finds it can't prove. > The error message you are seeing is GHC's way of saying, I cannot assert that > (Id a ~ Id b) holds. No, I meant can't it derive that equality when matching (Id a) against (Id b)? As you say, it can't derive (a ~ b) at that point, but (Id a ~ Id b) is known, surely? > > Generally speaking, is there any way to give a signature to foo'? > Sorry, but in the heat of explaining what GHC does, I missed the > probably crucial point. Your function foo is useless, as is foo'. > Not only can't you rename foo (to foo'), but you generally can't use > it. It's signature is ambiguous. Try evaluating (foo (1::Int)). The > problem is related to the infamous (show . read). My real code is somewhat analogous to (foo :: (Id Int -> Id Int)) (1::Int). Isn't that unambiguous in the same way as (show.read) is if I give show or read a signature? Cheers, Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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Re: HXT 7.4/7.5 leaking TCP connections
by Paul Brown 10 Apr '08

10 Apr '08

Hi, Bjorn and Uwe -- > > this is a known problem with HTTP package (version 3001.0.4). > > Paul Brown has described this somewere in his blog. > > (http://mult.ifario.us/t/haskell) but my firefox only shows > > an incomplete page of this blog, the solution is missing. > > Paul promissed in his blog to send a patch to Björn Bringert. Here's the link to the article about SimpleHTTP; for some reason, it wasn't showing up. (Guess I have a bug to fix there...) http://mult.ifario.us/p/a-short-adventure-with-simplehttp > > P.S. to Björn: It would be nice, if you could include this change into the > > HTTP module and upload a new version to hackage. > Thank you Daniel and Uwe for reporting this (again). Strangely, I > thought that I had already applied Paul's patch, but it seems I > haven't. Paul: I seem to recall us talking about this, but can't > remember the conclusion, nor find any e-mail about it. Do you remember > anything about this? I can't remember either; I'm sure that the darcs patch is lurking on one of my boxes. I'll track it down and send it your way so we can squash this one. Best. -- Paul -- paulrbrown(a)gmail.com http://mult.ifario.us/

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Fighting the monad stack, MonadIO
by Adam Smyczek 10 Apr '08

10 Apr '08

For a small webapi binding I try to implement a session like monad by building a stack including BrowserAction from Network.Browser module as following: newtype RBAction a = RBAction { exec :: ErrorT String (StateT RBState BrowserAction) a } deriving (Functor, Monad, MonadState RBState) I would like the RBAction to implement MonadIO as well, but fight with the liftIO function for hours now, without success. Any idea how the implementation of liftIO could look like? Thanks for help, Adam

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