December 2007 - Haskell-Cafe

when I am rebuilding a package that has package dependencies
by Galchin Vasili 04 Dec '07

04 Dec '07

Hello, Which document discusses how to first build all dependencies and then finally the target package? I prefer to read relevant documentation so I get fully up to speed rather than take up bandwidth on this newsgroup asking one question at a time. Regards, Vasya

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isSpace
by Ryan Bloor 04 Dec '07

04 Dec '07

hi I am having trouble with a function that is supposed to eliminate spaces from the start of a String and return the resulting string. I reckon a dropWhile could be used but the isSpace bit is causing me problems... words :: String -> String words a = case dropWhile isSpace a of "" -> s:ss -> (s:word) : words rest where (word,rest) = break isSpace ss Thanks Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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Re: [Haskell] IVar
by Isaac Dupree 04 Dec '07

04 Dec '07

>> A pure readIVar would be just like lazy I/O, with similar drawbacks. >> With readIVar, the effect that lets you observe the evaluation order >> is writeIVar; with hGetContents it is hClose. Conclusion: it's >> probably no worse than lazy I/O. > > Actually, it's considerably better. +: implementation may not evaluate something that may terminate/block, anyway IVar may change from blocking to non-blocking, unlike most pure expressions. This is only meaningful if trying to evaluate it would prevent it from gaining a value. It can only have such an effect (helping the IVar gain a value) if the same thread would do any IO (if it wasn't for the unnecessary evaluation). IO is generally observable, so getting stuck on a normal non-terminating expression wouldn't be any more acceptable than getting stuck on an IVar in that case. ... all assuming unsafeInterleaveIO doesn't exist, because it's unsafe, so the semantics are undefined of when it gets evaluated relative to later normally-sequenced IO actions, if it will be eventually evaluated for sure -- right? Nevertheless its intended use generally assumes that it will be evaluated as if its evaluation might not terminate, i.e. as late as possible. If it is in fact (a finite amount of) non-blocking IO, then evaluating it early will only have consequences on when the IO happens (and therefore its effect/result), not directly on the program's non-termination or actions. It gets more confusing the more I think about it! to modify Simon's example, it looks to me like treating readIVar as potentially non-terminating, and writeIVar as potentially having side-effects, is enough: main = do --irrelevant x <- newIVar let y = last [1..] print "test" --was irrelevant writeIVar x 3 print y Isaac

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RFI: Link errors with random pkg on 6.8.1
by Jim Stuttard 04 Dec '07

04 Dec '07

Hi, Getting same error in 2 diff. apps * built with ghc-6.8.1. * random-1.0.0.0 also built with 6.8.1 * random-1.0.0.0 is registered # HAppS -6.8.1/lib/random-1.0.0.0/HSrandom-1.0.0.0.o: unknown symbol `oldzmtimezm1zi0zi0zi0_SystemziTime_a99_info' ghc-6.8.1: unable to load package `random-1.0.0.0' # Haskell$ ghci DonsGame-0.1.hs GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Game ( DonsGame-0.1.hs, interpreted ) Ok, modules loaded: Game. *Game> main Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package filepath-1.1.0.0 ... linking ... done. Loading package directory-1.0.0.0 ... linking ... done. Loading package unix-2.2.0.0 ... linking ... done. Loading package process-1.0.0.0 ... linking ... done. Loading package random-1.0.0.0 ... linking ... <interactive>: /usr/local/lib/ghc-6.8.1/lib/random-1.0.0.0/HSrandom-1.0.0.0.o: unknown symbol `oldzmtimezm1zi0zi0zi0_SystemziTime_a99_info' ghc-6.8.1: unable to load package `random-1.0.0.0' # ubuntu gutsy 2.6.22-14-server #1 SMP Sun Oct 14 23:34:23 GMT 2007 i686 GNU/Linux Jim I

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NY Functional Programmers Network: F# Talk by Don Syme, Monday De cember 10th at 7pm
by Mansell, Howard 04 Dec '07

04 Dec '07

Credit Suisse will be hosting a talk by Don Syme from Microsoft Research on Monday December 10th. He will talk about F# and Microsoft's plans for it. This talk is arranged by the New York Functional Programmers Network, a group of individuals in the New York area who are interested in functional programming. Many of us also use functional programming in our work life. Despite the URL below, we are typically interested in statically typed functional languages. If you are interested in coming, please RSVP at the meetup site below: http://lisp.meetup.com/59/ <http://lisp.meetup.com/59/> Regards Howard Mansell Global Modelling & Analytics Group Credit Suisse ============================================================================== 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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Nofib modifications
by Neil Mitchell 04 Dec '07

04 Dec '07

Hi, Some of the nofib suite are messed up by Yhc/nhc because of the monomorphism restriction. Take imaginary/bernouilli as an example: powers = [2..] : map (zipWith (*) (head powers)) powers Hugs and GHC both see powers :: [[Integer]] and a CAF. Yhc (and nhc) both see powers :: (Enum a, Num a) => [[a]] and no CAF. This completely destroys the performance in Yhc/nhc. Since this is not so much a performance aspect but a compiler bug, based on a feature whose future in Haskell' is as yet unclear, perhaps it would be wise to patch nofib to include an explicit type signature where this matters. I am happy to send in a patch (or just apply it) - but I have no idea who "maintains" the suite. I've CC'd those people who make substantial use of the nofib suite. Thanks Neil

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isSpace
by Ryan Bloor 04 Dec '07

04 Dec '07

HI I will try and explain it better. I am meaning to write a function that takes a string, " apple" and eliminates the spaces at the start ONLY. called........ removeSpace :: String -> String I decided to use the function 'dropWhile' and another one 'isSpace' in the 'removeSpace' function. removeSpace:: String -> String removeSpace a = case dropWhile isSpace a of "" -> I get stuck here... I am not sure how to go about this function. Any ideas. Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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Advice for clean code.
by David McBride 04 Dec '07

04 Dec '07

I am still in the early stages learning haskell, which is my first foray into functional programming. Well there's no better way to learn than to write something, so I started writing a game. Mostly the thing looks good so far, far better than the C version did. However, my problem is that code like the following is showing up more often and it is becoming unwieldy. gameLoop :: World -> IO () gameLoop w = do drawScreen w action <- processInput let (result, w') = processAction action w case result of MoveOutOfBounds -> putStrLn "Sorry you can't move in that direction." MoveBadTerrain a -> case a of Wall -> putStrLn "You walk into a wall." Tree -> putStrLn "There is a tree in the way." otherwise -> putStrLn "You can't move there." otherwise -> return () let w'' = w' { window = updateWindowLocation (window w') (location $ player w')} unless (action == Quit) (gameLoop w'') Where world contains the entire game's state and so I end up with w's with multiple apostrophes at the end. But at the same time I can't really break these functions apart easily. This is error prone and seems pointless. I have been reading about control.monad.state and I have seen that I could run execstate over this and use modify but only if each function took a world and returned a world. That seems really limiting. I'm not even sure if this is what I should be looking at. I am probably just stuck in an imperative mindset, but I have no idea what to try to get rid of the mess and it is only going to get worse over time. Any suggestions on what I can do about it?

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Haskell interface file (.hi) format?
by Rob Hoelz 04 Dec '07

04 Dec '07

Hello fellow Haskellers, Does anyone know if/where I can find a specification for the .hi files generated by GHC? I ask because I want to write an omni-completion plugin for Vim to make Haskell hacking a bit nicer. Thanks, Rob Hoelz

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Re: [Haskell] IVars
by Bertram Felgenhauer 04 Dec '07

04 Dec '07

[redirecting to haskell-cafe] Simon Peyton-Jones wrote: > But since the read may block, it matters *when* you perform it. For > example if you print "Hello" and then read the IVar, you'll block > after printing; but if you read the IVar and then print, the print > won't come out. If the operation was pure (no IO) then you'd have > a lot less control over when it happened. Well, the same can happen with any pure computation that does not terminate. It's the compiler's job to avoid that, and as far as I know ghc is pretty careful not to reorder pure computations and IO operations too much. I'd argue for adding both an IO and a pure version of readIVar to an IVar implementation. Both have their uses; the firts gives you a little extra control, while the second one can easily be evaluated on demand, at the risk of creating a mine field of potential deadlocks. Bertram

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