July 2010 - Haskell-Cafe

Infinite types should be optionally allowed
by Job Vranish 22 Feb '11

22 Feb '11

There are good reasons against allowing infinite types by default (mostly, that a lot of things type check that are normally not what we want). An old haskell cafe conversation on the topic is here: http://www.nabble.com/There%27s-nothing-wrong-with-infinite-types!-td771373… However, I think infinite types should be allowed, but only with an explicit type signature. In other words, don't allow infinite types to be inferred, but if they are specified, let them pass. I think it would be very hard to shoot yourself in the foot this way. Newtype is the standard solution to situations where you really need an infinite type, but in some cases this can be a big annoyance. Using newtype sacrifices data type abstraction and very useful type classes like Functor. You can use multiparameter type classes and functional dependencies to recover some of the lost abstraction, but then type checking becomes harder to reason about and the code gets way more ugly (If you doubt, let me know, I have some examples). Allowing infinite types would fix this. I'm imagining a syntax something like this: someFunctionThatCreatesInfiniteType :: a -> b | b = [(a, b)] Thoughts? Opinions? Am I missing anything obvious? - Job

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trouble with HDBC-mysql on Mac OS X
by Mark Lentczner 21 Nov '10

21 Nov '10

I'm having trouble with HDBC-mysql on Mac OS X (10.6, Snow Leopard). It compiles and builds fine, but when loaded (into ghci), the dynamic loader complains about not being able to find libmygcc.dylib. The issue is, MySQL's libmygcc is a static lib, not a dynamic one, so of course it won't find it. I've checked the build sequence that HDBC-mysql uses, and it calls upon mysql_config to generate the lib options. I've checked those and they are correct, expressly listing libmygcc statically (!). I've tried building against both the 64bit and 32bit versions of MySQL libs. (Suspect that only the 32bit should work.) Anyone have HDBC-mysql running on 10.6? Any ideas for me to try would be appreciated. - Mark Mark Lentczner http://www.ozonehouse.com/mark/ IRC: mtnviewmark

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IORef memory leak
by Jim Snow 15 Oct '10

15 Oct '10

I'm having some trouble with excessive memory use in a program that uses a lot of IORefs. I was able to write a much simpler program which exhibits the same sort of behavior. It appears that "modifyIORef" and "writeIORef" leak memory; perhaps they keep a reference to the old value. I tried both ghc-6.8.3 and ghc-6.10.1. Is this a known limitation, or is this a ghc bug, or am I using IORefs in the wrong way? -jim module Main where import Data.IORef import Control.Monad -- Leaks memory leakcheck1 ior = do go 1000000000 where go 0 = return () go n = do modifyIORef ior (+1) go (n-1) -- Leaks memory leakcheck2 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior writeIORef ior (x+1) go (n-1) -- Runs in constant memory leakcheck3 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior go (n-1) main :: IO () main = do ior <- newIORef 0 leakcheck2 ior compiled with: ghc -O2 --make Leak.hs -o Leak

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BelHac: A Hackaton in Belgium, 5-7 November
by Jasper Van der Jeugt 27 Sep '10

27 Sep '10

Hello all, We are very pleased to officially announce an international Hackaton/Get-together in Ghent, Belgium, on 5, 6 and 7 November 2010. If you are interested in attending, we have put some information on the wiki [1]. We will soon put up more details about transportation, accommodation and food. (And Belgian beer, of course!) You can register here [2]. WHEN Friday November 5: 2pm - 7pm Saturday November 6: 10am - 6pm Sunday November 7: 10am - 6pm WHERE In “The Therminal”, in Ghent, Belgium. Please see the wiki [1] for more details. We hope to see you in Belgium! [1]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHac [2]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHa… On behalf of the GhentFPG organizing committee, Kind regards, Jasper Van der Jeugt

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OpenGL Speed!
by Eitan Goldshtrom 20 Sep '10

20 Sep '10

I'm having an unusual problem with OpenGL. To be honest I probably shouldn't be using OpenGL for this, as I'm just doing 2D and only drawing Points, but I don't know about any other display packages, so I'm making due. If this is a problem because of OpenGL however, then I'll have to learn another package. The problem is speed. I have a list of points representing the color of 800x600 pixels. All I'm trying to do is display the pixels on the screen. I use the following: renderPrimitive Points $ mapM_ display list flush where display [] = return () display ((x,y,i):n) = do color $ Color3 i i i vertex $ Vertex2 x y display n But, for some reason this takes FOREVER. I don't know how to use debugging hooks yet without an IDE -- and I don't use an IDE -- but I used a cleverly placed putStrLn to see that it was actually working, just really really slowly. Is there a solution to this speed problem or should I use a package that's more suited to 2D applications like this? Also, if I should use another package, are there any suggestions for which to use? Thanks for any help. -Eitan

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cabal problem on OS X
by Bill Atkins 19 Sep '10

19 Sep '10

When I run "cabal update" on my Mac (Snow Leopard, Intel), I get: % cabal update Downloading the latest package list from hackage.haskell.org cabal: Codec.Compression.Zlib: incompatible zlib version Anyone else seeing this? Reinstalling the Haskell Platform hasn't helped. Thanks!

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CnC Haskell
by Vasili I. Galchin 08 Sep '10

08 Sep '10

Hello, I have been reading work done at Rice University: http://habanero.rice.edu/cnc. Some work has been done by http://www.cs.rice.edu/~dmp4866/ on CnC for .Net. One component that David wrote a CnC translator that translates CnC textual form to the underlying language, e.g. F#. Is anybody working on a CnC textual form translator for Haskell so a Haskell user of CnC Haskell can write in a higher level?? Thanks, Vasili

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Handling absent maintainers
by Mark Wotton 19 Aug '10

19 Aug '10

Hello all, I've recently had problems with haskell-src-meta. While it's a great package, it doesn't currently compile on GHC 6.12, and Matt Morrow doesn't seem to be around to push the version that does to Hackage. Our "one-world" approach with cabal seems to discourage forking as a casual act, so when a package that others rely on goes AWOL, it's very awkward to fix it. I can think of a few ways to get around my current problems: 1. upload haskell-src-meta-placeholder, or haskell-src-meta-mwotton, or something similar - this could be said to pollute the namespace, but would solve my immediate problem. I'd have to similarly specialise the chain of packages up to the one I actually want to use as well, though. 2. run my own hackage server and tell my users to use that instead. 3. ... profit? Ideally, I'd like to be able to say something like "cabal install my-hacked-package --as original-package" - are there fundamental reasons that wouldn't be possible, or a bad idea? mark

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Re: [Haskell-cafe] Heavy lift-ing
by aditya siram 18 Aug '10

18 Aug '10

I wouldn't-it was a bad example. My only point was that because of the way (>>=) is implemented for lists the order of the arguments 'a' and 'b' in 'liftM2 f a b' matters. -deech On Sat, Jul 24, 2010 at 1:37 AM, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: > Why would you expect swapped operands to (-) ? > > > Sent from my iPad > > On Jul 23, 2010, at 20:12, aditya siram <aditya.siram(a)gmail.com> wrote: > >> Lists are non-deterministic, but the function taken by liftM2 does not >> necessarily generate all possible outcomes. In the case of (+) it >> does, not in the case of (-): >> liftM2 (-) [0,1] [2,3] => [0-1,0-2,1-2,1-3] => [-2,-3,-1,-2] >> if all possible cases were generated between the two lists we have to >> include also: >> [2-0,2-1,3-0,3-1] >> >> -deech >> >> On Fri, Jul 23, 2010 at 3:44 PM, Alex Stangl <alex(a)stangl.us> wrote: >>> On Fri, Jul 23, 2010 at 11:43:08AM -0700, michael rice wrote: >>>> What does it mean to "promote a function to a monad?" >>>> >>>> It would seem that the monad values must understand the function that's being promoted, like Ints understand (+). >>>> >>>> Prelude Control.Monad> liftM2 (+) (Just 1) (Just 1) >>>> Just 2 >>>> >>>> But how does one add [0,1] and [0,2] to get [0,2,1,3]? >>> >>> It depends upon the semantics of the particular monad. List monads >>> represent nondeterminism. So, for example, [0,1] represents a 0 or >>> 1, and [0,2] represents a 0 or 2. >>> >>> When you add 0 or 1 to 0 or 2, your possible answers are [0,2,1,3]. >>> >>> Alex >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Deprecated gtk2hs functions
by Alex Rozenshteyn 16 Aug '10

16 Aug '10

I recently started playing around with gtk2hs. I noticed that `onClicked`, `afterClicked`, etc. functions have been deprecated, presumably in favor of the `on` and `after` functions in the Glib signals module, but I couldn't find a collection of the appropriate signals to replace the functionality. Am I simply being blind? -- Alex R

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