February 2009 - Haskell-Cafe

Why does sleep not work?
by John Ky 11 Feb '09

11 Feb '09

Hi Haskell Cafe, I wrote very short program to sleep for 5 seconds compiled with the -threaded option in ghc on the Mac OS X 1.5. I am finding that using the sleep function doesn't sleep at all, whereas using threadDelay does: main = do putStrLn "Waiting for 5 seconds." threadDelay 5000000 -- works putStrLn "Done." main = do putStrLn "Waiting for 5 seconds." sleep 5 -- doesn't sleep at all putStrLn "Done." Anybody know what's happening? Thanks -John

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Painting logs to get a coloured tree
by Joachim Breitner 10 Feb '09

10 Feb '09

Hi, I have a problem that, it seems, I can not solve cleanly without resorting to imperative programming (via ST): Assume you have a tree (and you can think of a real tree here), defined by something like this: data Tree a = Bud | Branch a Double Tree Tree -- | ` Lenght of this branch -- ` General storage field for additional information Now, I have a nice algorithm that calulates something for each branch, but it only works on lists of branches, so I have to cut them apart first, remembering their position in space, and then work on these, well, logs. data Point = Point Double Double data Log = Log Point Point type Info = ... noInfo :: Info cutTreeApart :: Tree a -> [(Log, a)] someAlgorithm :: [(Log,a)] -> [(a, Info)] Conveniently, the algorithm allows me to tag the logs with something, to be able to keep track at least somewhat of the logs. Unfortunately, I need this information in the storage field in my Tree, as the list of logs is not sufficient for later calculations. Idea: Using ST ============== annotateTree :: Tree a -> Tree Info annotateTree tree = runSt $ do -- Put an STRef in each node treeWithPointer <- mapM const (newSTRef noInfo) tree -- Cut this tree apart let logsWithPointers = cutTreeApart treeWithPointer -- Run the algorithm let informations = someAlgorithm logsWithPointers -- Put the information back, via the ST ref mapM (\(stRef, info) -> writeSTRef stRef info) informations -- Read the ST refs mapM readIORef tree Note that I assume a instance Traversable Tree here, and mapM is Data.Traversable.mapM. Now while this works, and while ST is still somewhat pure, I’m wondering if there is no better way of expressing "This piece of information came from the point in a data structure, so something else can be put here easily". Some ideas where numbering the Nodes and then using this number as the tag on the log, but this is not much different from using STRefs, it seems. Thanks, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Additonal types for Foreign.C.Types
by Maurício 10 Feb '09

10 Feb '09

Hi, After reading an ISO draft for standard C, I found a few types that could be usefull when binding to libraries (these are from <stdint.h>): int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t What about if they were included in the next version of GHC Foreign.C.Types module? For instance, as: CInt8, CUInt8, CInt16 etc. Some libraries (e.g. sqlite) define function parameters that are supposed to always have a given size (e.g., sqlite defines sqlite3_int64). In order to have a portable binding to those libraries, it would be nice to have types in Haskell that also offer that guarantee. There are also a few other nice types, although I'm not sure they belong to standard modules. If they did, however, they could make life easier for those people writing higher level Haskell modules after standard C functions: complex, float complex, double complex (from <complex.h>) are used in functions like ccos, csin, cexp, csqrt etc. struct lconv (from <locale.h>) struct tm (from <time.h>) From <wchar.h> and <wctype.h>: mbstate_t, wint_t, wctrans_t, wctype_t. Do you think I could open a ticket for GHC proposing that? Best, Maurício

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Re: [Haskell-cafe] ANN: convertible (first release)
by Daniel Schüssler 10 Feb '09

10 Feb '09

Hello, it is a real code snippet, but I failed to include the necessary pragma (which is: {-# OPTIONS_GHC -fglasgow-exts #-} or {-# OPTIONS_GHC -XMagicHash #-} at the beginning of the file). the "#" suffix is for unboxed types: http://www.haskell.org/ghc/docs/latest/html/users_guide/primitives.html http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#m… Greetings, Daniel On Wednesday 11 February 2009 00:19:56 Dylan Tisdall wrote: > Maybe I'm just a noob falling for someone's joke here, but I can't > make that sample, or anything else I tried with "#" on the end of > names, to compile with ghci. In the example below, I get a parse error > at the line defining d2f. Is this a real code snippet? > > > Cheers, > Dylan > > On Feb 10, 2009, at 2:46 AM, Daniel Schüssler wrote: > > Hi, > > > > On Wednesday 28 January 2009 04:30:07 John Goerzen wrote: > >> On Tue, Jan 27, 2009 at 09:41:30PM -0500, wren ng thornton wrote: > >>> I once again point out that realToFrac is *wrong* for converting > >>> from > >>> Float or Double. > >>> > >>>> realToFrac (1/0::Float) ::Double > >>> > >>> 3.402823669209385e38 > >> > >> Yes, I understand what you are saying and agree with you. But there > >> is nothing better in the standard library > > > > don't know whether you consider GHC as standard library, but if you > > do...: > > > > import GHC.Types > > import GHC.Prim > > > > -- | Double to Float > > d2f (D# d) = F# (double2Float# d) > > > > -- | Float to Double > > f2d (F# f) = D# (float2Double# f) > > > > > > ghci> f2d (1/0) > > Infinity > > it :: Double > > > > :) > > > > -- > > Daniel > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Monad explanation
by Tim Newsham 10 Feb '09

10 Feb '09

I put up a small Monad explanation (I wouldn't quite call it a tutorial): http://www.thenewsh.com/~newsham/haskell/monad.html The intent here is to is to have a short description of what a Monad is that is approachable by Haskell beginners or non- Haskell programmers who are looking for an intuitive definition but may not yet have the background or the inclination to jump into full tutorial to tackle the subject. Tim Newsham http://www.thenewsh.com/~newsham/

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evaluation semantics of bind
by Gregg Reynolds 10 Feb '09

10 Feb '09

I think I've just about got monads figured out, but there's one detail that still escapes me. As I understand it, a monad is a kind of programming trick the uses data dependency to force evaluation order. x >>= f means apply f to x; since the value of f x depends on the value of x, the evaluator must evaluate x before f x. However, consider: getChar >>= \x -> getChar An optimizer can see that the result of the first getChar is discarded and replace the entire expression with one getChar without changing the formal semantics. But that would change the behavior, so to get the desired behavior, there must be some principle that prevents this from happening, ensuring that x >>= f always evaluates f x. I can see that the monad laws ensure this But I haven't found anything that states this. Am I missing something? Thanks, gregg

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Re: [Haskell] Google Summer of Code 2009
by Michael Litchard 10 Feb '09

10 Feb '09

I would love a mentor to help me with a Haskell binding to libnova. This is part of a larger project I have in mind, but the libnova binding seems like the first step. I don't expect this to be picked as an official GSoC, but this seemed like a good time to look for a mentor for this project. Michael Litchard On Tue, Feb 10, 2009 at 6:26 AM, Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> wrote: > Gentle Haskellers, > > The Google Summer of Code will be running again this year. Once again, > haskell.org has the opportunity to bid to become a mentoring > organisation. (Although, as always, there is no guarantee of > acceptance.) > > If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing > lists of your choice. We especially encourage students to communicate > with the wider community: if you keep your ideas private, you have a > much worse chance of acceptance than if you develop ideas in > collaboration with those who will be your "customers", end-users, or > fellow-developers. This is the open-source world! > > The timeline is that Haskell.org will apply for GSoC membership between > 9-13 March, and if we are successful, students can submit applications > between 23 March - 3 April. > > If you wish to help publicise GSoC amongst students, there are official > posters/fliers available (not specific to haskell.org) > > http://code.google.com/p/google-summer-of-code/wiki/GsocFlyers > > Regards, > Malcolm > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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ANN: Bug fix to regex-tdfa, new version 0.97.3
by ChrisK 10 Feb '09

10 Feb '09

To Haskell and Libraries and Haskell-Cafe, Whilst improving regex-tdfa I have run across new bugs. Some patterns were getting compiled wrong and others were affected by an execution bug. As this package has actual users, I wanted to make sure they get these fixes immediately. Three Cheers For QuickCheck! The new version is 0.97.3 at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-tdfa And this version passes all the unit tests I have, including coverage for the new bugs. This is no warranty that regex-tdfa is bug free, since I made that same claim last release. For instance: I suspect 0.97.3 may be buggy if used in the optional left-associative mode. The new improved version of regex-tdfa is still a long way off. Cheers, Chris Kuklewicz

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Asking the GHC garbage collector to run
by Mads Lindstrøm 10 Feb '09

10 Feb '09

Hi all, Is it possible to ask the GHC garbage collector to run ? Something like a collectAllGarbage :: IO() call. Greetings, Mads Lindstrøm

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Re: [Haskell] Google Summer of Code 2009
by Don Stewart 10 Feb '09

10 Feb '09

Malcolm.Wallace: > Gentle Haskellers, > > The Google Summer of Code will be running again this year. Once again, > haskell.org has the opportunity to bid to become a mentoring > organisation. (Although, as always, there is no guarantee of > acceptance.) > > If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing > lists of your choice. We especially encourage students to communicate > with the wider community: if you keep your ideas private, you have a > much worse chance of acceptance than if you develop ideas in > collaboration with those who will be your "customers", end-users, or > fellow-developers. This is the open-source world! > And I'll just note that since December we've been running a proposal submission site here, where you can vote and comment on ideas, http://www.reddit.com/r/haskell_proposals/top/ A great place to suggest ideas! -- Don

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