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ANNOUNCE: hs-ffmpeg 0.3.2 - Bindings to FFMpeg library
by Vasyl Pasternak 15 Oct '09

15 Oct '09

Hello, I am glad to announce next release of hs-ffmpeg library. Now you could download it from the Hackage along with the ffmpeg-tutorials, which show capabilities of this library. The installation process is a bit tricky now, so welcome to my blog post http://progandprog.blogspot.com/2009/10/video-processing-on-haskell-easy.ht…, where I am describing how to build and install library. Best regards, Vasyl Pasternak

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(no subject)
by Stavenga, G.C. 15 Oct '09

15 Oct '09

Hi, I'm just started to learn Haskell. Coming from a programming contest background (where it is important to be able to solve problems in a small amount of code) I'm wondering what the best way is for simple IO. A typical input file (in a programming contest) is just a bunch of numbers which you want to read one by one (sometimes interspersed with strings). In C/C++ this is easily done with either scanf or cin which reads data separated by spaces. In Haskell I have not found an equally satisfactionary method. The methods I know of 1) Stay in the IO monad and write your own readInt readString functions. A lot of code for something easy. 2) Use interact together with words and put the list of lexemes in a State monad and define getInt where at least you can use read. 3) Use ByteString.Char8 which has readInt (but I couldn't find a readString). But one has to put it also in a State monad. I think that there must be standard function that can do this. What do experienced Haskellers use? Thanks in advance

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Monotype error
by Martijn van Steenbergen 15 Oct '09

15 Oct '09

Dear café, > {-# LANGUAGE Rank2Types #-} > {-# LANGUAGE ImpredicativeTypes #-} > > type Void = forall a. a > > newtype Mono a = Mono { runMono :: [Void] } > > beep :: Mono a -> Mono a > beep (Mono vs) = Mono (map undefined vs) Compiling this with GHC results in: > Monotype.hs:9:28: > Cannot match a monotype with `Void' > Expected type: Void > Inferred type: a What does this error mean and why does the code not compile? Thanks! Martijn.

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newtype deriving Alternative
by Martijn van Steenbergen 15 Oct '09

15 Oct '09

Hello café, I've never written an Alternative instance for a newtype yet that doesn't look like this: > instance Alternative T where > empty = T empty > T x <|> T y = T (x <|> y) Why does newtype deriving not work for Alternative? (It works fine for Monoid.) Thanks, Martijn.

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Fuzzy Logic / Linguistic Variables
by Neal Alexander 15 Oct '09

15 Oct '09

So i was reading "Programming Game AI by Example" by Mat Buckland (http://www.ai-junkie.com/books/toc_pgaibe.html) and decided to rewrite his chapter on Fuzzy logic in haskell (from C++). My initial impression: its one of those scenarios where OOP grossly over complicates things Heres an example taken from the book: An agent needs to decide what weapon to use based on his distance from the target and the amount of ammo each has. The result is in the desirability domain (0-100). http://code.haskell.org/~hexpuem/fuzzyLogic/AI/Logic/Fuzzy/Example.hs An excerpt: ------------------------------------ weapons = [ Weapon "RocketLauncher" 9 rocketLauncher, Weapon "ShotGun" 13 shotgun, Weapon "AssaultRifle" 120 assaultRifle, Weapon "SniperRifle" 7 sniperRifle, Weapon "Knife" 1 knife ] chooseWeapon dist = maximum ratings where ratings = [ (f dist ammo, n) | Weapon n ammo f <- weapons ] ------------------------------------ shotgun :: Double -> Double -> Double shotgun dist ammo = unfuzz desireDom $ rules (fuzz distDom dist) (fuzz ammoDom ammo) where rules :: Fuzzy Distances -> Fuzzy Ammo -> FL Desirability rules distance ammo = do distance `is` SniperSuited =>> Pointless distance `is` Far =>> Undesirable distance `is` Medium =>> Undesirable distance `is` Melee =>> Undesirable (distance `fairly` Near) & (ammo `fairly` High) =>> VeryDesirable (distance `fairly` Near) & (ammo `fairly` Good) =>> Desirable (distance `fairly` Near) & (ammo `is` Low) =>> Undesirable ------------------------------------ Full code at http://code.haskell.org/~hexpuem/fuzzyLogic/. Suggestions welcome - maybe it'd be useful to upload to hackage at some point. It only supports triangle, shoulder, and singleton memberships at the moment.

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Sharing Subexpressions: Memoization of Fibonacci sequence
by SimonK77 14 Oct '09

14 Oct '09

Hallo everyone, the last few weeks I was trying to get used to memoization in haskell. As I found out in a previous post, memoization in haskell is, due to the graph reduction strategy used in haskell, almost always implemented by sharing subexpressions in an expression. In examples as the following this is quite easy to see. fibs = 0:1:zipWith (+) fibs (tail fibs) But as I browsed through the search results from google for this topic, I encountered the following implementation: memoized_fib :: Int -> Integer memoized_fib = let fib 0 = 0 fib 1 = 1 fib n = memoized_fib (n-2) + memoized_fib (n-1) in (map fib [0 ..] !!) You'll find it at Haskell.org. Here's the http://www.haskell.org/haskellwiki/Memoization link Let's assume we have the following implementation of the higher-order-function map: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x : map f xs The reduction sequence of memoized_fib 5 would start as follows: //Reduction of memoized_fib 5 memoized_fib 5 = (map fib [0..] !!) 5 = (fib 0 : map fib [1..] !!) 5 = (0 : fib 1 : map fib [2..] !!) 5 = (0 : 1 : fib 2 : map fib [3..] !!) 5 = (0 : 1 : (memoized_fib 0 + memoized_fib 1) : fib 3 : map fib [4..] !!) 5 = (0 : 1 : (map fib [0..] !! 0 + map fib [0..] !! 1) : (memoized_fib 1 + memoized_fib 2) : map fib [4..] !!) 5 . . . . and so on! I can't see where the sharing of subexpressions happens here. Any help is highly appreciated. Best regards Simon -- View this message in context: http://www.nabble.com/Sharing-Subexpressions%3A-Memoization-of-Fibonacci-se… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Setting environment variables on Windows
by Sönke Hahn 14 Oct '09

14 Oct '09

Hi! I need to set an environment variable from Haskell and i would like to do that cross-platform. There is System.Posix.Env.setEnv, which does exactly, what i want on Linux. There is the module System.Environment, which seems to be cross-platform, but it does not contain functions to manipulate the environment (, just to inspect it). At first glance, I didn't find anything relevant in the sub-modules of System.Win32. Is this just not implemented? How could i implement it myself? Grateful for any comment, Sönke

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Why does replicateM (10^6) $ return 0 produce output in the IO monad, but overflow the maybe monad?
by Thomas Hartman 14 Oct '09

14 Oct '09

-- Why does replicateM (10^6) $ return 0 produce output in the IO monad, but overflow the maybe monad? iterateNTimes i f x = foldr (.) id (replicate i f) $ x tntIO :: IO Int -- same as replicateM (10^6) $ return 0, same as sequence . replicate (10^6) $ return 0 tntIO = return . head =<< (iterateNTimes (10^6) (mcons . return $ 0) (return [])) -- produces output tntMb :: Maybe Int -- overflows tntMb = return . head =<< (iterateNTimes (10^6) (mcons . return $ 0) (return [])) -- stack overflow -- equivalently: mcons m ms = (:) <$> m <*> ms mcons m ms = ap (liftM (:) m) $ ms I guess the maybe version builds up a huge chain of unevaluated thunks somewhere, that's usually the reason. But specifically where and why, and why doesn't IO do the same thing? Equivalent, with 3 element list: (mcons $ return $ 0 ) $ (mcons $ return $ 0) $ (mcons $ return $ 0) $ (return []) (ap . liftM (:) . return $ 0 ) $ (ap . liftM (:) . return $ 0) $ (ap . liftM (:) . return $ 0) $ (return []) (ap $ liftM (:) $ return $ 0 ) $ (ap $ liftM (:) $ return $ 0) $ (ap $ liftM (:) $ return $ 0) $ (return []) t.

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example of PortMidi use
by Michael Mossey 14 Oct '09

14 Oct '09

Can someone give me an example of Sound.PortMidi use? I'm having trouble. This program has bugs---makes sound only intermittently, and seems to have set up some kind of loop that is sending midi messages continuously even after terminating the program: import Sound.PortMidi import Foreign.C msgs = [ (0::CULong,PMMsg 0x9c 0x40 0x40) , (500, PMMsg 0x8c 0x40 0x40) , (1000, PMMsg 0x9c 0x41 0x40) , (1500, PMMsg 0x8c 0x41 0x40) ] main = do let deviceId = 12 initialize >>= print getDeviceInfo deviceId >>= print startTime <- time let evts = map (\(t,msg) -> PMEvent msg (t+startTime)) msgs result <- openOutput deviceId 10 case result of Right err -> putStrLn ("After open: " ++ show err) Left stream -> do result <- writeEvents stream evts putStrLn ("After write: " ++ show result) close stream return () terminate >>= print

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(no subject)
by oleg＠okmij.org 14 Oct '09

14 Oct '09

Martin Sulzmann wrote: > "Undecidable" instances means that there exists a program for which there's > an infinite reduction sequence. I believe this may be too strong of a statement. There exists patently terminating type families that still require undecidable instances in GHC. Here is an example: > {-# LANGUAGE TypeFamilies #-} > > type family I x :: * > type instance I x = x > > type family B x :: * > type instance B x = I x GHC 6.8.3 complaints: Application is no smaller than the instance head in the type family application: I x (Use -fallow-undecidable-instances to permit this) In the type synonym instance declaration for `B' But there cannot possibly be any diverging reduction sequence here, can it? The type family I is the identity, and the type family B is its alias. There is no recursion. The fact that type families are open is not relevant here: our type families I and B are effectively closed, because one cannot define any more instance for I and B (or risk overlap, which is rightfully not supported for type families). The reason GHC complains is because it checks termination instance-by-instance. To see the termination in the above program, one should consider instances I and B together. Then we will see that I does not refer to B, so there are no loops. But this global termination check -- for a set of instances -- is beyond the abilities of GHC. This is arguably the right decision: after all, GHCi is not a full-blown theorem prover. Thus there are perfectly decidable type programs that require undecidable instances. Indeed, there is no reason to be afraid of that extension.

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Reverse dependencies in Hackage
by haskell＠kudling.de 14 Oct '09

14 Oct '09

Nice, thank you for the great work. Browsing the reverse dependencies of popular packages like "bytestring" http://bifunctor.homelinux.net/~roel/cgi-bin/hackage-scripts/revdeps/bytest… can be improved a bit. 1) Can you please sort the reverse dependent package names? That makes it easier to find packages with certain names. 2) I found the columns "Direct" and "Indirect" confusing until i found out that they show the number of reverse dependencies for those packages themselves and are not related to direct/indirect dependencies of the current package. I don't think it is necessary to provide those data here and i would be in favor of reducing the information overload by leaving those data to each package detail page. 3) Can you try to print the reverse packages horizontally instead of vertically in tables? Browsing two long tables of direct/indirect reverse dpependencies like for "bytestring" makes it tedious to get an overview. Besides that, i am quite happy. Thanks, Lenny

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Exponential complexity of type checking (Was: Type-level naturals & multiplication)
by Brad Larsen 14 Oct '09

14 Oct '09

On Tue, Oct 13, 2009 at 3:37 AM, Simon Peyton-Jones <simonpj(a)microsoft.com> wrote: > | > Is there any way to define type-level multiplication without requiring > | > undecidable instances? > | > | No, not at the moment. The reasons are explained in the paper "Type > | Checking with Open Type Functions" (ICFP'08): > | > | http://www.cse.unsw.edu.au/~chak/papers/tc-tfs.pdf > | > | We want to eventually add closed *type families* to the system (ie, > | families where you can't add new instances in other modules). For > | such closed families, we should be able to admit more complex > | instances without requiring undecidable instances. > > It's also worth noting that while "undecidable" instances sound scary, but all it means is that the type checker can't prove that type inference will terminate. We accept this lack-of-guarantee for the programs we *run*, and type inference can (worst case) take exponential time which is not so different from failing to terminate; so risking non-termination in type inference is arguably not so bad. > > Simon > I have written code that makes heavy use of multi-parameter type classes in the ``finally tagless'' tradition, which takes several seconds and many megabytes of memory for GHCI to infer its type. However, that example is rather complicated, and I am not sure its type inference complexity is exponential---it is at least very bad. Are there any simple, well-known examples where Haskell type inference has exponential complexity? Or Hindley-Milner type inference, for that matter? (Haskell 98 is not quite Hindley-Milner?) Sincerely, Brad Larsen

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ANN: fp-southwales, the South Wales Functional Programming User Group
by Andy Gimblett 14 Oct '09

14 Oct '09

Dear friends, It is my pleasure to announce the formation of fp-southwales, a user group for anybody interested in functional programming in the area of south Wales, UK. We're based out of Swansea University's Computer Science department, where there are a few of us using Haskell for our research, but we welcome anyone who wants to join in, from academia or industry, from Swansea, or Cardiff, or indeed anywhere in south Wales. As the name of the group also suggests, we're interested in all aspects of functional programming, not just Haskell, although we expect that to be a central topic, for now at least. We exist online as a google group: http://groups.google.com/group/fp-southwales The first point of business is: hello, what shall we do, and when shall we do it? :-) http://groups.google.com/group/fp-southwales/browse_thread/thread/ddb2d352a… All welcome! -Andy

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why does "iterateNTimes (10^6) (ap . liftM (:) . return $ 0) (return [])" produce output in the IO monad, but stack overflow in the maybe monad?
by Thomas Hartman 13 Oct '09

13 Oct '09

Can someone explain why the one stack overflows and the other one doesn't? iterateNTimes i f x = foldr (.) id (replicate i f) $ x tntIO :: IO Int -- same as replicateM (10^6) $ return 0 , and same as sequence . replicate (10^6) $ return 0 tntIO = iterateNTimes (10^6) (ap . liftM (:) . return $ ) (return []) -- produces output tntMb :: Maybe Int -- overflows tntMb = iterateNTimes (10^6) (ap . liftM (:) . return $ ) (return []) -- stack overflow

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Type-level naturals & multiplication
by Brad Larsen 13 Oct '09

13 Oct '09

Suppose we implement type-level naturals as so: data Zero data Succ a Then, we can reflect the type-level naturals into a GADT as so (not sure if ``reflect'' is the right terminology here): data Nat :: * -> * where Zero :: Nat Zero Succ :: Nat a -> Nat (Succ a) Using type families, we can then proceed to define type-level addition: type family Add a b :: * type instance Add Zero b = b type instance Add (Succ a) b = Succ (Add a b) Is there any way to define type-level multiplication without requiring undecidable instances? Sincerely, Brad

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a library for control of terminal driver modes?
by Iain Barnett 13 Oct '09

13 Oct '09

I'm looking for a library like Perl's Term-Readkey, so that I can turn on and off the echo for secure password input from a terminal. Anyone know which library I need to use for this? Iain

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<**> for nested applicative functors?
by Kim-Ee Yeoh 13 Oct '09

13 Oct '09

Does anyone know if it's possible to write the following: <**> :: (Applicative m, Applicative n) => m (n (a->b)) -> m (n a) -> m (n b) Clearly, if m and n were monads, it would be trivial. Rereading the original paper, I didn't see much discussion about such nested app. functors. Any help appreciated. -- View this message in context: http://www.nabble.com/%3C**%3E-for-nested-applicative-functors--tp25858792p… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: ANN: CmdArgs - easy command line argument processing
by Ben Franksen 13 Oct '09

13 Oct '09

Neil Mitchell wrote: > I am pleased to announce CmdArgs v0.1. CmdArgs is a library for easy > command line argument processing - taking the arguments passed into > your program from getArgs and converting them into a structured value > for use in your program. Compared to the System.Console.GetOpts > library there are two key advantages: > > 1) The command line argument processors are shorter - in a typical > case they are about 1/3 the size, 90 lines with getopts vs 30 simpler > lines with CmdArgs (for HLint). > > 2) CmdArgs can support multiple mode command lines, such as those > found in hpc, darcs and cabal. > > The guiding principle of CmdArgs is to try and write each piece of > information only once, which makes the command line processing > shorter, and eliminates many of the accidental copy-and-paste style > bugs that are easy with getopts. CmdArgs requires GHC, but other than > this restriction, I do not foresee any reason for anyone to use > getopts over CmdArgs. I very much like the general idea of this thing, but have a question regarding the interface. This is from the docs: (&=) :: a -> Attrib -> a Add attributes to a value. Always returns the first argument, but has a non-pure effect on the environment. Take care when performing program transformations. value &= attrib1 & attrib2 To expose an impure function (!) in an API, I don't know... I mean, couldn't one just wrap the value like this data Attributed a -- opaque (&=) :: a -> Attrib -> Attributed a mode :: Data a => Attributed a -> Mode a and thus retain a purely functional interface? Cheers Ben

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Haskell-tan competition
by Svein Ove Aas 13 Oct '09

13 Oct '09

I say "competition", but.. at the moment I'm only aware of a single Haskell-tan, namely the one at http://www.reddit.com/r/programming/comments/9ss7n/haskell%E3%82%BF%E3%83%B…. This cannot stand. Haskell needs an anthropomorphized personification, like any other modern language. Anyway, have any of you seen any others? -- Svein Ove Aas

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Re: MTL vs Transformers?
by John Lato 13 Oct '09

13 Oct '09

> From: Erik de Castro Lopo <mle%2Bhs(a)mega-nerd.com> > > Well Iteratee built with MTL passes all the tests that shipped with it > so I suppose it must be correct. > Unfortunately the iteratee tests aren't exhaustive, but if it builds with MTL it should work. I'm more surprised that it built just by changing the .cabal file; I would expect you'd have to hack up the imports in the source code. John

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MTL vs Transformers?
by Erik de Castro Lopo 13 Oct '09

13 Oct '09

Hi all, I've just received the following error message: headers.hs:6:7: Could not find module `Control.Monad.Identity': it was found in multiple packages: transformers-0.1.4.0 mtl-1.1.0.2 I'm trying to use the Iteratee module which depends on Transformers but I use MTL in other stuff. Whats the preferred solution here? Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Why MTL and Transformers?
by Gregory Crosswhite 13 Oct '09

13 Oct '09

Something that I have been wondering for a while is: why are there *still* two monad transformer libraries with seemingly identical functionality, especially given that they have conflicting namespaces? It creates stupid problems like the one that Erik encountered and had to work around. I recognize that this situation most likely came about via historical accident rather than by intention, but why does it continue to be true? Cheers, Greg

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Haskell Weekly News: Issue 134 - October 10, 2009
by jfredett＠gmail.com 13 Oct '09

13 Oct '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091010 Issue 134 - October 10, 2009 --------------------------------------------------------------------------- Welcome to issue 134 of HWN, a newsletter covering developments in the [1]Haskell community. What with Don Stewart's [2]call to [3]arms to lead Haskell to conquest over (E)DSL-land, I've once again tried to highlight discussion of EDSL's this week. Fortunately, it was actually more difficult choosing what _not_ to include this week, since there was so much discussion about DSLs and Syntax extensions (a related notion, in my opinion). Also, this week Bryan O'Sullivan put his Criterion Library to good use on the `text` package, leading to [4]code which is more than ten times faster than before! With all this fantastic news, I won't hold you up any longer, Haskellers, the Haskell Weekly News! Announcements CfPart: FMICS 2009, 2-3 November 2009, Final Call. FMICS 2009 workshop chair [5]announced the final call for particpaction for FMICS 2009 ICFP videos now available. Wouter Swierstra [6]announced the availablity of videos from the International Conference on Functional Programming (ICFP) GPipe-1.0.0: A functional graphics API for programmable GPUs. Tobias Bexelius [7]announced the first release of GPie, a functional graphics API for programmable GPUs. text 0.5, a major revision of the Unicode text library. Bryan O'Sullivan [8]announced a new, major version of the text package. New API features, and huge improvments in speed, as Bryan says, 'Get it while it's fresh on Hackage, folks!' vty-ui 0.2. Jonathan Daugherty [9]announced a new version of the vty-ui package, with fewer bugs, more widgets, and cleaner code due to new more powerful abstractions. htzaar-0.0.1. Tom Hawkins [10]announced HTZAAR, a Haskell implementation of TZAAR Graphalyze-0.8.0.0 and SourceGraph-0.5.5.0. Ivan Lazar Miljenovic [11]announced To keep this editor happy, Ivan released two new packaged in one announcement. This time, he's added Legend support to Graphalyze, but also many new changes to SourceGraph, including a legend so you can see what all the symbols mean, Better color support, and much more. TxtSushi 0.4.0. Keith Sheppard [12]announced a new version of TxtSushi, a set of command line utilities for processing CSV and TSV files. Discussion Applicative do? Philippa Cowderoy [13]asked about a `do` like syntax for Applicative functors. How to add use custom preprocessor in cabal. Bernd Brassel [14]asked how to add a custom preprocessor to the build chain of a cabal file. On DSLs - one last time. Gunther Schmidt [15]summarized his impressions on al the recent discussion of DSLs What is a DSL? Oleg [16]offered some insight into different [17]properties that can be part of a single tagless framework. He also pointed to some slides and other materials such as a website [18]here and slides [19]here about DSL implementations and definitions. What is a DSL? Gunther Schmidt [20]posed the question, 'What is a DSL', and with some further questions added by yours truly, a lively discussion about the definition of a DSL ensued. Finally tagless - stuck with implementation of 'lam'. Gunther Schmidt [21]asked another question about Finally Tagless DSLs and resolving an issue with the implementation of 'lam' Blog noise [22]Haskell news from the [23]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Darcs: [24]darcs weekly news #43. * JP Moresmau: [25]What client for an Haskell Multi Player Game?. * Mikael Vejdemo Johansson (Syzygy-): [26][MATH198] Third lecture is up. * Bryan O'Sullivan: [27]Announcing a major revision of the Haskell text library. * Eric Kow (kowey): [28]darcs hashed-storage work merged (woo!). * David Amos: [29]Symmetries of PG(n,Fq). * The GHC Team: [30]Parallelism /= Concurrency. * >>> Nefigah: [31]Fake World Haskell. Nefigah, a recent addition to the community, has been working through RWH, and is providing some excellent examples. Though, This editor prefers the title 'Real Life Haskell' as opposed to his choice. * Tom Schrijvers: [32]Release 0.6 of Monadic Constraint Programming. * Neil Brown: [33]Concurrency Can Be Deterministic (But The Type System Doesn't Know It). * Clint Moore: [34]Curiously Parallel. * Galois, Inc: [35]Tech Talk: Constructing A Universal Domain for Reasoning About Haskell Datatypes. * Neil Brown: [36]Terminal Concurrency: The Printing Process. * Sean Leather: [37]'Upwards and downwards accumulations on trees' translated into Haskell. * Mikael Vejdemo Johansson (Syzygy-): [38][MATH 198] Second lecture. * Chris Smith: [39]View Patterns as Pattern Matching for Records. * Chris Smith: [40]Playing With Records. * FP Lunch: [41]Left Kan extensions of containers. Quotes of the Week * Baughn: Blum Blum Shub, a PRNG derived from poking around R'Lyeh. * ksf: * lambdabot locks up ksf in a Monad <ksf> mmmmh it's warm and fuzzy in here. * monochrom: Don't wrap your head around Haskell. Immerse! Wrap Haskell around your head. * chak: ... In other words, FP is inevitable. * robreim: I'm in your base hacking all your lambdas * gwern: RAM is overrated, swap is where it's at ;) * idnar: [to gwern] swap to a ramdisk! ;P About the Haskell Weekly News New editions are posted to [42]the Haskell mailing list as well as to [43]the Haskell Sequence and [44]Planet Haskell. [45]RSS is also available, and headlines appear on [46]haskell.org. To help create new editions of this newsletter, please see the information on [47]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [48]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.haskell.org/pipermail/haskell-cafe/2009-October/067203.html 3. http://www.reddit.com/r/haskell/comments/9qk54/haskell_should_own_the_edsl_… 4. http://www.serpentine.com/blog/2009/10/09/announcing-a-major-revision-of-th… 5. http://article.gmane.org/gmane.comp.lang.haskell.general/17555 6. http://article.gmane.org/gmane.comp.lang.haskell.general/17552 7. http://article.gmane.org/gmane.comp.lang.haskell.general/17546 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64591 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64529 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64516 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64399 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64301 13. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64616 14. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64558 15. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64552 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64533 17. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/ 18. http://dsl09.blogspot.com/ 19. http://dsl09.blogspot.com/2009/07/panel.html 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64474 21. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64343 22. http://planet.haskell.org/ 23. http://haskell.org/haskellwiki/Blog_articles 24. http://blog.darcs.net/2009/10/darcs-weekly-news-43.html 25. http://jpmoresmau.blogspot.com/2009/10/what-client-for-haskell-multi-player… 26. http://blog.mikael.johanssons.org/archive/2009/10/math198-third-lecture-is-… 27. http://www.serpentine.com/blog/2009/10/09/announcing-a-major-revision-of-th… 28. http://koweycode.blogspot.com/2009/10/darcs-hashed-storage-work-merged-woo.… 29. http://haskellformaths.blogspot.com/2009/10/symmetries-of-pgnfq.html 30. http://ghcmutterings.wordpress.com/2009/10/06/parallelism-concurrency/ 31. http://my.life-is-virtual.com/2009/10/07/fake-world-haskell-part-1/ 32. http://tomschrijvers.blogspot.com/2009/10/release-06-of-monadic-constraint.… 33. http://chplib.wordpress.com/2009/10/07/concurrency-can-be-deterministic/ 34. http://www.l2mlogistics.com/2009/10/curiously-parallel.html 35. http://www.galois.com/blog/2009/10/06/huffman-universal/ 36. http://chplib.wordpress.com/2009/10/06/the-printing-process/ 37. http://feedproxy.google.com/~r/splonderzoek/~3/4E8TZPDZ-aM/upwards-and-down… 38. http://blog.mikael.johanssons.org/archive/2009/10/math-198-second-lecture/ 39. http://cdsmith.wordpress.com/2009/10/04/view-patterns-as-pattern-matching-f… 40. http://cdsmith.wordpress.com/2009/10/03/playing-with-records/ 41. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=237 42. http://www.haskell.org/mailman/listinfo/haskell 43. http://sequence.complete.org/ 44. http://planet.haskell.org/ 45. http://sequence.complete.org/node/feed 46. http://haskell.org/ 47. http://haskell.org/haskellwiki/HWN 48. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

6 5

Graph Library Using Associated Types
by Lajos Nagy 13 Oct '09

13 Oct '09

Hi, I understand that one of the original motivations for introducing associated types to Haskell was the survey of support for generic programming done by Garcia et al. where they compared the implementation of the Boost Graph Library in various languages (C++, Java, Haskell, ML, C#, Eiffel). Haskell got full marks in all categories, except for associated types which made the resulting implementations more verbose than necessary (and exposed implementation details). Since then GHC added support for associated types but I was wondering if anybody re-did the experiment with the new features enabled (the original code is available at: http://www.osl.iu.edu/research/comparing/) Regards, Lajos

2 1

QuickCheck2 question
by Patrick Perry 13 Oct '09

13 Oct '09

I'm having some trouble with QuickCheck2 and Control.Applicative. Specifically, I have the following functions (slightly simplified): copyVector :: IOVector -> Vector -> IO () freezeVector :: IOVector -> IO (Vector) I have a test, part of which looks like monadicIO $ do run $ copyVector <$> pure dst <*> freezeVector src -- more stuff... When I try running the test (using test-framework-quickcheck2), the `copyVector` function never gets called. If I re-write the test as monadicIO $ do src' <- run $ freezeVector run $ copyVector dst src' -- etc. then everything works fine (i.e., the call to `copyVector` gets executed). Does anyone have any clues as to what is going on? I suspect the problem is related to Test.QuickCheck.Monadic using `unsafePerformIO` without a `{-# NOINLINE -#}` pragma [1], but I'm not completely sure. Adding such a pragma to `monadicIO` and `run` in Monadic.hs does not fix the problem. I'm using Quickcheck-2.1.0.2 and GHC 6.10.1. Thanks in advance for any help, Patrick [1] http://hackage.haskell.org/packages/archive/QuickCheck/2.1.0.2/doc/html/src…

1 0

Stupid error, probably
by b1g3ar5 13 Oct '09

13 Oct '09

I can't get the following to work in Leksah - but it works OK in GHC. Can anyone spot the error? I wondered if it was becasue the libraries loaded are different - but I'm just a Haskell beginner ... I have: myGroupBy :: Int→ [a]→ [[a]] myGroupBy = takeWhile not . null . (unfoldr (Just . (splitAt 3))) and I am getting the error: Couldn't match expected type `Int' against inferred type `[a]' In the second argument of `(.)', namely `(splitAt 3)' In the first argument of `unfoldr', namely `(Just . (splitAt 3))' In the second argument of `(.)', namely `(unfoldr (Just . (splitAt 3)))' But I think that splitAt :: Int->[a]->([a],[a]) so: splitAt 3 :: [a]->([a], [a]) and: Just.(splitAt 3) :: [a]->Maybe ([a], [a]) which seems OK as the first argument of unfoldr: unfoldr :: (b-> Maybe(a,b))->b->[a] with the a and b of unfoldr being [a] and [a]. The mention of Int in the error makes me wonder if I've got the wrong splitAt - with the arguments reversed maybe. Any ideas? Thanks. N

2 1

Need help with ghc static compile for cgi using mysql
by Austin King 12 Oct '09

12 Oct '09

I'm trying to host a cgi I've written. It uses Database.HDBC, Database.HDBC.MySQL, Network.CGI, and Text.XHtml.Transitional Here is the command I'm using to compile ghc --make -optl-static -optl-pthread -static -o test.cgi -package cgi -package xhtml -package HDBC-mysql -package HDBC -optl -lz Main.hs and I get "(.text+0x120): undefined reference to `compress'" Searching around I found that mysql recommended passing "lz" to the linker... so I added -optl -lz Complete command and output: ghc --make -optl-static -optl-pthread -static -o test.cgi -package cgi -package xhtml -package HDBC-mysql -package HDBC -optl -lz Main.hs [1 of 5] Compiling Config ( Config.hs, Config.o ) [2 of 5] Compiling Model ( Model.hs, Model.o ) [3 of 5] Compiling View ( View.hs, View.o ) [4 of 5] Compiling Controller ( Controller.hs, Controller.o ) [5 of 5] Compiling Main ( Main.hs, Main.o ) Linking test.cgi ... /opt/ghc-6.10.4/lib/ghc-6.10.4/network-2.2.1.2/libHSnetwork-2.2.1.2.a(HsNet.o): In function `my_inet_ntoa': HsNet.c:(.text+0x20): multiple definition of `my_inet_ntoa' /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_net.o):(.text+0x0): first defined here /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(mf_pack.o): In function `unpack_dirname': (.text+0x75b): warning: Using 'getpwnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(libmysql.o): In function `read_user_name': (.text+0x6081): warning: Using 'getpwuid' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(mf_pack.o): In function `unpack_dirname': (.text+0x76a): warning: Using 'endpwent' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /opt/ghc-6.10.4/lib/ghc-6.10.4/network-2.2.1.2/libHSnetwork-2.2.1.2.a(HsNet.o): In function `hsnet_getaddrinfo': HsNet.c:(.text+0x15): warning: Using 'getaddrinfo' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(ssl.o): In function `yaSSL::read_file(yaSSL::SSL_CTX*, char const*, int, yaSSL::CertType)': (.text+0x29f3): warning: memset used with constant zero length parameter; this could be due to transposed parameters /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_gethostbyname.o): In function `my_gethostbyname_r': (.text+0x3c): warning: Using 'gethostbyname_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /opt/ghc-6.10.4/lib/ghc-6.10.4/network-2.2.1.2/libHSnetwork-2.2.1.2.a(BSD__213.o): In function `s9bc_info': (.text+0x55): warning: Using 'getprotobyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(libmysql.o): In function `mysql_server_init': (.text+0x6b22): warning: Using 'getservbyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_compress.o): In function `my_uncompress': (.text+0x6b): undefined reference to `uncompress' /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_compress.o): In function `my_compress_alloc': (.text+0x120): undefined reference to `compress' collect2: ld returned 1 exit status I'm using ghc 6.10.4 on Ubuntu 9.04 i386 I have libmysqlclient15-dev installed.... Thanks, Austin

1 0

Simple program. Simple problem?
by michael rice 12 Oct '09

12 Oct '09

What is going wrong here? Michael ======= import System.Random coinToss :: StdGen -> IO () coinToss gen = putStrLn "What's your guess, heads or tails ('h' or 't')?" >> getChar >>= \c -> let (randInt, _) = randomR(0,1) gen :: (Int, StdGen) in if c == ((!!) "ht" randInt) then putStrLn "You win!" else putStrLn "You lose!" main = do gen <- getStdGen coinToss gen gen <- newStdGen main ======= [michael@localhost ~]$ runhaskell cointoss.hs What's your guess, heads or tails ('h' or 't')? h You win! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? h You lose! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? ^Ccointoss.hs: cointoss.hs: interrupted [michael@localhost ~]$

4 9

Can type be changed along a >>= chain?
by michael rice 12 Oct '09

12 Oct '09

The first of these works, but not the second. It would seem that the type cannot change along a >>= chain, but I may be missing something in the code. Is the second example illegal? If so, is there a different way to change a String to an Int along the >>= chain? Michael =========== import Data.Char {- transform :: IO () transform = putStrLn "What is your word?" >> getLine >>= \str -> return ('Q':str) >>= \str -> return ('Z':str) >>= \str -> putStrLn $ "Transformed word is " ++ show str -} transform :: IO () transform = putStrLn "What is your digit string?" >> getLine >>= \str -> return ('9':str) >>= \str -> return (read str :: Int) >>= \i -> putStrLn $ "The number is " ++ show i

4 6

statistics package and randomness
by Michael Mossey 12 Oct '09

12 Oct '09

I'm trying to learn how to use randomness in Haskell and it seems very non-straightforward and complex. I could do a lot of things using 'split' from System.Random, but apparently it's broken. There is the statistics package here: http://hackage.haskell.org/package/statistics Is this a better solution? It uses the ST monad in the RandomVariate module. Can someone point me to a tutorial explaining ST, and/or a tutorial in the RandomVariate module? Pseudorandomness seems like one case where it would just be a hell of a lot simpler to have a global generator--never split the state. Is the ST monad some way to accomplish this? Thanks, Mike

3 3

"safe" way to use Rand?
by Michael Mossey 12 Oct '09

12 Oct '09

I'm looking at Control.Monad.Random which provides the Rand monad. I would like to know how to use this for generating multiple infinite series, while trusting that the implementation never uses "split" behind the scenes. (Note: I'm on Windows XP, and there appears to be a bug in getStdGen. It does NOT return an arbitrary generator, but rather the same one every time I run the program. However, newStdGen DOES return an arbitrary generator. So I'm using that, even though I know it accesses split behind the scenes. My thinking is that this only happens once so it is okay.) For example, is this code split-free? simple :: Rand StdGen [Int] simple = getRandomRs (0::Int, 10) main1 = do gen <- newStdGen let answer = (flip evalRand) gen $ do xs <- simple ys <- simple return $ (take 5 xs) ++ (take 3 ys) print answer Then, to elaborate on my specific problem, I need to create special types of infinite series. For example, I might need to create one that looks like this: 0 0 5 0 0 0 2 0 0 0 0 0 5 0 9 0 0 8 ... The pattern here is that there is some random number of zeros followed by a single non-zero value, followed again by a random number of zeros, etc. forever. This is one way to implement this. Does all look well here? makeSeries :: [Int] -> [a] -> a -> [a] makeSeries (i:is) (f:fs) zero = replicate i zero ++ [f] ++ makeSeries is fs zero lessSimple :: Rand StdGen [Int] lessSimple = do counts <- getRandomRs (1::Int , 5 ) values <- getRandomRs (1::Int , 9 ) return $ makeSeries counts values 0 main2 = do gen <- newStdGen let answer = evalRand lessSimple gen print . take 20 $ answer We could even have several of these series zipped together. Is this split-free? main3 = do gen <- newStdGen let fs = (flip evalRand) gen $ do s1 <- lessSimple s2 <- lessSimple return $ zip s1 s2 print . take 20 $ fs

1 0

SDL build problem
by Henk-Jan van Tuyl 11 Oct '09

11 Oct '09

L.S., I am trying to install SDL, but I get error messages about multiple definition of `main'; before I spend many hours of debugging, does anybody know why this happens? See the log below. Regards, Henk-Jan van Tuyl C:\Programs\Haskell\Libs\SDL-0.5.5>runhaskell Setup configure --extra-include-dirs=C:\Programs\MSYS\1.0\include\SDL Setup.lhs:2:2: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you need Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Warning: defaultUserHooks in Setup script is deprecated. Configuring SDL-0.5.5... checking for sdl-config... /usr/bin/sdl-config configure: creating ./config.status config.status: creating config.mk config.status: creating SDL.buildinfo config.status: creating includes/HsSDLConfig.h config.status: includes/HsSDLConfig.h is unchanged C:\Programs\Haskell\Libs\SDL-0.5.5>runhaskell Setup build Setup.lhs:2:2: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you need Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Preprocessing library SDL-0.5.5... dist\build\Graphics\UI\SDL\General_hsc_make.o:General_hsc_make.c:(.text+0x0): multiple definition of `main' C:\Programs\ghc\ghc-6.10.4/gcc-lib/libmingw32.a(main.o):main.c:(.text+0x0): first defined here C:/Programs/MinGW/lib/libSDLmain.a(SDL_win32_main.o): In function `console_main': /Users/hercules/trunk/SDL-1.2/./src/main/win32/SDL_win32_main.c:246: undefined reference to `SDL_main' collect2: ld returned 1 exit status linking dist\build\Graphics\UI\SDL\General_hsc_make.o failed command was: C:\Programs\ghc\ghc-6.10.4\gcc.exe -BC:\Programs\ghc\ghc-6.10.4\gcc-lib -IC:\Programs\ghc\ghc-6.10.4\include\mingw -lSDL -L/usr/lib -lmingw32 -lSDLmain -lSDL -mwindows -LC:\Programs\ghc\ghc-6.10.4\base-4.1.0.0 -lwsock32 -luser32 -lshell32 -LC:\Programs\ghc\ghc-6.10.4\integer-0.1.0.1 -LC :\Programs\ghc\ghc-6.10.4\ghc-prim-0.1.0.0 -LC:\Programs\ghc\ghc-6.10.4 -LC:\Programs\ghc\ghc-6.10.4/gcc-lib -lm -lffi -lgmp -lwsock32 dist\build\Graphics\UI\SDL\General_hsc_make.o -o dist\build\Graphics\UI\SDL\General_hsc_make.exe --

1 0

The Monad.Reader (14) - Call for copy
by Brent Yorgey 11 Oct '09

11 Oct '09

Call for Copy: The Monad.Reader - Issue 14 ------------------------------------------ Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 14 will be: **Friday, January 8, 2010** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out.

1 1

ANN: text 0.5, a major revision of the Unicode text library
by Bryan O'Sullivan 11 Oct '09

11 Oct '09

Get it while it's fresh on Hackage, folks! Details of the changes here: http://bit.ly/1u4UOT

5 9

How do I get this done in constant mem?
by mf-hcafe-15c311f0c＠etc-network.de 11 Oct '09

11 Oct '09

Hi all, I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position. -- test.hs import Random import System.Environment (getArgs) import System.IO (putStr) samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3) maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples) main = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . show $ maxima x I would expect this to take constant memory (foldr as well as foldl), but this is what happens: $ ghc -prof --make -O9 -o test test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 100 +RTS -p [9.881155955344708e8,9.910336352165401e8,9.71000686630374e8,9.968532576451201e8,9.996200333115692e8] $ grep 'total alloc' test.prof total alloc = 744,180 bytes (excludes profiling overheads) $ ./test 10000 +RTS -p [9.996199711457872e8,9.998928358545277e8,9.99960283632381e8,9.999707142123885e8,9.998952151508758e8] $ grep 'total alloc' test.prof total alloc = 64,777,692 bytes (excludes profiling overheads) $ ./test 1000000 +RTS -p Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $ so... does sequence somehow force the entire list of monads into evaluation before the head of the result list can be used? what can i do to implement this in constant memory? thanks! matthias

5 9

Snow Leopard, gtk2hs
by Arne Dehli Halvorsen 11 Oct '09

11 Oct '09

This may be a little off-topic, but if someone could help me, I'd be grateful.I am trying to get to a working gtk2hs environment in MacOSX Snow Leopard I have a Macbook Pro 2.1 with Snow Leopard. While I had Leopard, I had the Haskell Platform installed, and it worked fine. I also had the MacPorts edition of gtk, and compiled gtk2hs by hand. After I got Snow Leopard, I installed the Haskell Platform. I have made the necessary changes to make it compile (flags to ghc), and it compiles normal programs. I installed the standalone version of the GTK+ platform, and then tried to compile gtk2hs from /gtk2hs . ./configure works, but make results in this: adhmac:gtk2hs arne$ make make all-am rm -rf gtk/Graphics/UI/Gtk/Types.o gtk/Graphics/UI/Gtk/Types_split/; mkdir -p gtk/Graphics/UI/Gtk/Types_split /usr/bin/ghc +RTS -RTS -split-objs -c gtk/Graphics/UI/Gtk/Types.hs -o gtk/Graphics/UI/Gtk/Types.o -O -XForeignFunctionInterface -fglasgow-exts -igtk -package-conf package.conf.inplace -hide-all-packages -ignore-package gtk -package base-4.1.0.0 -package mtl-1.1.0.2 -package containers-0.2.0.1 -package array-0.2.0.0 -package glib-0.10.1 -package cairo-0.10.1 -package-name gtk-0.10.1 '-#include<gtk/hsgtk.h>' -I/Library/Frameworks/GLib.framework/Headers -I/Library/Frameworks/Cairo.framework/Headers -I/Library/Frameworks/Gtk.framework/Headers -I. /var/folders/G8/G8LY9MSBEGuyatB+JjSWq++++TI/-Tmp-/ghc4162_0/ghc4162_0.split__2870.s:35:0: invalid character '_' in mnemonic /var/folders/G8/G8LY9MSBEGuyatB+JjSWq++++TI/-Tmp-/ghc4162_0/ghc4162_0.split__2870.s:953:0: FATAL:Symbol L_gtk_misc_get_type$stub already defined. make[1]: *** [gtk/Graphics/UI/Gtk/Types.o] Error 1 make: *** [all] Error 2 I also have gtk installed via macports recently, but I am not sure whether this makes a difference. I don't think there is any point currently in trying to go MacPorts all the way, and install ghc+gtk2hs via MacPorts. Does anyone have a recipe for getting ghc + gtk2hs + gtk working on MacOSX Snow Leopard? Best regards, Arne D Halvorsen

4 7

how to determine the dependency versions before packaging and submitting to hackage
by Andrew U. Frank 11 Oct '09

11 Oct '09

i plan to submit a package to hackage. i run it on my machine with a cabal file which has for all dependencies "any" (except for base, where it says >=4.0 -- see previous discussion on haskell-cafe). i am not certain, if my code runs agains any version, but i fear it is too restrictive to say >= "current version", which could exclude too much? testing against all previous versions of the packages seems complicated and very time consuming. what is the advice in this case? thank you andrew

3 2

STM in .NET
by Andrew Coppin 10 Oct '09

10 Oct '09

Apparently they like what they see, and they want it to: http://kskky.info/wiki/STM.NETInFramework4 (I notice that it relies on the user manually annotating anything that isn't safe to revert... This makes me nervous.)

2 2

HDBC PostgreSQL
by Patrick Brannan 10 Oct '09

10 Oct '09

Hello, I'm getting back into Haskell after an absence of a few years. I'm in the process of trying to connect to postgresql with hdbc-postgresql on a Windows XP box. Seemingly, things installed without a problem, but at the command prompt I get the following: Prelude> :module Database.HDBC Database.HDBC.PostgreSQL module `Database.HDBC.PostgreSQL' is not loaded If I run ghci this way with package options here's what I get: C:\work\haskell\HDBC-postgresql-2.1.0.0>ghci -package HDBC -package HDBC-postgresql GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package HUnit-1.2.0.3 ... linking ... done. Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package QuickCheck-1.2.0.0 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package Win32-2.2.0.0 ... linking ... done. Loading package time-1.1.4 ... linking ... done. Loading package convertible-1.0.5 ... linking ... done. Loading package filepath-1.1.0.2 ... linking ... done. Loading package directory-1.0.0.3 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package testpack-1.0.2 ... linking ... done. Loading package utf8-string-0.3.5 ... linking ... done. Loading package HDBC-2.1.1 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package HDBC-postgresql-2.1.0.0 ... linking ... done. Prelude> module HDBC HDBC.PostgreSQL Now I would think that the line "Loading package HDBC-postgresql-2.1.0.0 ... linking ... done." means that the module is installed correctly, but I still can't execute the :module statement. Does anyone have any ideas about where I should start? It's probably something stupid, but I'm a little worn out on searching. Thank you, Patrick

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How to add use custom preprocessor in cabal
by Bernd Brassel 10 Oct '09

10 Oct '09

Hi folks, I am trying to integrate my own preprocessor into a cabal build process. But there are several points that I get stuck with. Could someone help me, please? A simplification of my problem: I have files "Abc.foo" and each of them should be transformed into "Abc.hs" by calling function "transform". The resulting "Abc.hs" files should then be exposed as Haskell library modules. 1. Question: How do I tell cabal that the file Abc.foo belongs to the package? After reading the User's Guide I had the impression that I should write something like Library Eposed-Modules: Abc.foo But this leads to a parse error. Adding a Data-Files: Abc.foo to the heading, on the other hand, does not seem to do anything. 2. Question: How to add the preprocessor? I have tried main = defaultMainWithHooks simpleUserHooks{hookedPreProcessors=[("foo",transformation)]} transformation :: BuildInfo -> LocalBuildInfo -> PreProcessor But under which circumstances will this function be called? Up to now I have not succeeded in making cabal call this function. Thanks for your time! Bernd

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ANN: Extensions to Vec uploaded (useful for GPipe programs)
by Tobias Bexelius 10 Oct '09

10 Oct '09

Hi, I've added two extensions to the Vec package: Vec-Transform and Vec-Boolean. These two packages are more or less essential when programming GPipe programs, but can also be useful for other applications as well. * Vec-Transform provides some 4x4 transform matrices such as perspective projection and rotation. * Vec-Boolean provides Data.Boolan instances for the Vec data types. Cheers /Tobias _________________________________________________________________ Windows Live: Håll dina vänner uppdaterade om vad du gör online. http://www.microsoft.com/windows/windowslive/see-it-in-action/social-networ…

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Ghci :ctags or hasktags print to standard out instead of file
by Johannes 10 Oct '09

10 Oct '09

Hi, I want to use Vim with Haskell. Therefore ctags generation is really useful. The TagList plugin for Vim reads the ctags info from the command line instead of from the file. I could not figure out how to make ghci :ctags or hasktasks to print the ctags info to the command line. Is there a way to do that? Any hints? Kind regards Johannes

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What does "mkUsageInfo: internal name?" mean? (ghc 6.10.1, Template Haskell)
by Serguey Zefirov 10 Oct '09

10 Oct '09

I get a bunch of messages like "mkUsageInfo: internal name? WritebackCmd{tc a1eZY}" while transforming my program using Template Haskell. It displayed for every data type and for every constructor - I generate some instances for my data types.

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Re: Applicative do?
by oleg＠okmij.org 10 Oct '09

10 Oct '09

It seems the applicative do is almost identical to the form ML and Scheme has had since the beginning. Perhaps that semantic similarity might inform the syntactic debate. > do a <- f > g > b <- h > pure $ foo a b > > into this: > > (\a b -> pure $ foo a b) <*> (f <*> g *> h) This form, although very similar in appearance to the monadic do, is very different in the scoping of bound identifiers. To me, that is quite unfortunate, greatly increasing confusion. In the monadic do, do x <- e1 y <- e2 e3 the identifier 'x' may appear in e2 and e3. To be more precise, the scope of each bound identifier extends to the rest of the do-block and can be used on the right-hand-side of other bindings in the rest of the block. In the proposed applicative do above, the identifier 'a' may not appear within g and h. Such a significant difference in scoping calls for a different name at the very least; furthermore, it calls for a compiler supports so that errors, such as using the identifier 'a' in 'h' by mistake (common during editing and refactoring) could be reported with an intelligent error message. Also, in monadic-do, it makes sense to bind the same identifier several times, as in do {x <- e1; x <- e2; e3}. It makes little sense doing that in applicative-do, and probably should be disallowed. Bob Atkey wrote: > applicatively > let x = foo > let y = bar > in <pure stuff> But OCaml already has the right syntax -- and so does Scheme. That syntax is let. In Scheme expression (let ((x e1) (y e2) (_ e11)) e3) only the expression e3 is in scope of x and y. Emphatically, e2 is not in scope in x; so the value bound to x cannot be used during the evaluation of e2. The expression e11 is evaluated only for its side-effect (underscore is just an identifier, I could've used `dummy'). Again, the expression e11 is not in scope of x and y. All identifiers bound by the let-clause must be distinct. The equivalent OCaml construction is let x = e1 and y = e2 and _ = e11 in e3 If, by mistake, we do mention x or y in e11 (and there are no global binding to x and y), OCaml will report the error about the unbound identifier. It seems the parallel let of Scheme or OCaml fall short of Applicative in one respect: the order of evaluating binding is not specified. If we agreed that the binders of a parallel let must be evaluated in order, we would get exactly the desired `applicative-do'. Perhaps, a better form in OCaml would be let seq x = foo and y = bar in e3 or let x = foo andalso y = bar in e3 Perhaps it may make sense to use the keyword `let' rather than 'do' in Haskell as well? P.S. For completeness, both Scheme and OCaml have the exact equivalent of monadic do. Haskell's do x <- e1 y <- e2 x <- e3 e4 e5 can be written in Scheme as (let* ((x e1) (y e2) (x e3) (_ e4)) e5) and in OCaml as let x = e1 in let y = e2 in let x = e3 in let () = e4 in e5 The latter pattern is very common in OCaml.

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Haskell Weekly News
by Joe Fredette 09 Oct '09

09 Oct '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091003 Issue 134 - October 03, 2009 --------------------------------------------------------------------------- Welcome to issue 134 of HWN, a newsletter covering developments in the [1]Haskell community. I have a nasty sinus infection this week, so we're somewhat light on content. Lots of good discussion about DSL related stuff this week. Bryan O'Sullivan also release 'Criterion' this week, a new benchmarking library that Don Stewart described (on reddit) as 'awesome and game changing.' A new TMR editor -- someone familiar -- was announced. Also, there was some talk about homework policies on the mailinglists and in the irc channels. There is a [2]page on the Haskell wiki about this, but to sum it up in a maxim, remember, 'Help, don't do'. Until next week, the Haskell Weekly News! Announcements New TMR editor. Wouter Swierstra [3]announced that he would be stepping down from the editorship of 'The Monad Reader', with former HWN editor Brent Yorgey taking his place. Much thanks for Wouter's hard work and good luck to Brent on his new editor job! SourceGraph 0.5.{0,1,2}.0. Ivan Lazar Miljenovic [4]announced three new releases of the SourceGraph packages, this links to the latest release. json-b-0.0.4. Jason Dusek [5]announced a new version of the json-b package, which fixes defective handling of empty objects and arrays. rss2irc 0.4 released. Simon Michael [6]announced a new release of rss2irc, with many new improvements and features. vty-ui 0.1. Jonathan Daugherty [7]announced vty-ui, which is an extensible library of user interface widgets for composing and laying out Vty user interfaces. atom-0.1.1. Tom Hawkins [8]announced Atom, a Haskell DSL for designing hard real-time embedded applications. Graphalyze-0.7.0.0. Ivan Lazar Miljenovic [9]announced (in an apparent effort to take over hackage by submitting dozens of quality packages at absurdly high speed), Graphalyze, a library for using graph-theoretic techniques to analyse the relationships inherent within discrete data. Criterion. Bryan O'Sullivan [10]announced (without tacking on an 'ANN' tag, I might add, I almost missed it!) Criterion, a benchmarking library he describes [11]here. ListTree 0.1. yairchu(a)gmail.com [12]announced ListTree, a package for combinatorial search and pruning of trees. usb-0.1. Bas van Dijk [13]announced a library for interacting with usb modules from userspace. (Deadline extended to October 5th) APLAS 2009 Call for Posters. Kiminori Matsuzaki [14]announced a deadline extension to the call for posters for the APLAS conference. graphviz-2999.6.0.0. Ivan Lazar Miljenovic [15]announced a new version of the graphviz library, which features various new features and small changes. Discussion Testing polymorphic properties with QuickCheck. Jean-Philippe Bernardy [16]gave an excellent overview about how to use QuickCheck to test polymorphic properties. Designing a DSL? Gunther Schmidt [17]asked about different methods employed for designing a DSL. DSL and GUI Toolkits. Gunther Schmidt [18]also asked about different DSLs for working with GUIs error on "--++ bla bla bla". Hong Yang [19]asked about why '--++' wasn't being parsed in the way he thought it was. Haskell for Physicists. edgar [20]requested name suggestions for the talk he is giving about Physics and Haskell. Blog noise [21]Haskell news from the [22]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Sean Leather: [23]'Extensibility and type safety in formatting: the design of xformat' at the Dutch HUG. * Martijn van Steenbergen: [24]let 5 = 6. * Lee Pike: [25]Writer's unblock. * Manuel M T Chakravarty: [26]NVIDIAs next generation GPU architecture has a lot for HPC to love. * David Amos: [27]Finite geometries, part 4: Lines in PG(n,Fq). * Bryan O'Sullivan: [28]New criterion release works on Macs. * Neil Brown: [29]Poison: Concurrent Termination. * The GHC Team: [30]Heads up: what you need to know about Unicode I/O in GHC 6.12.1. * Galois, Inc: [31]Tech Talk: Roll Your Own Test Bed for Embedded Real-Time Protocols: A Haskell Experience. * Bryan O'Sullivan: [32]Criterion, a new benchmarking library for Haskell. * Tom Schrijvers: [33]Monadic Constraint Programming. * Neil Brown: [34]Growing Sort Pump. Quotes of the Week * dekudekuplex: (Unfortunately (unless intentional)) the preceding (by ksf (in the 'Quotes of the Week' section)) quote had mismatched (one too many opening) parentheses (although it was still funny (even though it could have been edited (to make the parentheses match (even though that is not an important issue)))). * pozic: I think if you want to contact dons, you have to say that you found a bug in ByteString. * Veinor: [about dibblego kicking a whole bunch of spammers] crouching dibblego, hidden op * allbery_b: [on UndecidableInstances] 'this exceeds my easy threshold, turn on wizard mode' (at which point it becomes a lot smarter but may start contemplating its navel without warning) * byorgey: a bus error? try recompiling with -fsubway, perhaps * jafet: 'Zygomorphism' sounds like a reproductive disorder About the Haskell Weekly News New editions are posted to [35]the Haskell mailing list as well as to [36]the Haskell Sequence and [37]Planet Haskell. [38]RSS is also available, and headlines appear on [39]haskell.org. To help create new editions of this newsletter, please see the information on [40]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [41]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.haskell.org/haskellwiki/Homework_help 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64213 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64259 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64254 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63838 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64070 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64035 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63989 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63985 11. http://www.serpentine.com/blog/2009/09/29/criterion-a-new-benchmarking-libr… 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63853 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64206 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17529 15. http://article.gmane.org/gmane.comp.lang.haskell.general/17520 16. http://thread.gmane.org/gmane.comp.lang.haskell.general/17535 17. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64267 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64244 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64161 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64128 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://feedproxy.google.com/~r/splonderzoek/~3/BHqNHIZFF7U/extensibility-an… 24. http://martijn.van.steenbergen.nl/journal/2009/10/02/let-5-6/ 25. http://leepike.wordpress.com/2009/09/30/writers-unblock/ 26. http://justtesting.org/post/201508784 27. http://haskellformaths.blogspot.com/2009/09/finite-geometries-part-4-lines-… 28. http://www.serpentine.com/blog/2009/09/30/new-criterion-release-works-on-ma… 29. http://chplib.wordpress.com/2009/09/30/poison-concurrent-termination/ 30. http://ghcmutterings.wordpress.com/2009/09/30/heads-up-what-you-need-to-kno… 31. http://www.galois.com/blog/2009/09/29/pike-haskell0/ 32. http://www.serpentine.com/blog/2009/09/29/criterion-a-new-benchmarking-libr… 33. http://tomschrijvers.blogspot.com/2009/01/monadic-constraint-programming.ht… 34. http://chplib.wordpress.com/2009/09/28/growing-sort-pump/ 35. http://www.haskell.org/mailman/listinfo/haskell 36. http://sequence.complete.org/ 37. http://planet.haskell.org/ 38. http://sequence.complete.org/node/feed 39. http://haskell.org/ 40. http://haskell.org/haskellwiki/HWN 41. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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Non-traversable foldables
by Martijn van Steenbergen 09 Oct '09

09 Oct '09

Hallo café, Can anyone think of datatypes that are Foldable but not Traversable? If not, what is the purpose of having a separate Foldable class? Thanks, Martijn.

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Different semantics in "identical" do statement?
by staafmeister 09 Oct '09

09 Oct '09

In my program do x <- do blah <- someFunc return blah return $ Constructor x behaves differently from do blah <- someFunc return $ Constructor blah where the dots are identical. I would think that these programs should behave identically, by the monad laws. The result of my program is that the second gives correct behaviour, while the first loops forever. Greetings, Gerben -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Applicative do?
by Philippa Cowderoy 09 Oct '09

09 Oct '09

I do a lot of work with parsers, and want to do more using Applicatives. That said, I'm finding it a little tedious being forced to use pointless style for a task that's well-suited to having a few names around. The idea of an applicative do notation's been kicked around on #haskell a few times (I can't find any trace of it on the mailing list, but I confess to not having searched too hard), so I thought I'd propose it here. The basic idea is to turn this: do a <- f g b <- h pure $ foo a b into this: (\a b -> pure $ foo a b) <*> (f <*> g *> h) Aside from changing >>= and >> into <*> and *>, the most significant difference from monadic do is that all the generated lambda abstractions go in front of the final "return" statement which is then fmapped across the rest of the code. Bindings are thus only in scope in the "return" statement. I believe sugared let statements can be handled similarly so long as they respect the binding discipline. This leads us to the bikeshed topic: what's the concrete syntax? The obvious way is to replace do with a new keyword - for example, ado for "applicative do". There's a nice alternative though: we can check whether a do statement meets the binding rules for an applicative block and treat it as one if so, or a monadic one if not. While not all Monads are Applicatives, code can readily be changed back using the WrappedMonad newtype - whereas existing code needn't turn on the appropriate extension in the first place. Thoughts, comments? -- flippa(a)flippac.org

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Num instances for 2-dimensional types
by Sönke Hahn 09 Oct '09

09 Oct '09

Hi! I often stumble upon 2- (or 3-) dimensional numerical data types like (Double, Double) or similar self defined ones. I like the idea of creating instances for Num for these types. The meaning of (+), (-) and negate is clear and very intuitive, i think. I don't feel sure about (*), abs, signum and fromInteger. I used to implement fromInteger n = (r, r) where r = fromInteger n , but thinking about it, fromInteger n = (fromInteger n, 0) seems very reasonable, too. Any thoughts on that? How would you do it? btw: These are two examples i looked at in hackage: Data.Complex.Complex (which is special, cause it is mathematically defined, what (*), abs, signum and fromInteger should do (i think)) and Physics.Hipmunk.Common.Vector (http://hackage.haskell.org/packages/archive/Hipmunk/5.0.0/doc/html/Physics- Hipmunk-Common.html#9) Sönke

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ANNOUNCE: htzaar-0.0.1
by Tom Hawkins 09 Oct '09

09 Oct '09

HTZAAR is a Haskell implementation of TZAAR, a great little two-player abstract strategy game designed by Kris Burm. TZAAR won Games Magazine's Game-of-the-Year in 2008. TZAAR has some interesting game dynamics. At each turn a player must decide whether to attack or strengthen his own pieces. The unbalanced number of tzaars, tzarras, and totts pieces leads to several phase transitions in the game. At first tzaars are targeted, but once they're fortified, tzarras become the targets, then totts. It also has a rather distinct transition from mid to end game. Midway through, most effort is on positioning power pieces for attack. But as more pieces come off the board, movement is restricted to the point where groups become isolated from others, which can work for or against you. Anyway, I thought the game would be a fun AI problem. HTZAAR comes with a very simple AI opponent (lame), but I did my best to structure the interface to make it easy to create new AI strategies. I hope a few people try to build and contribute AI strategies to HTZAAR. And if there's anyway I can improve the AI interface, let me know. HTZAAR can also pit AI against AI. The following is a game between two instances of the "lame" AI. For an idea of the complexity of TZAAR, the number in parentheses indicates the number of possible moves at each turn. TZAAR has a large number of choices at each turn because each turn consists of two different moves (n^2). Typical games are between 15 to 25 turns. $ htzaar -s 22 lame lame 1. white (lame) : D7 -> E7 (42) 2. black (lame) : F6 -> F5 G1 -> F2 (6355) 3. white (lame) : D6 -> E6 A5 -> A4 (5867) 4. black (lame) : F3 -> E4 H2 -> G2 (5227) 5. white (lame) : I2 -> I1 C5 -> C6 (4992) 6. black (lame) : F5 -> G4 H1 -> H3 (4345) 7. white (lame) : H5 -> G6 C6 -> C7 (4176) 8. black (lame) : F7 -> E7 D4 -> C3 (4265) 9. white (lame) : G6 -> E5 H4 -> G5 (3464) 10. black (lame) : H6 -> I5 F2 -> E2 (2870) 11. white (lame) : E5 -> G4 D2 -> D1 (2303) 12. black (lame) : B2 -> C2 G7 -> I5 (2742) 13. white (lame) : G5 -> I5 E6 -> I3 (1965) 14. black (lame) : E7 -> B5 G3 -> E3 (2055) 15. white (lame) : E1 -> F1 I4 -> I3 (1303) 16. black (lame) : E2 -> D1 C4 -> D5 (1656) 17. white (lame) : I5 -> F8 C7 -> I3 (913) 18. black (lame) : B5 -> A4 E4 -> B4 (1480) 19. white (lame) : D3 -> C2 B1 -> C2 (608) 20. black (lame) : B4 -> B6 C3 -> C2 (526) 21. white (lame) : D8 -> E8 F1 -> F4 (185) 22. black (lame) : H3 -> G4 B3 -> B6 (314) 23. white (lame) : C1 -> A1 F4 -> G4 (47) 24. black (lame) : B6 -> E8 E8 -> F8 (125) black (lame) Wins! Screenshots and links: http://tomahawkins.org Known bugs: Occasionally HTZAAR throws a Prelude.head: empty list. I'm still trying to track this down.

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