June 2009 - Haskell-Cafe

What happened to the installation chapter in the GHC User's Guide?
by Benjamin L.Russell 24 Jun '09

24 Jun '09

Up to GHC 6.10.2, there used to be an installation chapter in the GHC User's Guide (see "Chapter 2. Installing GHC" at http://www.haskell.org/ghc/docs/6.10.2/html/users_guide/installing-bin-dist…) This section was very useful, for example, in determining whether it was possible to move the GHC tree around in Windows, or, by extension, whether it was necessary to uninstall GHC in order to install a later version. However, starting with GHC 6.10.3, this chapter seems to be missing. When I asked about this issue earlier on this mailing list a few months ago, IIRC, one response was that the chapter was outdated. Are there any plans to update this chapter? Every once in a while, I forget whether GHC needs to be uninstalled in order to be updated on Windows, and am forced to consult an earlier version of this chapter. -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Weird thing happened when I use gtk2hs with concurrent.
by Magicloud Magiclouds 24 Jun '09

24 Jun '09

This is the weird code, please see line 61. 20 cpuNew :: IO CPU 21 cpuNew = do 22 self <- meterNew 23 usage <- newIORef $ CPUusage 0 0 0 0 0 0 0 0 24 self `on` realize $ do 25 forkIO $ redraw self usage 33 return () 34 return self 35 where 56 redraw self usage = do 57 newUsage <- getUsage 58 oldUsage <- readIORef usage 59 postGUISync $ meterDraw self "Cpu" $ computeIdle $ diff newUsage oldUsage 60 writeIORef usage newUsage 61 threadDelay 10000 -- ten seconds, but it just refresh crazy! 62 redraw self usage

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Using Parsec with ByteString ?
by Fernand 24 Jun '09

24 Jun '09

Hi, May be it's a frequently asked question, but I have been unable to find my way in the parsec-3.0.0 library when trying to write a small parser with Parsec that could work with ByteStrings. I mean, there is the Text.Parsec.ByteString module which allows to parse a file using ByteStrings, but the parser one needs to write must parse ByteStrings instead of Strings (that is, something like having a "Parsec Bytestring ()" type, unless I'm completely misunderstanding the situation). My problem is that I do not manage to write primitive parsing combinators (like string, satisfy, letter, etc.) to define my language's tokens. I'm surely missing some primitive somewhere (may be the Text.Parsec.Prim.token, though I do not really manage to use it). Thanks in advance, F.

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Type system trickery
by Andrew Coppin 23 Jun '09

23 Jun '09

I have a datatype with about a dozen constructors. I'd like to find a way to use the type system to prevent one of the constructors from being used in certain places. But I can't think of a way to do that. data Foobar = Foo Foobar | Bar Foobar | Zoo Foobar I want the type system to track whether or not Zoo has been used in a specific value. Sure, you can check for it at runtime, but I'd be happier if the type system can guarantee its absence when required. I tried this: data Zoo x = Zoo x | NoZoo x data GeneralFoobar f = Foo f | Bar f type Foobar = GeneralFoobar Foobar type FoobarZ = GeneralFoobar (Zoo FoobarZ) but the type checker seems to not like it. (Plus I now have to wade through miles of NoZoo wrappers everywhere.) The other alternative is to just duplicate the entire data declaration, sans the Zoo constructor... but I'd obviously prefer not to have to do that. (E.g., if I ever need to alter the declaration, I now have two copies to keep in sync.) Suggestions?

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(fwd) Haskell logo fail
by Benjamin L.Russell 23 Jun '09

23 Jun '09

On Tue, 16 Jun 2009 14:06:42 -0700 (PDT), in comp.lang.haskell Robert Hicks <sigzero(a)gmail.com> wrote: >http://blog.plover.com/prog/haskell/logo.html >From the site referenced at the above-mentioned URL: >Tue, 16 Jun 2009 > >Haskell logo fail >The Haskell folks have chosen a new logo. > ><image of new Haskell logo> > ><image of Amtrak logo> > >ouch The two logos do look disturbingly similar.... -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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How to determine if a FilePath is a directory name or regular file?
by Colin Paul Adams 23 Jun '09

23 Jun '09

I've been hoogling like bad to try to determine if a function like this exists. getDirectoryContents returns sub-directories as well as file names. I want only the latter, so I'm looking for a suitable filter. -- Colin Adams Preston Lancashire

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Generate Fortran 77 parser from a yacc grammar using Happy
by Cetin Sert 23 Jun '09

23 Jun '09

I have stumbled upon the following F77 yacc grammar: http://yaxx.cvs.sourceforge.net/viewvc/yaxx/yaxx/fortran/fortran.y?revision… . How can I make a Fortran 77 parser out of this file using Happy<http://www.haskell.org/happy/> ? Why is there some C?/C++? code in that .y file?

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HaRe (the Haskell Refactorer) in action - short screencast
by Claus Reinke 23 Jun '09

23 Jun '09

I've heard that many Haskellers know HaRe only as a rumour. It has been many years since the original project finished, and HaRe hasn't been maintained for quite some time, so just pointing to the sources isn't quite the right answer. The sources are still available, and build with GHC 6.8.3 (I had to fix one lineending issue on windows, iirc, and copy one old bug fix that hadn't made it into the latest release), but there is currently noone with the time or funding for maintenance, fixing bugs, making releases, or ironing out practical issues. If anyone would provide funding, people to do the work could be found, but the effort would probably be better spent on reimplementing the ideas in a GHC / Cabal environment (instead of the Haskell'98 environment targetted by our "Refactoring Functional Programs" project). If you've got the funding, please get in touch - even a three month run could get something started at least!-) In principle, the project experiences and lessons learned are quite well documented at the project site http://www.cs.kent.ac.uk/projects/refactor-fp/ but that doesn't give anyone an idea of what working with HaRe was like. With the recent interest in screencasts, I thought I'd make a little demo, for archival purposes. Nothing fancy, using only features that were already present in HaRe 0.3 (end of 2004), and not all of those, on a tiny 2-module example (screenspace is a bit crowded to keep the text readable on YouTube). I hope it might give a rough idea of what the papers, slides and reports are talking about, for Haskellers who weren't around at the time: http://www.youtube.com/watch?v=4I7VZV7elnY For the old HaRe team, Claus ------------------- YouTube video description: HaRe - the Haskell Refactorer (a mini demo) [4:10] The Haskell Refactorer HaRe was developed in our EPSRC project "Refactoring Functional Programs" http://www.cs.kent.ac.uk/projects/refactor-fp/ Building on Programatica's Haskell-in-Haskell frontend and Strafunski's generic programming library, it supported module-aware refactorings over the full Haskell'98 language standard. Interfaces to the refactoring engine were provided for both Vim and Emacs (this demo uses HaRe via GVim on Windows). While HaRe has continued to see occasional contributions by students and researchers, who use its Haskell program transformation API as a platform for their own work, it is not currently maintained. As the Haskell environment marches on, this demo is meant to record a snapshot of what working with HaRe could be like when it still built (here with GHC 6.8.3). The lessons learned (note, eg, the preservation of comments, and the limited use of pretty-printing, to minimize layout changes) are well documented at the project site, and should be taken into account when porting the ideas to the GHC Api, or other Haskell frontends.

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GHCi infers a type but refuses it as type signature
by Eric 23 Jun '09

23 Jun '09

Hi haskell helpers, I am new to haskell (but enthusiast). I have begun to play with State and StateT, but this very simple exercice has led me to a strange situation where GHCi recognises and accepts the type of a function but GHC won't allow it as a type signature. Here is the example (which is also a try at literate haskell). > import Control.Monad.State > > type Play = Char > type Game = [Play] -- a game is a series of plays Now, you play by issuing a char, and you win if you have played an already played char. That was my first "play" function, returning True if you win. > play :: Play -> State Game Bool > play p = do ps <- get > put (p:ps) > return $ p `elem` ps I tested this one this way: > play_abc :: State Game Bool > play_abc = do play 'a' > play 'b' > play 'c' > > play_abca :: State Game Bool > play_abca = play_abc >> play 'a' Now in GHCi, "runState play_abc []" yields (False, "cba") "runState play_abca []" yields (True, "acba") Good! I was happy, now trying StateT to add IO to get console input: > type IOGame = StateT Game IO > > run_io :: IOGame Bool -> IO (Bool, Game) > run_io x = runStateT x [] Now I wanted a version of "play" that reads a char, on the following model: > play_once :: IOGame Bool > play_once = do x <- liftIO getChar > play' x Of course I first tried to express play' using play, and failed. Eventually I copied-and-pasted play, only changing the type signature, and it worked: > play' :: Play -> IOGame Bool > play' x = do xs <- get > put (x:xs) > return $ x `elem` xs Now, on GHCi I was happy to type: "run_io $ play_once" or even "run_io $ play_once >> play_once >> play_once" However, how to avoid the code duplication? I just tried to remove the type signature, and yes, play2 suddenly works in both State Game and StateT Game IO: > play2 x = do xs <- get > put (x:xs) > return $ x `elem` xs > > play2_abc :: State Game Bool > play2_abc = play2 'a' >> play2 'b' >> play2 'c' > > play2_once :: IOGame Bool > play2_once = do x <- liftIO getChar > play2 x So what's the type of play2? :t play2 yields: (MonadState [a] m, Eq a) => a -> m Bool Wow, nice, I get it now! However if I try to add that type signature, or even the second, more specific one below, ghci fails (signatures commented out because of that): > -- play3 :: (MonadState [a] m, Eq a) => a -> m Bool > -- play3 :: (MonadState Game m) => Play -> m Bool > play3 x = do xs <- get > put (x:xs) > return $ x `elem` xs It seems that an extension is required: Non type-variable argument in the constraint: MonadState [a] m (Use -XFlexibleContexts to permit this) In the type signature for `play3': play3 :: (MonadState [a] m, Eq a) => a -> m Bool So how is it possible that GHCi can infer (and use) a type that you cannot use as signature? And is it really non standard to avoid such code duplication? I was a bit surprised that such simple example should require a compiler extension. Thanks! Eric

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ANN: New release of ZeroTH
by Robin Green 23 Jun '09

23 Jun '09

This announcement will only be of interest to Haskell programmers using, or thinking of using, Template Haskell. I am very pleased to announce a new release (2009.6.23.3) of ZeroTH (also known as zeroth), a tool for preprocessing Haskell code to run splices and remove Template Haskell dependencies: http://hackage.haskell.org/package/zeroth For example, you could use it to reduce the size of your Haskell program binaries, if you use Template Haskell. (zeroth now actually uses itself on itself, for this very purpose, and you can use the Distribution.ZeroTH module for that in your own cabal projects.) It's still very much a hack - in that it would probably be more efficient and clean to implement it as an option to GHC - but hopefully now at least it's a slightly *better* hack. Lemmih has kindly agreed to hand over the job of being maintainer of ZeroTH to me. So please send any patches or complaints my way. The new darcs repository for the project, which is a darcs 2 repository, is at: http://patch-tag.com/publicrepos/zerothDarcs2 For this release, the "cabal" command-line tool is the only officially supported means of installation. So you should do something like: cabal update cabal install zeroth to install it. To get a summary of command-line options, type: zeroth --help Major changes from previous releases: * Much more Haskell code is now supported. This is partly because the code has been updated to work with the just-released haskell-src-exts 1.0.0; and partly because it supports a wider variety of Template Haskell code, including code which refers to definitions declared in the same source file. * It is now compulsory to specify any Haskell language extensions you are using in the input file, with a LANGUAGE pragma. (For debugging purposes, zeroth prints to stderr the result of parsing the LANGUAGE pragma, if any. If haskell-src-exts doesn't understand the LANGUAGE pragma, this will be Nothing. But that shouldn't happen.) * Template Haskell imports are now *automatically* stripped by default by zeroth, and so cpphs doesn't need to be run if you aren't otherwise using CPP. * New command-line options have been added (see zeroth --help). * Error messages are now more informative. * Librification! Almost anything can benefit from being made into a library, and zeroth is no exception. Of course, the executable is still there. * You can still use it on certain code which doesn't compile in GHC (yes, really, I've tested this!) but you now have to use the --only-splices option if you want to do this. Obviously, the splices themselves have to compile. --only-splices also makes zeroth faster. It is very lightly tested, so it might not work for your code out of the box. Please let me know if you have any problems with it, no matter how small. Happy hacking, -- Robin

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