July 2006 - Haskell-Cafe

Re: [Haskell-cafe] The difficulty of designing a sequence class
by David Menendez 31 Jul '06

31 Jul '06

Brian Hulley writes: > 1) Did Edison choose MonadPlus just because this fitted in with the > lack of multi-parameter typeclasses in H98? Instances of Monoid (and your ISeq) have kind *. Instances of MonadPlus (and Edison's Sequence) have kind * -> *. Functions like map, zip, and their variants are best defined in terms of type constructors. With Sequence, you have zipWith :: (Sequence s) => (a -> b -> c) -> s a -> s b -> s c With ISeq, you'd have to do something like zipWith :: (ISeq s1 a, ISeq s2 b, ISeq s3 c) => (a -> b -> c) -> s1 -> s2 -> s3 which isn't able to make any assumptions about s1, s2, and s3 having the same structure. > 3) Is it worth bothering to derive ISeq from Monoid (with the > possible extra inefficiency of the indirection through the > definitions for append = mappend etc or does the compiler completely > optimize this out)? I would expect the compiler to inline append. > 4) Would it be worth reconsidering the rules for top level names so > that class methods could always be local to their class (ditto for > value constructors and field names being local to their type > constructor). Qualified module imports are the way to go, here. Do you really want to start writing "if x Eq/== y Num/+ 1 then ... "? -- David Menendez <zednenem(a)psualum.com> | "In this house, we obey the laws <http://www.eyrie.org/~zednenem> | of thermodynamics!"

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What is the state if Unicode in Haskell implementations?
by Olof Bjarnason 31 Jul '06

31 Jul '06

Hi there! I'm trying to user Haskell as a code-generating language, specifically generating C# code files. The wish list is 1) reading UTF-8 coded text files into unicode-enabled Strings, lets call them UString 2) writing UStrings to UTF-8 coded text files 3) using unicode strings in-code, that is in my .hs files I can live without 3), and with a little good will also 2), but 1) is harder since I cannot really hope my input files (meta-data-files) are coded in anything else than UTF-8. I've searched&browser the web for information on the current state of unicode in GHC/Hugs but the latest discussion I could find on the topic leaves me less than happy. BUT it is from january 2005 so I thought maybe you guys have more up-to-date answers to these questions. The discussion I found: http://groups.google.se/group/fa.haskell/browse_thread/thread/ccf1c6f32dbea… Thanks! /Olof

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Apache HTTP Server And Haskell
by Kaveh Shahbazian 31 Jul '06

31 Jul '06

Is there a mod_haskell (like mod_perl and mod_python) for Apache HTTP server? Does anyone know about it?

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Fwd: [Haskell-cafe] Apache HTTP Server And Haskell
by Kaveh Shahbazian 31 Jul '06

31 Jul '06

Thanks, But it seems to be dead! (Last news on 18 Apr 2002). Is there a more mature one?

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The possibility of a real OS based on Haskell zippers
by Andy Elvey 30 Jul '06

30 Jul '06

Hi everyone - I'm very much a Haskell newbie, but have followed Haskell (and the various Haskell sites) for quite a few months now, and I absolutely *love* the language. One thing that **fascinated** me a while ago was the site which gave Haskell code for a "Haskell-based fileserver/OS" using zippers (the URI follows) - http://okmij.org/ftp/Computation/Continuations.html#zipper That code looks amazing! So, I was thinking - would it *actually be (theoretically) possible* to create a "real" booting OS (a bit like "House") from the above code? By a "real" OS, I mean something that uses (say) GRUB, and at least has a command-line of some kind. ( A GUI could come later ... ). I think that a "Plan 9-ish" OS (with all its "namespace/URL stuff, and its network-centric approach) would seem to be *particularly well suited* to being done using Haskell. ( I have to say that I love Plan 9's elegance, but I just *can't* manage its controls/GUI, they have me flummoxed ... ) If it were possible, I would *love* to see it - I've actually seriously considered even coughing up say $100 to $200 or so for a contest to create such a thing.... Anyway, "first things first" :-) - is such an OS possible? Many thanks for your time - bye for now - - Andy

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Serializing Functions and Actions for Distributed Programming
by Brian Sniffen 30 Jul '06

30 Jul '06

I'm very excited by the ability to pass functions or IO actions between threads of the same program. But I don't see any language or library support for doing so between programs, or between sessions with the same program. OCaml provides a partial solution: <http://caml.inria.fr/pub/docs/manual-ocaml/libref/Marshal.html> Though all it's really sending is an address and a hash of the binary program. Even SerTH doesn't help with functional types. I seek the knowledge of the Haskell Cafe: is there a reasonable way of addressing this problem? -- Brian T. Sniffen bts(a)alum.mit.edu or brian.sniffen(a)gmail.com http://www.evenmere.org/~bts

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Baffled by Disk IO
by Matthew Bromberg 30 Jul '06

30 Jul '06

I have run into a baffling distinction between the behavior of GHCi and the compiled binary from GHC. I suspect it's something pretty stupid on my part. I have the following test program <test.hs> import Matrix main = let f1 = bRgauss 4 3 f2 = bRgauss 3 2 fu = f1 *. f2 in bsave "fumat" fu ---------------------------------- The type signature of bsave is -- | save a matrix to a file bsave :: String -> m -> IO() bsave str z = do k <- matindx z withCString str (\x -> bmsave x k) and bmsave is a C routine that does a simple write to disk using fprintf. If I compile using GHC and run this as test.exe it does absolutely nothing, no file is actually saved. In fact no file is saved even if I try to print components of fu in the IO monad. Whereas if I load it into GHCi and run main, everything works as expected. Well sort of. Apparently everytime I extract something out of fu and use it in an IO monad it executes all the IO actions in fu, so it's easy to get unwanted behavior with incautious use.

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Cabal in ghc 6.4.2
by allan 30 Jul '06

30 Jul '06

Hello all, (before I start I should say I'm relatively new (a few months) to haskell, so I'm not exactly savvy with the use of the compilers) I have a problem using the cabal with version 6.4.2 of ghc, the same thing occurs with two projects which I wish to compile, I'll use the simplest one since the problem seems to be the same in both cases. The program is a very simple printer thing for haskell the source of which you can download at: http://homepages.inf.ed.ac.uk/s9810217/software/haskell_print/haskellprint.… <http://homepages.inf.ed.ac.uk/s9810217/software/haskell_print/haskell_print…> (but you probably won't need to, in order to diagnose this problem) I wish to compile this with the sequence of commands: ./Setup.hs configure --user --prefix=/home/allan/install ./Setup.hs build this works perfectly on version 6.4 of ghc, however with version 6.4.2 I get the following error: haskellprint$ ./Setup.hs build Preprocessing executables for haskellprint-0.1... Building haskellprint-0.1... Chasing modules from: Main.hs Could not find module `Text.ParserCombinators.Parsec': use -v to see a list of the files searched for (imported from ./Parser.hs) haskellprint$ Now if I run haskellprint$ ./Setup.hs build --verbose then I can see the command line used to compile the program which is: /home/allan/install/bin/ghc -Idist/build -o dist/build/haskell_print/haskell_print --make -Wall -hide-all-packages -i -idist/build/autogen -i. -odir dist/build/haskell_print/haskell_print-tmp -hidir dist/build/haskell_print/haskell_print-tmp -package base-1.0 Main.hs If I copy and paste this into the command line and remove the argument [ -hide-all-packages ] then the program compiles fine. So basically my question is: what am I doing wrong? I assume my problem is in the .cabal file, so I include the contents here: name: haskellprint version: 0.1 license: GPL license-file: LICENSE author: Allan Clark maintainer: a.d.clark(a)ed.ac.uk build-depends: base executable: haskell_print main-is: Main.hs include-dirs: c-sources: other-modules: extra-libraries: extensions: ghc-options: -Wall -- ghc-options: -fwarn-incomplete-patterns -fwarn-missing-fields -- ghc-options: -prof -auto-all -Wall -fglasgow-exts -O -funbox-strict-fields -fasm -optl-Wl,-s thanks in advance for any help. allan

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Is id strict?
by David House 30 Jul '06

30 Jul '06

Hi all. I've seen two definitions of a 'strict function', which I'm trying to unite in my mind: 1) f is strict iff f _|_ = _|_. 2) f is strict iff it forces evaluation of its arguments. There is a large sticking point that in my minds seems to fit (1) but not (2): id. Clearly, id undefined is undefined, but I also don't think id forces evaluation of its argument. There was a mini-discussion concerning this topic last night on #haskell, but if there was a consensus conclusion, it passed me by. Thanks in advance. -- -David House, dmhouse(a)gmail.com

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Sending Ctrl-C under Windows
by Neil Mitchell 29 Jul '06

29 Jul '06

Hi, I want to send Ctrl-C to a process opened by runInteractiveCommand, under Windows. I have found System.Posix.Signals, but on Windows none of those methods exist. For further information, the processes I want to send Ctrl-C to are ghci and hugs, so doing something like terminateProcess isn't what i want, because I don't want the process to be terminated, just the current computation. If there is another way to programmatically terminate the computation started by hugs/ghci (but not the process) via runInteractiveCommand, that would be great too. Thanks Neil

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