September 2007 - Haskell-Cafe

Re: [Haskell] Records
by Justin Bailey 17 Sep '07

17 Sep '07

On 9/16/07, Barney Hilken <b.hilken(a)ntlworld.com> wrote: > Now that I have a version of ghc with type classes, I have had a go > at implementing records based on the ideas I mentioned on this list a > few months ago. The code of my first attempt is available at http:// > homepage.ntlworld.com/b.hilken/files/Records.hs That's pretty interesting stuff, but it's hard to see how ti works without some examples. Would you want to include some in your comments? I do appreciate the thorough documentation otherwise, though. Justin p.s. replied via haskell-cafe.

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Cross-compiling of GHC
by L.Guo 17 Sep '07

17 Sep '07

Hi all: How to build a GHC which can run in a embed linux system ? I have toolchain for the targer system. Using which, I can compile on PC and run the program on the embed system. Is this means, if I change CC env-var to the toolchain compiler and compile GHC manually, I can get a program for the embed system ? I have read Building/Porting for GHC[1], that does not mention anything about how to build a GHC on a target machine using a cross-compiler like xxx-linux-gcc. *or* What do I need to do besides the stages that page mentioned ? Regards [1] http://hackage.haskell.org/trac/ghc/wiki/Building/Porting -------------- L.Guo 2007-09-17

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Re: [Haskell] Linear Equation Solver Using Arrays
by Andrew Wagner 17 Sep '07

17 Sep '07

(Replying on haskell-cafe) Let me start with a disclaimer: I haven't looked at your code extensively. That said, the feeling that I get from it is one of listening to a non-native speaker. The things in your program are obviously not completely inaccurate, or they wouldn't have type-checked. And, presumably, you've already done some testing, so it probably produces the right answers. Similarly, there are many things in English that you _could_ say, which would be entirely grammatically correct, but which native speakers would never say. For example, in your code, I noticed a pattern which I've just recently learned: <code> do ... tmp <- readArray scaleInfo j writeArray scaleInfo imax tmp </code> Now, if you were to de-sugar the last 2 lines, it would look something like this: <code> readArray scaleInfo j >>= \tmp -> writeArray scaleInfo imax tmp </code> ...but as soon as you see that lambda expression, you should (hopefully) say "oh, that can be written more easily: <code> readArray scaleInfo j >>= writeArray scaleInfo imax </code> Of course, a native speaker will probably not even think about the intermediate lambda expression, but the point is, this is an idiom. And, really, this is a very tiny example. Your entire program is written in a very imperative style. "Do this, then do that, then do the other thing." And this is to be expected. After all, you said that you were directly porting the algorithm from an imperative language. However, if you're going to do this idiomatically, you really need to think very differently. I suggest reading and pondering this well-known blog entry for more ideas about the differences between the approaches of functional and imperative programming: http://blogs.nubgames.com/code/?p=22 Sorry, I don't have more specific suggestions, but I suspect this is what most people thought when they read your code: it's hard to make specific suggestions, when you're really approaching the problem in a non-idiomatic way. On 9/16/07, Xiao-Yong Jin <xj2106(a)columbia.edu> wrote: > Dear Haskellers, > > My thanks to people on the irc channel, especially `int-e'. > With their help, I managed to write a linear equation solver > using STUArrays. > > > It is basically a direct rewrite of a C version, which > adopts Crout's LU decomposition method with implicit > pivoting, in chapter 2.3 of the book Numerical Recipes. > > I must admit that the code is in a really bad style, > somewhat ugly, because of the limit of my Haskell knowledge. > The code is attached in this email for your amusement. I > would be really thankful if you can give me any kind of > comments, idea, improvement, criticism, because I sincerely > want to make the code better. > > Best regards, > Xiao-Yong > -- > c/* __o/* > <\ * (__ > */\ < > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > > >

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Parsec Scanner Question
by Jude 16 Sep '07

16 Sep '07

Haskell Folks, I have an existing Parsec CharParser parser that I would like to extend to include line continuation support. When there's a backslash-newline combination anywhere in the token stream, I'd like to remove it so it's not read by the rest of the parser, even if its in the middle of a keyword. Currently I'm doing this with a separate "preprocess" function that weeds out all the '\\\n' strings from the input before passing it to the parser. The problem with this is it messes up the line position when there's an error below the '\\\n' point, since the newline is never seen by the parser. So I've been trying to figure out how to handle this better with Parsec. I think I need something that acts just like CharParser, except it skips over the backslash-newline tokens, It looks like section 2.11 in the Parsec manual (Advanced: separate scanners") covers this, but I'm not having much luck turning it into working code. Could someone who's written their own scanner spell out how it's done in a little more detail ? Thanks ! - jude

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Data.Binary Endianness
by Adrian Neumann 16 Sep '07

16 Sep '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 I try to write a Windows Bitmap File using Data.Binary, but I have some trouble. For example, the internet states, that the magic number, that puts 'BM' in the first two bytes of the file is 19778. But when I > put (19778::Word16) I get 'MB' instead. I read on the german Wikipedia, that bmp uses little endian encoding, but Data.Binary uses big endian. Obviously this can't work. Is there a way to switch endianness? Or at least a function that converts between the two? I looked at the Data.Binary Haddock documentation, but found nothing like that. Thanks in advance, Adrian -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFG5Q1Z11V8mqIQMRsRA/V6AJoC/R2k5rz0LjQI4iGoi79zLYZmgwCdEx/g cZOR5jseUcQnc6+I09N+xj8= =K5ww -----END PGP SIGNATURE-----

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unknown RTS option: -N2
by Gregory Propf 16 Sep '07

16 Sep '07

I've built a program with the -threaded option using ghc. This option is supposed to link your program to the threaded runtime with support for multicore CPUS (mine is a dual core). The program pukes with the message in the subject line when I try to use the -N option to tell it to use both CPUs. I'm reasonably sure that I actually have the threaded runtime because initially I got a runtime message from the program about how GTK is single threaded and can only be used with the threaded runtime using runtimeunsafeInitGUIForThreadedRTS instead of the standard initGUI. I made the change in the code and now the program runs but still does not know the -N RTS option. This problem seems to affect all my programs. It is as if the threaded runtime is there but doesn't know any of its special flags. This is the line I'm compiling the program with. It's a life game clone in case you're curious. Compiler is ghc-6.4.2 on 64 bit AMD dual core running Gentoo Linux. - Greg ghc -threaded -package gtk -package glade -o hlife hlgtk.o Hlife.o ____________________________________________________________________________________ Check out the hottest 2008 models today at Yahoo! Autos. http://autos.yahoo.com/new_cars.html

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Re: [Haskell] question about a failure to generalize
by Tim Chevalier 16 Sep '07

16 Sep '07

[replying to haskell-cafe] On 9/15/07, Norman Ramsey <nr(a)eecs.harvard.edu> wrote: > Dear Haskellers, > > I've had a Haskell program rejected for reasons I don't understand. > Here's the relevant bit of code; the problem is that I expected the > type-inference engine to generalize the abbreviation 'fold' to an > overloaded function, but it doesn't---to make the code work, I had to > expand 'fold' into 'foldRegsUsed' everywhere it appears. I'm baffled > because 'fold' isn't mutually recursive with anything, so I would have > expected ordinary Hindley-Milner style inference to generalize it to > something of the type > > UserOfLocalRegs a => (b -> LocalReg -> b) -> b -> a -> b > > But that's not what happens. This code fails to compile because the > compiler is willing to use 'fold' at only one type (CmmExpr as it happens): > > class UserOfLocalRegs a where > foldRegsUsed :: (b -> LocalReg -> b) -> b -> a -> b > > instance UserOfLocalRegs Middle where > foldRegsUsed f z m = middle m > where middle (MidComment {}) = z > middle (MidAssign _lhs expr) = fold f z expr > middle (MidStore addr rval) = fold f (fold f z addr) rval > middle (MidUnsafeCall tgt _ress args) = fold f (fold f z tgt) args > middle (CopyIn _ _formals _) = z > middle (CopyOut _ actuals) = fold f z actuals > fold = foldRegsUsed > > > What rule of the language have I overlooked? > Monomorphism restriction? Replacing fold with foldRegsUsed would work because there's a type signature for foldRegsUsed. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "...There is no mystery; there is only paradox, the incontrovertible union of contradictory truths." -- Edward Abbey

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Sequencing Operations in a Monad
by Dominic Steinitz 15 Sep '07

15 Sep '07

> If you arrange the types to try to do all the operations inside the IO > monad you can't chain together more than 1 binary operation. eg. > > do > S <- A + B > Z <- Q * S > > vs > > do > S <- Q * (A + B) > > Are there any suggestions for this dilemma? Am I using the wrong monad for > this task? I'm not sure if this is what you are asking but isn't liftM2 or some variant what you need? Dominic.

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haskell on llvm?
by brad clawsie 15 Sep '07

15 Sep '07

has anyone ever considered using llvm as a infrastructure for haskell compilation? it wold seem people are looking at building frontends for scheme, ocaml, etc. i don't know if an alternate backend is appropriate, but it would seem to be an interesting way to aggregate the best thinking for various optimizations over a more diverse group of developers.

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Is "take" behaving correctly?
by PR Stanley 15 Sep '07

15 Sep '07

Hi take 1000 [1..3] still yields [1,2,3] I thought it was supposed to return an error. Any ideas? Thanks, Paul

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