February 2006 - Haskell-Cafe

question about type lambda and decidability of typechecking
by Robert Dockins 10 Feb '06

10 Feb '06

OK. I've been doing a little thinking about type lambda in Haskell. Now, I understand the prevailing wisdom is that adding type lambda and/or partially applied type synonyms to the haskell type system would make type checking/inference undecidable. The reason given is that higher-order unification is undecidable. I have to admit that I don't fully understand this reason. Setting aside typeclasses for now, it seems to me that type expressions together with the kind system are just the simply-typed lambda calculus with unit, which is well known to be strong normalizing. So any type with kind * has a normal form with (by definition) no internal redexes. I think this is sufficient to guarantee that all type lambdas are removed. Now you can proceed using first-order unification, which is decidable. Of course, all valid expressions have kind * (ignoring unboxing and other trickiness for now). So where have I gone wrong? Do typeclasses complicate the matter? Or have I missed something more basic? Thanks, Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG

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Numerical methods in Haskell
by Rob Tougher 10 Feb '06

10 Feb '06

I'm currently working on a mathematics library in Haskell. I'm not a mathematician -- I'm basically writing this library as an excuse to learn mathematics and Haskell at the same time. Feel free to check out the code: http://robtougher.com/ My main focus is financial mathematics (options pricing, etc), but I took a detour last week and implemented basic matrix stuff. I might work on some Linear Programming algorithms (simplex, interior point) before returning to finance. - Rob

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Matching constructors
by Creighton Hogg 10 Feb '06

10 Feb '06

Hi, If I have something like data Patootie = Pa Int | Tootie Int and I want to pull out the indices of all elements of a list that have type constructor Tootie, how would I do that? I thought I might be able to use findIndices, but I don't know how to express the predicate.

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multiple defentions of .. fst defined here errors.
by Marc Weber 10 Feb '06

10 Feb '06

Why do these error occur? After hiding the packages com (beeing in comlib) and hdirect (beeing in lib) and adding -package com -package hdirect it works fine. I haven't tried without hiding but adding -package options yet. ghc --make -H16m -O -fglasgow-exts -syslib com -fno-warn-missing-methods daotest.hs Chasing modules from: daotest.hs Skipping DAO ( ./DAO.hs, ./DAO.o ) Skipping DB ( ./DB.hs, ./DB.o ) Skipping Main ( daotest.hs, daotest.o ) Linking ... c:\haskell\myfptoolsdirectory\fptools\hdirect\comlib/libHScom.a(PointerPrim.o)(.text+0x696):ghc2648.hc: multiple definition of `__stginit_PointerPrim_' c:\haskell\myfptoolsdirectory\fptools\hdirect\lib/libHShdirect.a(PointerPrim__12.o)(.text+0x0):ghc2036.hc: first defined here c:\haskell\myfptoolsdirectory\fptools\hdirect\comlib/libHScom.a(PointerPrim.o)(.text+0x6da):ghc2648.hc: multiple definition of `__stginit_PointerPrim' c:\haskell\myfptoolsdirectory\fptools\hdirect\lib/libHShdirect.a(PointerPrim__12.o)(.text+0x44):ghc2036.hc: first defined here Marc

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Combinatory logic interpreter?
by Colin Paul Adams 10 Feb '06

10 Feb '06

Is there any Haskell code around that can interpret combinatory logic expressions? -- Colin Adams Preston Lancashire

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Language shootout (reloaded)
by dons＠cse.unsw.edu.au 10 Feb '06

10 Feb '06

Just so we can feel that we're doing the right things :) On the great language shootout, as of last night, we're: * Ranked overall number 1, by a good margin: * Ranked number 1 on lines of code * Ranked number 2 on speed. http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=all Thanks to everyone who's contributed to the entries over the last few weeks, on the wiki, and on this list. Cheers, Don P.S. I remember having a discussion on #haskell 2 weeks ago where we all agreed that Haskell placing #1 was pretty much impossible. Did we have an inferiority complex?

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RE: [Haskell-cafe] Language shootout (reloaded)
by Simon Peyton-Jones 10 Feb '06

10 Feb '06

Congratulations, guys! An amazing, and (maybe it should have been!) to me surprising, achievement. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Donald Bruce Stewart | Sent: 10 February 2006 01:35 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Language shootout (reloaded) | | Just so we can feel that we're doing the right things :) | | On the great language shootout, as of last night, we're: | | * Ranked overall number 1, by a good margin: | * Ranked number 1 on lines of code | * Ranked number 2 on speed. | | http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=all | | Thanks to everyone who's contributed to the entries over the last few | weeks, on the wiki, and on this list. | | Cheers, | Don | | P.S. I remember having a discussion on #haskell 2 weeks ago where we all | agreed that Haskell placing #1 was pretty much impossible. Did we have | an inferiority complex? | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: Betreft: Re: [Haskell-cafe] Elementary HaXml question
by Malcolm Wallace 10 Feb '06

10 Feb '06

Koen.Roelandt(a)mineco.fgov.be writes: > foldXml (txt ?> keep :> > fontSize4 /> tag "em" ?> mkSubtitle :> > children) > now transforms > <font size="4"><em>Hello World</em></font> > into > <h1 class="subtitle"><em><Hello World</em></font> > > I still have a problem with the other example: > <em><font size="4">Hello World</font></em> Ah, I see. In the first case, you simply change the <font> tag into <h1>, keeping the <em> tag exactly the same. But in the second case, you not only want to change <font> into <h1>, but also to move the <em> tag from outside the <font> to inside. I think you need something like this: mkSubtitle `o` (tag "em" /> fontSize4 /> mkElem "em") |>| children i.e. build an <em> around the children of the <font> tag, and then build the <h1> subtitle around the outside of that. Regards, Malcolm

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Numerical methods in Haskell
by Creighton Hogg 10 Feb '06

10 Feb '06

Hi, Between google searching and looking through the activity report, I take it that no one has really developed serious libraries for matrix manipulations, diff eqs, etc. Are there any practical reasons for this or is it just a matter of the haskell community being small and there not being many people interested in something so specialized?

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FFI Wiki Page (was: FFI question)
by Dimitry Golubovsky 09 Feb '06

09 Feb '06

Bulat wrote: > we need to establish FFI > page on the wiki and give at least links to all this packages and > small info about using FFI with ghc/hugs There is such a page (to be more precise, a section of a page): http://haskell.org/hawiki/LibrariesAndTools section "FFI Preprocessors" Also there are http://haskell.org/hawiki/FfiCookbook and http://haskell.org/hawiki/FfiTutorial pages. -- Dimitry Golubovsky Anywhere on the Web

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