December 2007 - Haskell-Cafe

Creating a type for a subset of the integers
by Brad Larsen 20 Dec '07

20 Dec '07

Hi there list, How would one go about creating a new type for a subset of the integers, for (contrived) example just the even integers? I was thinking of making a new type newtype EvenInt = EvenInt Integer but the problem with this is that it accepts any integer, even odd ones. So to prevent this, the module does not export the constructor for it---rather, the module exports a function `mkEvenInt' that creates an EvenInt if the given value is acceptable or raises an error otherwise. What's the right way to do this? Thanks! Brad Larsen

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Re: [Haskell-cafe] [HXT] Simple question
by Miguel Mitrofanov 20 Dec '07

20 Dec '07

> I can't really reproduce this: > A simple ghci session gives the following: > ----------------------------------- > uwe@si:~/haskell/hxt/curr/examples/arrows/HelloWorld> ghci HelloWorld.hs > GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help > Loading package base ... linking ... done. > [1 of 1] Compiling Main ( HelloWorld.hs, interpreted ) > Ok, modules loaded: Main. > *Main> runX $ (readString [(a_validate,v_0)] "<x> </x>" >>> writeDocumentToString []) > Loading ... > ... > ["<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<x> </x>"] Try "<x><y> </y></x>" and a_indent writing option.

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#haskell works
by Andrew Coppin 20 Dec '07

20 Dec '07

Many people have written words to the effect that the #haskell IRC channel is just bursting with helpful Haskellers who are endlessly friendly and seemingly able to help solve any problem. Personally, this has never been my experience. (More like there's 300 people idling and nobody ever actually speaks...) I am pleased to report that last night I was on #haskell and I did in fact get lots of useful help, from a number of people. So first of all, thanks for that guys! It all relates to this simple program: http://hpaste.org/4438 I pointed out that you can write a complex explicit loop in Java, and that you can translate the same thing into Haskell, and it works. But in Haskell, you can also do it by composing a couple of functions, and it's much easier to read. Somebody else countered that while this is all very cute, it can never be efficient. To which I obviously countered that stream fusion *makes* it efficient. The numbers generated are thus: Program with no particular optimisations: 0.35 seconds. Program with stream fusion [and GHC HEAD]: 0.25 seconds. Program with stream fusion and ByteString: 0.05 seconds. Surely you'd have to work pretty hard to get that kind of speed even in C. ;-) ...erm, actually no. Somebody sat down and wrote something in five minutes that takes 0.005 seconds. Oops! So it seems even with ByteStrings and stream fusion, we're still 10x slower than naive C. :-( You can console yourself that maybe 5 milliseconds is too short a time span to reliably measure, and maybe the speed difference is just down to the OS caching the file in RAM or something. Maybe a benchmark of something that takes tens of seconds would be better. All I know is that once again, it seems Haskell isn't as fast as I thought... *sigh* Well anyway, a few years ago we didn't have fusion, and we didn't have ByteString. A few years ago, the program would have taken 0.35 seconds. The end. Today, with a few import statements and compiler switches, the exact same code takes 0.05 seconds. Tomorrow, who knows? Maybe I'm being overly optimistic, but... ;-)

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Storable types
by Clerton Filho 20 Dec '07

20 Dec '07

Hi, I'm newbie in Haskell, and I have some doubts... In this programming language, do we have storable values? Case affirmative, what are the storable types in Haskell, and how can I implement then... thanks! -- Clerton Ribeiro de Araujo Filho Graduando em Ciência da Computação Integrante do PET - Programa de Educação Tutorial Integrante do LIA - Laboratório de Inteligência Artificial Universidade Federal de Campina Grande - UFCG

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Re: [Haskell-cafe] [HXT] Simple question
by Miguel Mitrofanov 20 Dec '07

20 Dec '07

Seems rather strange for me, I've just installed HXT and got this: Prelude Text.XML.HXT.Arrow> runX $ readString [(a_validate,"0")] "<this> </this>" >>> writeDocumentToString [] ["<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<this> </this>"] > Everything works fine except for the fact that all the nodes <this> > </this> > (that is, a space (an XML text node whose contents are a single space > character) > within a <this> element node) get transformed to a <this/> element node,

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[HXT] Simple question
by Fernand 20 Dec '07

20 Dec '07

Hi, I'm using HXT (7.4) with GHC to make some XML transformations (BTW, congratulations to the maintainers of that package for their impressive work). Everything works fine except for the fact that all the nodes « <this> </this> » (that is, a space (an XML text node whose contents are a single space character) within a <this> element node) get transformed to a « <this/> » element node, which I do not want (I want to keep those nodes as they are). Note that my transformations do not change the <this> nodes or their children (my transformations works on others parts of the document). This seems to be related to the XML parsing or writing of the XML file, and I have tried several read/write options (using a_remove_whitespace and a_canonicalize with v_0, for both writing and reading) to prevent this behaviour, without success. I have even tried to add text childs nodes to all the empty <this> nodes, but the issue seems to lie in the XML output options. Has anybody an idea ? Excuse me if this is a trivial issue, Sincerely yours Fernand -- Excuse my french, it's my mother tongue.

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Re: type trickery
by oleg＠okmij.org 20 Dec '07

20 Dec '07

Adrian Neumann wrote: > I figured I'd need something like this > data GF = GF Integer Integer > so that each element of the finite field would remember p. However I > can't think of a way to use the typesystem to ensure that p is always > the same. You might like: Vectro: Haskell library for "statically typed linear algebra" http://ofb.net/~frederik/stla/ which marks each element of a vector space with its dimension. The type system makes sure that you can only add vectors of the same dimension (the type system does even more: it computes the dimension of a result of multiplying a vector by a non-square matrix, for example). > I think that would need an infinite number of different types, You think correctly. Haskell already has the infinite number of different types (e.g., the infinite number of function types).

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Class deriving in GHC 6.8
by Emil Axelsson 20 Dec '07

20 Dec '07

Hello all! How come in GHC 6.6 I could to write > {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} > data Foo = Foo deriving Show > data Bar c = Bar (c Foo) deriving Show but in GHC 6.8.2 I get the error > No instance for (Show (c Foo)) > arising from the 'deriving' clause of a data type declaration > at Ert.hs:3:0-37 > Possible fix: add an instance declaration for (Show (c Foo)) > When deriving the instance for (Show (Bar c)) Thanks, / Emil

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type trickery
by Adrian Neumann 20 Dec '07

20 Dec '07

Hello haskell-cafe! After making "data Number = Zero | Succ Number" an instance of Integral, I wondered how I could do the same with galois fields. So starting with Z mod p, I figured I'd need something like this data GF = GF Integer Integer so that each element of the finite field would remember p. However I can't think of a way to use the typesystem to ensure that p is always the same. I think that would need an infinite number of different types, but the type hackers here probably know something better. Adrian

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Re: [xmonad] Re: XMonad.Layout.NoBorders
by gwern0＠gmail.com 19 Dec '07

19 Dec '07

On 2007.12.19 10:57:33 -0500, David Roundy <daveroundy(a)gmail.com> scribbled 0.3K characters: > No, I don't have time. And I'm not sure why one would want no borders > on floating windows... > > David Perhaps I don't fully understand the issues, but wouldn't such a thing be nice for things like MPlayer? The other day, I happened to play a video through MPlayer, and I noticed (using smartBorders/NoBorders) that there was an ugly border on the left and upper edges, and that it detracted from the cinematic experience. It was otherwise fullscreen and nice, but not as good as it could've been. -- gwern picking FID DES Mole Cap-Stun timers Z7 NAAP 2.6.2. ARC

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