March 2007 - Haskell-Cafe

Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"?
by Andrzej Jaworski 26 Mar '07

26 Mar '07

Daniel Fischer has cared to inform me that: >Diversity is generated by mutations. With due respect, but this is hardly a revelation. My point was that you need two competing components in relative balance to grow something meaningful. Cancer growth is based solely on mutation! Also I was not theological. It is the advice to multiply Prelude and use time to verify them rather cosmic in scale. It implies the assumption that the world will freeze and wait for the verdict.

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Newbie: a parser for a list of objects?
by Dmitri O.Kondratiev 26 Mar '07

26 Mar '07

Please see my questions inside comments {-- --} : Thanks! --- module Parser where import Data.Char type Parse a b = [a] -> [(b, [a])] {-- Newbie: a parser for a list of objects? I am working with the section 17.5 "Case study: parsing expressions" of the book "Haskell The Craft of Functional Programming", where a parser for a list of objects is defined. I called this function pList in order to avoid confusion with 'list' as a term for data structure. Please help me to understand how pList works (please, see the rest of the code at the end of this message): --} pList :: Parse a b -> Parse a [b] pList p = (succeed []) `alt` ((p >*> pList p) `build` (uncurry (:))) {-- First of all, I don't quite understand why there must be a choice ('alt') between the function ('succeed') that always returns an empty list and the other part? This results in adding [] to the front, why? I thought that 'simplified' version of pList should still work fine. Trying to prove this I wrote : --} pL1 :: Parse a b -> Parse a [b] pL1 p = (p >*> pL1 p) `build` (uncurry (:)) {-- Which, as expected, does not work correctly - just gives an empty list [] - but I don't understand why: *Parser> t1 "12345" [] *Parser> Also, I don't understand why the textbook version of pList gives this result: *Parser> test "12345" [("","12345"),("1","2345"),("12","345"),("123","45"),("1234","5"),("12345","")] *Parser> In particular, I don't understand where the first element ("","12345") of the resulting list comes from? I am trying to figure out how pList recursively unfolds. To my mind operators in the expression: (succeed []) `alt`((p >*> pList p) `build` (uncurry (:))) has the following execution order: 1) >*> 2) 'build' 3) 'alt' It seems that operation >*> should be done as many times as many elements the input list has. Right? Signature: (>*>) :: Parse a b -> Parse a c -> Parse a (b, c) implies that second argument of the expression: p >*> pList p should be of type 'Parse a c' but in this application it is of type 'Parse a b -> Parse a [b]' How can that be? How recursion termination conditinon is expressed in pList? --} none :: Parse a b none inp = [] succeed :: b -> Parse a b succeed val inp = [(val, inp)] suc:: b -> [a] -> [(b, [a])] suc val inp = [(val, inp)] spot :: (a -> Bool) -> Parse a a spot p [] = [] spot p (x:xs) | p x = [(x, xs)] | otherwise = [] alt :: Parse a b -> Parse a b -> Parse a b alt p1 p2 inp = p1 inp ++ p2 inp bracket = spot (=='(') dash = spot (== '-') dig = spot isDigit alpha = spot isAlpha infixr 5 >*> (>*>) :: Parse a b -> Parse a c -> Parse a (b, c) (>*>) p1 p2 inp = [((x,y), rem2) |(x, rem1) <- p1 inp, (y, rem2) <- p2 rem1] build :: Parse a b -> (b -> c) -> Parse a c build p f inp = [ (f x, rem) | (x, rem) <- p inp] test = pList dig t1 = pL1 dig

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What ever happened to Haskell 98 as a "stable branch"?
by Stefan O'Rear 26 Mar '07

26 Mar '07

Upon more reflection... >From the Preface to the Haskell 98 Language and Libraries Report: > Haskell 98 was conceived as a relatively minor tidy-up of Haskell > 1.4, making some simplifications, and removing some pitfalls for the > unwary. It is intended to be a "stable" language in sense the > implementors are committed to supporting Haskell 98 exactly as > specified, for the foreseeable future. In other words, we already have a stable branch of Haskell. Why, then, are we so paranoid about introducing breaking changes in the development branch of haskell? Why are we stuck without the class system extension proposal? Why is Num so still so horribly mangled? Why can I not 'map' over a Set? Why must I use lists of characters if I desire standard sorting? Why can I not get a good error message from read? (If you answer "because H98 is obsolete", then file this away as a must-read after H' is released.) Stefan

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A question about functional dependencies and existential quantification
by Jean-Marie Gaillourdet 26 Mar '07

26 Mar '07

Hi, I am trying to do something like the following: > {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} > module TestCase where > > data Any root = forall pos sel . T root pos sel => ANY pos > > class T root pos sel | pos -> root, root -> sel where > f :: pos -> sel -> Bool > > instance T root (Any root) sel where > f (ANY p) s = f p s There is a multi-parameter type class T with some functional dependencies. And I want do define an almost general type for T, Any root. I would like to have that type in T as well. But ghc is not able to type check f. It gives: > TestCase.hs:10:7: > Couldn't match expected type `sel1' (a rigid variable) > against inferred type `sel' (a rigid variable) > `sel1' is bound by the pattern for `ANY' at TestCase.hs:10:7-11 > `sel' is bound by the instance declaration at TestCase.hs:9:0 > When using functional dependencies to combine > T root pos sel, arising from use of `f' at TestCase.hs:10:18-22 > T root pos sel1, > arising from is bound by the pattern for `ANY' > at TestCase.hs:10:7-11 > at TestCase.hs:10:7-11 > In the pattern: ANY p > In the definition of `f': f (ANY p) s = f p s I think sel1 and sel are equal, because the root in the type of "(ANY p)" on the left side is the same as the root of the type of "f p s" on the right side. From that should follow with help of the functional dependency that sel1 and sel are the same types. Do I misunderstand the type system or is that a weakness of GHC? Or am I trying to do something silly? I have a solution that involves requiring sel to be in Typeable and using cast and I never came across a runtime error, so far. Is there a better way to express that? Any enlightenment is appreciated. Regards, Jean-Marie

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Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"?
by Daniel Fischer 26 Mar '07

26 Mar '07

> -----Ursprüngliche Nachricht----- > Von: "Andrzej Jaworski" <himself(a)poczta.nom.pl> > Gesendet: 26.03.07 15:00:47 > An: "Simon Peyton-Jones" <simonpj(a)microsoft.com> > CC: Haskell-Cafe(a)haskell.org > Betreff: Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"? > >Haskell is rather a Darwinian sort of place. > > With whole respect. You need two components for evolution to work: the > survival of the fitness and Generator Of Diversity (GOD). Diversity is generated by mutations. Go ahead, be mutagenous. And don't worry about theology, Simon's not GOD's prophet, he's (both of them, actually) just a doctor venerabilis. > > Now, Haskell attracts originality and easily accommodates changes but nobody > burns tires in testing anything so that complexity and learning curve grow > while deficiencies remain dormant. > > Recent threads are a kind of healthy evolutionary pressure (survival of the > fitness), but you insist that Haskell should be committed to GOD;-) > > With great respect, > -Andrzej > With TIC, Daniel

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AI Strike Force!
by Andrew Wagner 26 Mar '07

26 Mar '07

The time has come! Calling all Haskell programmers interested in AI! I've established a new home base at http://www.haskell.org/haskellwiki/AI . Functional programming has long been recognized as an excellent paradigm for Artificial Intelligence. It's time to make Haskell an important language in that area! The goal is to provide a toolkit of AI tools in Haskell that can easily be applied to a wide variety of applications: e.g., Neural Networks, genetic algorithms, natural language processing tools, etc.

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Link error in ALUT "Hello, World"
by Nobuhito Mori 25 Mar '07

25 Mar '07

Hi, I installed OpenAL and ALUT bindings downloaded from http://hackage.haskell.org/packages/archive/pkg-list.html and tried compilation of "HelloWorld.hs" example which I got from http://darcs.haskell.org/packages/ALUT/ But it returns errors as follow and I can not get an '.exe' file. Though there are clearly "link errors", I can not understand why it happens. By option "-package ALUT", libalut.a (which made by pexports and dlltool because I do not know original "alut.lib" can be used by mingw) and other necessary libraries are automatically linked, I think. What is "@8" of "alutInit@8"? How can I avoid these errors? My environment is Windows XP SP2, GHC6.6, MinGW/MSYS (so some of mingw tools are duplicated. one is GHC's, the other is MinGW's) --------------- >ghc -package ALUT HelloWorld.hs C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x18): undefined reference to `alutInit@8' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xdc): undefined reference to `alutInitWithoutContext@8' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x19d): undefined reference to `alutExit@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x1d9): undefined reference to `alutGetError@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x210): undefined reference to `alutCreateBufferFromFile@4' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x2b0): undefined reference to `alutCreateBufferFromFileImage@8' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x36d): undefined reference to `alutCreateBufferHelloWorld@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x3f7): undefined reference to `alutCreateBufferWaveform@16' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x584): undefined reference to `alutLoadMemoryFromFile@16' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x6b8): undefined reference to `alutLoadMemoryFromFileImage@20' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x7e8): undefined reference to `alutLoadMemoryHelloWorld@12' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x92f): undefined reference to `alutLoadMemoryWaveform@28' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xb2c): undefined reference to `alutGetMIMETypes@4' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xbc9): undefined reference to `alutGetMajorVersion@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xc01): undefined reference to `alutGetMinorVersion@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xc55): undefined reference to `alutSleep@4' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xd89): undefined reference to `alutGetErrorString@4' --------------------- -------------------- Nobuhito _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

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Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stable branch"?
by Andrzej Jaworski 25 Mar '07

25 Mar '07

>If you answer "because H98 is obsolete", then file this away as a >must-read after H' is released Ideas always originate in a single mind. Good ideas are only footnotes to the best idea that determine them. Now: a team of people with different views on the same thing can achieve their best decorating a Christmas Tree. Unless, it is Haskell;-) Regards, -Andrzej

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newbie "concatenating" monad question
by Leandro Penz 25 Mar '07

25 Mar '07

Hi I'm new here and this is my first mail to the list, so be gentle :) I have some functions that build big strings by calling other functions and appending the result, like buildStuff = func1 ++ func2 ++ func3 ++ func4 My idea is to have a monad with a concatenating >>, so that I can: bulidStuff = do func1 func2 func3 func4 I am thinking about using the writer monad, but I would still have to use "tell". The list monad is not an option either, as it does not concatenate. Is there a proper way of doing something like this (efficiently) ? Leandro Penz

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Application of iterate
by Logesh Pillay 24 Mar '07

24 Mar '07

Ordinarily I would not post to a list when I've got a program running. But I've found learning Haskell so tough (this is my 4th try in the last two years) that I feel I have to bore everyone with my first non-trivial program. It is Newton's method adapted to give the square root of a number. And all in just one line which I think is an amazing demonstration of the power of Haskell. And it took no more than a couple of minutes to write (directly to the screen too as opposed to the usual many paper drafts with C). It works too, correct to 13 decimal points on my GHC system It is :- my_sqrt t = last (take 20 (iterate (\n -> n/2 + t/(2 * n)) t)) It is a bit crude though. 20 iterations is a bit arbitrary. I don't suppose there is a easy way to iterate until the results stop changing.

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