October 2001 - Haskell

tuple component functions
by trb＠eastpac.com.au 03 Jan '03

03 Jan '03

S.D.Mechveliani writes: > As Haskell has the standard functions fst, snd to decompose (a,b), > maybe, it worths to provide also > tuple31, tuple31, tuple31, > ... > tuple51, tuple52, tuple53, tuple54, tuple55 > > for the tuples of n = 3,4,5 ? I've found some of these useful, except I named them differently: > fst3 :: (a,b,c) -> a > fst3 (x,_,_) = x > snd3 :: (a,b,c) -> b > snd3 (_,x,_) = x > thd3 :: (a,b,c) -> c > thd3 (_,_,x) = x .... never got around to quadruples etc. I also defined (some of these may be untested): > tripl :: a -> (a,a,a) > tripl x = (x,x,x) > curry3 :: ((a, b, c) -> d) -> a -> b -> c -> d > curry3 f x y z = f (x, y, z) > uncurry3 :: (a -> b -> c -> d) -> ((a, b, c) -> d) > uncurry3 f t = f (fst3 t) (snd3 t) (thd3 t) > applyTriple :: (a -> b, a -> c, a -> d) -> a -> (b,c,d) > applyTriple (f,g,h) x = (f x, g x, h x) > cross3 :: (a -> b, c -> d, e -> f) -> (a,c,e) -> (b,d,f) > cross3 (f,g,h) (x,y,z) = (f x, g y, h z) > mapTriple :: (a -> b) -> (a, a, a) -> (b, b, b) > mapTriple = cross3 . tripl > applyFst3 :: (a -> b) -> (a, c, d) -> (b, c, d) > applyFst3 f = applyTriple (f . fst3, snd3, thd3) > applySnd3 :: (a -> b) -> (c, a, d) -> (c, b, d) > applySnd3 f = applyTriple (fst3, f . snd3, thd3) > applyThd3 :: (a -> b) -> (c, d, a) -> (c, d, b) > applyThd3 f = applyTriple (fst3, snd3, f . thd3) > applyArgs3 :: (a -> b) -> (c -> d) -> (e -> f) -> (b -> d -> f -> g) -> (a -> c -> e -> g) > applyArgs3 af1 af2 af3 f = \x y z -> f (af1 x) (af2 y) (af3 z) > rotl :: (a -> b -> c -> d) -> (b -> c -> a -> d) > rotl f y z x = f x y z > rotr :: (a -> b -> c -> d) -> (c -> a -> b -> d) > rotr f z x y = f x y z > rotl4 :: (a -> b -> c -> d -> e) -> (b -> c -> d -> a -> e) > rotl4 f y z t x = f x y z t > rotr4 :: (a -> b -> c -> d -> e) -> (d -> a -> b -> c -> e) > rotr4 f t x y z = f x y z t Some more combinators (only for pairs so far): > twin :: a -> (a,a) > twin x = (x,x) > swap :: (a,b) -> (b,a) > swap (x,y) = (y,x) -- apply a pair of functions to one argument > applyPair :: (a -> b, a -> c) -> a -> (b, c) > applyPair (f, g) x = (f x, g x) -- apply a pair of functions to a pair > cross :: (a -> b, c -> d) -> (a, c) -> (b, d) > cross (f, g) = applyPair (f . fst, g . snd) -- compose a pair of functions onto one function > dotPair :: (a -> b, a -> c) -> (d -> a) -> (d -> b, d -> c) > dotPair (f, g) h = (f . h, g . h) -- compose a pair of functions onto a pair of functions > dotCross :: (a -> b, c -> d) -> (e -> a, f -> c) -> (e -> b, f -> d) > dotCross (f, g) (h, i) = (f . h, g . i) > mapPair :: (a -> b) -> (a, a) -> (b, b) > mapPair = cross . twin > applyFst :: (a -> b) -> (a, c) -> (b, c) > applyFst f = applyPair (f . fst, snd) > applySnd :: (a -> b) -> (c, a) -> (c, b) > applySnd f = applyPair (fst, f . snd) > applyArgs :: (a -> b) -> (c -> d) -> (b -> d -> e) -> (a -> c -> e) > applyArgs af1 af2 f = \x y -> f (af1 x) (af2 y) > betweenFst :: (a -> b -> c) -> (a, d) -> (b, e) -> c > betweenFst = applyArgs fst fst > betweenSnd :: (a -> b -> c) -> (d, a) -> (e, b) -> c > betweenSnd = applyArgs snd snd I find these combinators useful, and probably other people have their own versions - it would be nice to get them standardised, so we all speak the same language. By standardised, I mean a prominent version that will be adopted by users, not necessarily a committee process. Tim

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ICFP programming contest
by trb＠eastpac.com.au 03 Jan '03

03 Jan '03

Michael Hobbs writes: > Just took a look at the results for the ICFP contest. Am I correct in > assuming that "LA" stands for Lennart Augustsson and that "Si^3" stands > for Simon Marlow, Simon Peyton-Jones, Sigbjorn Finne? Is there a URL for the results ? Tim

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Lazy Sound File IO
by Carl McTague 02 Dec '01

02 Dec '01

Please let me refine my question. Is there a general way to coerce an [Int] or [Double] etc. into a String, so that it may be written out to a binary file with hPutStr? There is toEnum, but this doesn't seem like a real solution, since it only works for Int values <256. Thanks, Carl

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Re: Student Programming Projects
by Yoann Padioleau 16 Nov '01

16 Nov '01

Johannes Waldmann <joe(a)isun.informatik.uni-leipzig.de> writes: > > Next Semester, I am supposed to teach a short course in Haskell. > > Can anyone recommend interesting programming projects which can > > be completed in about a month? Thank you very much. > > apart from "pure" programming exercise > (see Haskell textbooks http://haskell.org/bookshelf/) > here are a few suggestions in the area of experimental mathematics. > also, check with your introductory discrete mathematics course (or equivalent). > a lot of combinatorial problems on finite structures > can be illustrated by finding (counter-)examples by computer. > > > say Ramsey colorings, Monochromatic arithmetic subsequences > (van der Waerden's theorem), words avoiding patterns, and so on. > http://www-igm.univ-mlv.fr/~berstel/Lothaire/ChapitresACW/C3.ps > > next, perhaps some game theory? > nim (and variants), other take-and-break games (octal games). > you can safely ignore all of the Sprague-Grundy theory, > and just define the game, and ask for some method > to determine quickly who wins. > > or graph theory: finding hamiltonian paths > (knights tours on a x b chessboards, for instance) > > if the students alreay took some computability theory, > then a nice topic is Posts Correspondence Problem, see > http://www.cs.ualberta.ca/~zhao/PCP/intro.htm > http://www.informatik.uni-leipzig.de/~pcp/pcpcontest_en.html > > also string rewriting: finding/avoiding loops. > this (open!) problem is easily stated: http://www.lri.fr/~rtaloop/95.html Wow, what some nice exercise, are you really serious ? do you think computer science student enjoy that kind of programs (except for the AI-related stuff)? why not make them programming something they would like to use !! why not a tetris/pong/reversi/any_video_game, see for example the book from Paul hudak (multimedia ... in haskell). this man understand what makes student happy : graphism,video games,compiler,operating systems, AI games and not this shit of mathematics/computer science theorist problem. This is the kind of things that make student think that functionnal programming is for doing useless things, and that C is cool cos you can do graphism, program your own operating systems, ... ICFP 2000 ask for an interesting problem: a raytracer, a thing that many student like cos you can then play with it and describe your own scene, and that contest show that ocaml/haskell were really cool for doing graphism-related work. > > > hope this helps. I'm happy to provide more detail. -- Yoann Padioleau, INSA de Rennes, France, http://www.irisa.fr/prive/padiolea Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

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RE: 'Forall' Polymorphism Question
by Simon Peyton-Jones 31 Oct '01

31 Oct '01

Any system that allows a parametric type variable to be instantiated by a for-all type is called "impredicative", I think. Example: Maybe (forall a. a->a), or as you have below (m (forall a. a->a)). I don't understand all (well, any) of the details, but I understand that impredicative systems make type inference nigh impossible. I don't know any programming language that has impredicative types. Haskell certainly does not, nor any extension I know of. I'm willing to be corrected! Simon | -----Original Message----- | From: Ashley Yakeley [mailto:ashley@semantic.org] | Sent: 31 October 2001 10:14 | To: Haskell List | Subject: 'Forall' Polymorphism Question | | | I note that GHC by default gives this type for "return id": | | (return id) :: forall m a. (Monad m) => m (a -> a) | | Wouldn't this be more general: | | (return id) :: forall m. (Monad m) => m (forall a. a -> a) | (return return) :: forall m. (Monad m) => m (forall m1 a. | (Monad m1) | => a -> m1 a) | | ...? | | Is this something GHC could ever do, or are there good reasons why it | would never work in Haskell? | | -- | Ashley Yakeley, Seattle WA | | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

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varying number of arguments restriction
by Hal Daume 31 Oct '01

31 Oct '01

I'm curious why the following code is invalid (from a language design point of view): > foo :: [(Int, String)] -> String > foo [] = "" > foo = snd . head ghc complains: Varying number of arguments for function `foo' I don't understand why this should be invalid? Basically, as I read it, I say, well, for a parameter s, calling foo s: does s match [] ? if so, return "" otherwise, try the next definition of foo (which must match in this case). obviously i can rewrite: foo [] = "" foo s = (snd . head) s but this is uglier. - hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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'Forall' Polymorphism Question
by Ashley Yakeley 31 Oct '01

31 Oct '01

I note that GHC by default gives this type for "return id": (return id) :: forall m a. (Monad m) => m (a -> a) Wouldn't this be more general: (return id) :: forall m. (Monad m) => m (forall a. a -> a) (return return) :: forall m. (Monad m) => m (forall m1 a. (Monad m1) => a -> m1 a) ...? Is this something GHC could ever do, or are there good reasons why it would never work in Haskell? -- Ashley Yakeley, Seattle WA

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Re: varying number of arguments restriction
by Ashley Yakeley 31 Oct '01

31 Oct '01

At 2001-10-30 11:01, Hal Daume wrote: >obviously i can rewrite: > >foo [] = "" >foo s = (snd . head) s > >but this is uglier. I'm not sure. I actually prefer it written out so that the number of arguments in the cases matches (as GHC enforces). -- Ashley Yakeley, Seattle WA

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functor
by Chen Yu 30 Oct '01

30 Oct '01

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Haskell Communities Survey - LAST CALL
by C.Reinke 29 Oct '01

29 Oct '01

Dear Haskellers, over the last weeks, more and more of you have (been;-) volunteered to provide short summaries of recent and planned activities in your area of work with or on Haskell (for details, and for the current status, see http://www.haskell.org/communities/) Those summaries, as far as they haven't reached me already, are due in this week (rather sooner than later), and after putting everything together, the first edition of the collective report should get out to this list next week. The web pages shows which areas are covered, and which summaries have reached me. So, please, have a last look at the web page, and check for: missing topics? If your favourite Haskell topic isn't covered yet, get in touch with me as soon as possible (preferably by sending in a summary, but at least by naming possible contacts) micro-reports? late-breaking news?-) If you don't have any big project/group/area to report on, you might still want to let the Haskell community know that you're there and what you are working on. Just drop me a line or two on your work, and a link to your home page. missing reports? If you've promised a report, remember to send it to me!-) missing emails? If I've asked for your help, could you please let me know whether or not you'll be able to provide a report?-( Cheers, Claus

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