- Haskell-Cafe - Haskell.org

New to Monads
by Tom Hawkins 07 Oct '05

07 Oct '05

I'm just starting to feel comfortable working inside IO; now I am trying to wrap my head around Control.Monad.State. Looking at the implementation, I came across some unfamiliar syntax... class (Monad m) => MonadState s m | m -> s where What is the meaning of "| m -> s"? I found no mention of it in the tutorial. And slight logical dilemma... With IO, I understand one your in it, you can't get out. I.e., any function that conducts IO must have IO _ in the type signature. But this must not apply to all monads, otherwise the type signature for "main" would be rather messy (IO + State + every other monad used in a program). But I don't see how a monad could escape this fate since bind and return only produce more monads with the same type constructor. Thanks for your help! -Tom

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Working inside the fold
by Greg Buchholz 07 Oct '05

07 Oct '05

Recently I've been browsing some of Oleg Kiselyov's articles entitled "Towards the best collection traversal interface"... http://okmij.org/ftp/Computation/Continuations.html#enumerator-stream A programming language system gives us typically one of the two interfaces to systematically access elements of a collection. One traversal API is based on enumerators -- e.g., for-each, map, filter higher-order procedures -- of which the most general is fold. The second approach relies on streams, a.k.a. cursors, lazy lists. ...where he argues that (since they're both intraconvertible) a default enumerator/fold choice is better than the lazy-list approach. I was sufficiently intrigued I thought I'd take up the challenge and see what could be done working inside the fold. This message is literate Haskell source, for those of you playing at home. The first thing to do is define a few folds which will take the place of lists. I'll start out with a finite and infinite one... >numsTo10 f unit = foldr f unit [1..10] >nats f unit = foldr f unit [1..] If you want the sum of the first ten naturals you'd invoke it as... *Main> numsTo10 (+) 0 55 ...and to see that you can convert back to a list... *Main> numsTo10 (:) [] [1,2,3,4,5,6,7,8,9,10] That's nice, but the first thing you'll notice is that while there are numerous list processing functions in the prelude, there are none for specifically working inside a fold. Let's try and define a few... >map_ :: (a -> b) -> (b -> c -> d) -> a -> c -> d >map_ f g a b = g (f a) b ...so if we want the sum of the first ten squares... *Main> numsTo10 (map_ (^2) (+)) 0 385 ...or the actual list... *Main> numsTo10 (map_ (^2) (:)) [] [1,4,9,16,25,36,49,64,81,100] ...Filter is also a nice function... >filter_ p f a b = if p a then f a b else b *Main> numsTo10 (filter_ odd (:)) [] [1,3,5,7,9] ...and we can also play nice with infinity... >takeWhile_ unit p f a b = if p a > then f a b > else unit *Main> nats (takeWhile_ [] (<15) (:)) [] [1,2,3,4,5,6,7,8,9,10,11,12,13,14] ...That's a little bit ugly because you have to supply the unit value (in this case "[]") for the fold to takeWhile_. The dropWhile_ function is also a little quirky, since it uses the tupling trick from... "A Tutorial on the Universality and Expressiveness of Fold" http://citeseer.ist.psu.edu/hutton93tutorial.html >dropWhile_ p f a (ys, xs) =((if p a > then ys > else f a xs), f a xs) *Main> numsTo10 (dropWhile_ (<5) (:)) ([],[]) ([5,6,7,8,9,10],[1,2,3,4,5,6,7,8,9,10]) *Main> numsTo10 (dropWhile_ (<5) (+)) (0,0) (45,55) ...where the first member of the pair is the desired answer. If you want to do more than one thing inside the fold, "fork" might be the solution (is there a better name?) >fork f g z (x,y) = (f z x, g z y) *Main> numsTo10 (fork (:) (+)) ([],0) ([1,2,3,4,5,6,7,8,9,10],55) *Main> numsTo10 (fork (filter_ odd (:)) (fork (+) (*))) ([],(0,1)) ([1,3,5,7,9],(55,3628800)) ...And you can mostly compose these operations, although for infinite lists you'll hit bottom if anything other than takeWhile_ is the first function... >c = nats (takeWhile_ ([],(0,1)) (<20) > (filter_ even (map_ (^2) (fork (:) (fork (+) (*)))))) > ([],(0,1)) *Main> c ([4,16,36,64,100,144,196,256,324],(1140,34519618525593600)) ...So a few generic munging functions can be defined for inside the fold. I couldn't think of how to define "take" or "drop" or figure out what a zipWith would mean. Are there any other interesting functions that could be defined? Is there a better way define these functions? Is there anything other than curiosity which would motivate someone to use these functions? FWIW, Greg Buchholz

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Emacs Haskell Mode: Indenting after "where"
by Joel Reymont 07 Oct '05

07 Oct '05

Folks, Does anyone know how to make the Haskell mode (Emacs) indent the line after "where" for this type of look: peek = if be then peek0 else peekR where peek0 a = fmap BigEndian $ peek (castAway a) peekR a = peekByteOff a 0 instead of this: peek = if be then peek0 else peekR where peek0 a = fmap BigEndian $ peek (castAway a) peekR a = peekByteOff a 0 Thanks, Joel -- http://wagerlabs.com/idealab

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Re: [Haskell-cafe] New to Monads
by Wolfgang Jeltsch 07 Oct '05

07 Oct '05

Am Freitag, 7. Oktober 2005 06:32 schrieb Cale Gibbard: > That | m -> s reads "where m determines s", and means that there can > be at most one s (here, the state type) for a given m (the monad > type). This is used in type inference and checking to ensure that the > type of state being carried around by a monad in MonadState is well > defined, and to prevent you from having to annotate the operations in > MonadState with type signatures. (To tell which type s to use.) What you can express via this syntax is called functional dependencies (in analogy to functional dependencies in database theory). One should note that functional dependencies and even multi-parameter type classes are an extension to Haskell 98 and not part of Haskell 98 which is why you haven't read about them in the tutorial. > [...] Best wishes, Wolfgang

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Confused about types
by Lanny Ripple 06 Oct '05

06 Oct '05

I thought I was doing ok with haskell until I tried to program in it. :/ I figured the first thing to tackle was something easy so I hacked up newton's method. Worked great for Double's. Then I figured I would extend it to work with different types of Fractionals. What a pain. I finally got that working for Floats and Rationals but I'm drawing a blank on Complex. I've got: <Newton.hs> import Data.Complex import Data.Ratio import Debug.Trace main = putStrLn $ "answer = " ++ show (newton myd $ -5) newton :: (Fractional a, Ord a) => (a -> a) -> a -> a newton f x = newton_h f x 1.0e-6 newton_h, next_x_h, dy_h :: (Fractional a, Ord a) => (a -> a) -> a -> a -> a newton_h f x h = until ((<= h) . abs . f) (next_x_h f h) x next_x_h f h x = trace ("next_x = " ++ show foo) foo where foo = x - (f x) / (dy_h f x h) dy_h f x h = ( (f $ x + h) - f x ) / h -- roundTo :: Fractional a => a -> a -> a -- roundTo x eps = ((fromRational . toRational) (round $ x / eps)) * eps myd :: Double -> Double myd x = ((x/100 + 1) * x + 1) * x - 10 myf :: Float -> Float myf x = ((x/100 + 1) * x + 1) * x - 10 myr :: Rational -> Rational myr x = approxRational myr' 1e-6 where myr' = ((x/100 + 1) * x + 1) * x - 10 myc :: RealFloat a => Complex a -> Complex a myc x = x * x + 1 </Newton.hs> When I try newton myc 5 in ghci I get a warning about newton wanting Ords. Complex aren't ordered. I figure I need to specify a newton_h that can handle complex newton_h f (Complex x) h = until ((<= h) . realPart . abs . f) (next_x_h f h) x laughs at me. So does newton_h f (:+ r im) h = until ((<= h) .realPart . abs . f) (next_x_h f h) (:+ r im) although I sure thought I had read that constructors could be used in pattern matching. I've been working on this for too long and getting frustrated. Can someone clue me in? Thanks, -ljr -- Lanny Ripple <lanny(a)cisco.com> CC Tools / Cisco Systems, Inc.

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RE: [Haskell-cafe] Papers from the 2005 Haskell Workshop (Tallinn)?
by Simon Marlow 06 Oct '05

06 Oct '05

On 05 October 2005 17:11, Dimitry Golubovsky wrote: > The papers presented at the Workshop are already available in the ACM > library which requires membership/subscription to read full text PDFs. > Are there any plans to make those papers available anywhere else on > the Web without subscription? The papers I co-authored are available from my web page: http://www.haskell.org/~simonmar/bib/bib.html Cheers, Simon

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Re: [Haskell-cafe] Papers from the 2005 Haskell Workshop (Tallinn)?
by Dimitry Golubovsky 06 Oct '05

06 Oct '05

Nils Anders Danielsson wrote: > > Most authors do put their papers on their web pages nowadays. > In particular, I would like to read the paper on "halfs" (haskell filesystem). Googling for <halfs haskell filesystem> gave nothing but the Workshop's schedule and ACM Library TOC. Dimitry Golubovsky Middletown, CT

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Define a data structure of a predicate
by Sara Kenedy 05 Oct '05

05 Oct '05

Hello you all, I am a newbie in Haskell. Now I am working on datatype structure of Haskell, especially on predicate. Today I try to search on Internet to find the reference document but I did not find the specific. If any of you know how to define a data structure for the abstract syntax of predicate logicI , for example : x^2 + 3*x + 2 > 0, if you don't mind, please share with me. Thanks in advance.

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CFP: PAPP 2006
by Ch. A. Herrmann 05 Oct '05

05 Oct '05

--------------------------------------------------------------------- Please accept our apologies if you have received multiple copies. Please feel free to distribute it to those who might be interested. --------------------------------------------------------------------- ---------------------------------------------------------- CALL FOR PAPERS PAPP 2006 Third International Workshop on Practical Aspects of High-level Parallel Programming http://graal.ens-lyon.fr/~abenoit/conf/papp2006.html part of ICCS 2006 The International Conference on Computational Science May 28-31, 2006, University of Reading, UK ---------------------------------------------------------- AIMS AND SCOPE Computational Science applications are more and more complex to develop and require more and more computing power. Parallel and grid computing are solutions to the increasing need for computing power. High level languages offer a high degree of abstraction which ease the development of complex systems. Moreover, being based on formal semantics, it is possible to certify the correctness of critical parts of the applications. Algorithmic skeletons, parallel extensions of functional languages such as Haskell and ML, parallel logic and constraint programming, parallel execution of declarative programs such as SQL queries, etc. have produced methods and tools that improve the price/performance ratio of parallel software, and broaden the range of target applications. The PAPP workshop focuses on practical aspects of high-level parallel programming: design, implementation and optimization of high-level programming languages and tools (performance predictors working on high-level parallel/grid source code, visualisations of abstract behaviour, automatic hotspot detectors, high-level GRID resource managers, compilers, automatic generators, etc.), applications in all fields of computational science, benchmarks and experiments. Research on high-level grid programming is particularly relevant. The PAPP workshop is aimed both at researchers involved in the development of high level approaches for parallel and grid computing and computational science researchers who are potential users of these languages and tools. TOPICS We welcome submission of original, unpublished papers in English on topics including: * high-level models (CGM, BSP, MPM, LogP, etc.) and tools for parallel and grid computing * high-level parallel language design, implementation and optimisation * functional, logic, constraint programming for parallel, distributed and grid computing systems * algorithmic skeletons, patterns and high-level parallel libraries * generative (e.g. template-based) programming with algorithmic skeletons, patterns and high-level parallel libraries * applications in all fields of high-performance computing (using high-level tools) * benchmarks and experiments using such languages and tools PAPER SUBMISSION AND PUBLICATION Prospective authors are invited to submit full papers in English presenting original research. Submitted papers must be unpublished and not submitted for publication elsewhere. Papers will go through a rigorous reviewing process. Each paper will be reviewed by at least three referees. The accepted papers will be published in the Springer-Verlag Lecture Notes in Computer Science (LNCS) series, as part of the ICCS proceedings. Submission must be done through the ICCS website: http://www.iccs-meeting.org/iccs2006/papers/upload.php We invite you to submit a full paper of 8 pages formatted according to the rules of LNCS, describing new and original results, no later than December 2, 2005. Submission implies the willingness of at least one of the authors to register and present the paper. An early email to Anne.Benoit at ens-lyon.fr with your intention to submit a paper would be greatly appreciated (especially if you have doubts about the relevance of your paper). Accepted papers should be presented at the workshop and extended and revised versions will be published in a special issue of Parallel and Distributed Computing Practices, provided revisions suggested by the referees are made. IMPORTANT DATES December 2, 2005 Full paper due January 31, 2006 Referee reports and notification February 10, 2006 Camera-ready paper due May 2006 Journal version due June 2006 Referee reports July-October 2006 Revision of papers, final notification November 2006 Final paper due PROGRAM COMMITTEE Marco Aldinucci (CNR/Univ. of Pisa, Italy) Olav Beckmann (Imperial College London, UK) Anne Benoit (ENS Lyon, France) Alexandros Gerbessiotis (NJIT, USA) Stephen Gilmore (Univ. of Edinburgh, UK) Clemens Grelck (Univ. of Luebeck, Germany) Christoph Herrmann (Univ. of Passau, Germany) Zhenjiang Hu (Univ. of Tokyo, Japan) Frédéric Loulergue (Univ. Orléans, France) Casiano Rodriguez Leon (Univ. La Laguna, Spain) Alexander Tiskin (Univ. of Warwick, UK) ORGANIZERS Dr. Anne BENOIT Laboratoire d'Informatique du Parallélisme Ecole Normale Supérieure de Lyon 46 Allée d'Italie 69364 Lyon Cedex 07 - France Pr. Frédéric LOULERGUE Laboratoire d'Informatique Fondamentale d'Orléans Université d'Orléans Bâtiment IIIA - Rue Léonard de Vinci - BP6759 45067 Orléans Cedex 2 - France

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RE: [Haskell-cafe] Problem with sparc distribution?
by Simon Marlow 05 Oct '05

05 Oct '05

On 29 September 2005 10:37, Christian Maeder wrote: > I've repacked the distribution and put it at: > http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/CoFI/ hets/solaris/ghc-6.4.1-sparc-sun-solaris2.tar.bz2 > > Can you copy it to http://www.haskell.org/ghc/dist/6.4.1/, Simon? Done. Simon

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