June 2004 - Haskell - Haskell.org

RE: [Haskell] IO, exceptions and error handling
by Tim Docker 14 Jun '04

14 Jun '04

Philippa Cowderoy wrote: > The ability to fail doesn't need the do notation, just use of > return for success - similar for propagating failure. I'm not sure I understand. Do you mean writing functions like: sqr x | x < 0 = fail "less than zero" | otherwise = return (sqrt x) If so, I don't see how I can use this without either do or >>=: test x = do v1 <- sqr x v2 <- sqr (x+1) return (v1+v2) I can't (easily) write text c = sqr x + sqr (x+1) which was what I was getting at. Tim

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[Haskell] Programming language shootout (completing the Haskell entry)
by Sean E. Russell 14 Jun '04

14 Jun '04

Hiya, I know it is silly, but I'm interested in helping to complete the Haskell entry in the Programming Language Shootout page[1] (http://www.bagley.org/~doug/shootout/craps.shtml) The Haskell entry is missing 5 (out of 13) entries, and since no points are "scored" for missing entries, this skews Haskell's rank downwards. Since I'm not the worlds Best Haskell Programmer -- and I'm certainly not adept at optimizing, and since I'd like to provide the fastest Haskell code that I can, I thought I'd run it through my code through this list and see if anybody would like to suggest improvements. The implementations for this particular test must be written in the "same way"; this is defined to be that the submissions "use the same logic and data structures", the goal being to "try to measure languages doing the same operations, as closely as possible." The "description" of the test is: "In this test, we create 10000 hash entries, then add them into a new hash N times. " The correct output for this code where N=10 must be: 1 9999 10 99990 I chose to use (lookup) and associative arrays. I create a *bunch* of hashes, rather than just two that I re-use because, honestly, I'm such a novice I don't know how to do it otherwise. Anyway, as I said, any suggestions for optimization are appreciated. This is just for fun. ===== BEGIN hash2.hs module Main where import System (getArgs) import Numeric (readDec) import Maybe main = do argv <- getArgs let h1 = hash1 9999 [] h2 = hash2 (fst $ head $ readDec $ argv!!0) h1 [] print h1 h2 where hash1 0 x = x hash1 n xs = hash1 (n-1) ((mkentry n):xs) mkentry y = ( "foo_"++(show y), y ) hash2 0 _ x2 = x2 hash2 n x1 x2 = hash2 (n-1) x1 [ (k1, v1+(get k1 x2)) | (k1, v1) <- x1 ] print x1 x2 = putStrLn ( (show $ get "foo_1" x1)++" "++ (show $ get "foo_9999" x1)++" "++ (show $ get "foo_1" x2)++" "++ (show $ get "foo_9999" x2) ) get :: String -> [(String, Int)] -> Int get key hash = fromMaybe 0 $ lookup key hash ===== END hash2.hs [1] The page is entirely tounge-in-cheek, but I find it interesting, nonetheless. -- ### SER ### Deutsch|Esperanto|Francaise|Linux|XML|Java|Ruby|Aikido ### http://www.germane-software.com/~ser jabber.com:ser ICQ:83578737 ### GPG: http://www.germane-software.com/~ser/Security/ser_public.gpg

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RE: [Haskell] IO, exceptions and error handling
by Tim Docker 14 Jun '04

14 Jun '04

Keith Wansbrough wrote: > Read the paper _A Semantics for Imprecise Exceptions_. The > problem is that the evaluation order of Haskell would have to > be fixed for this not to lose referential transparency. What > is the value of > > catchExcept (show (makeExcept "E1" + makeExcept "E2")) > (\x -> x) > > ? Haskell wouldn't be "purely functional" any more. As Graham Klyne indicated in a previous mail, in practice there are two ways of dealing with errors in purely functional code: - capture possible errors in the result type. This will probably force the use of a monad, with the subsequent sequencing of operation and coding style. - throw an exception, and hence be unable to recover from errors outside the IO monad. Both of these approaches seem fairly "invasive" in their effect on the code. Are people using haskell for real world tasks happy with having to choose from these? The former is more general, but any function that needs to be able to fail or propagate failure will end up being a do {} block. Tim

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Soft2004 workshop CFP
by Stefano Bistarelli 14 Jun '04

14 Jun '04

============================================================================ ======= 6th International Workshop on Soft Constraints and Preferences September 27th, 2004 Toronto, Canada Held in conjunction with 10th International Conference on Principles and Practice of Constraint Programming, CP2004 http://www.sci.unich.it/~bista/organizing/soft-2004/ ============================================================================ ======== Call for Paper ============================================================================ ======== Preferences are ubiquitous in real life: most problems are over-constrained and would not be solvable if we insist that all their requirements are strictly met. Moreover, many problems are more naturally described via preference rather than hard statements. Soft constraints are the way the constraint community has extended his classical framework to deal with the conceept of preferences. However, other frameworks for expressing preferences have been proposed in the last few years in AI or related disciplines, with different features and leading to different results. For example, both qualitative and quantitative preference frameworks are being studied and used to model and solve real-life probleems. This workshop would like to bring together researchers interested in all aspects of preferences and soft constraints, such as: - theoretical frameworks - problem modeling - solving algorithms - languages - preference aggregation - preference elicitation - multi-objective or qualitative optimization - combining/integrating different frameworks and algorithms - comparative studies - real-life applications Submissions This workshop is intended to build on the experience and success of the workshops on soft constraints that have been held at CP99, CP2000, CP2001, CP2002 and CP2003, and on the AAAI 2002 workshop on preferences in AI and CP. Its aim is to provide a forum where researchers currently working in the area of preferences or soft constraints can discuss their most recent ideas and developments and think together about the most promising new directions. Therefore we encourage the presentation of work in progress or on specialized aspects of soft constraints. Papers that bridge the gap between theory and practice are especially welcome. Prospective attendees can submit a paper, which can be up to 15 pages in length. We encourage authors to submit papers electronically in postscript or pdf format. Papers should be formatted using the Lecture Notes in Computer Science (LNCS) style. Please send your submissions by email to stefano.bistarelli(a)iit.cnr.it using the subject line Soft-2004 Workshop Submission. Although no agreement is yet in place, we will consider the publication of a selection of papers presented at this workshop in a special issue of a journal or in a volume of a serie. Important Dates The proposed schedule of important dates for the workshop is as follows: Paper Submission deadline June 26th Notification of acceptance July 24th Camera-ready version deadline August 14th Workshop Date September 27th Workshop Organizers: Stefano Bistarelli (Primary Contact) Dipartimento di Scienze Università degli studi "G. D'Annunzio" di Chieti-Pescara, Italy Email: bista(a)sci.unich.it Web: http://www.sci.unich.it/~bista/ and Istituto di Informatica e Telematica C.N.R. Pisa, Italy Email: stefano.bistarelli(a)iit.cnr.it and Francesca Rossi Dipartimento di Matematica Pura ed Applicata Universita' degli Studi di Padova, Italy Email: frossi(a)math.unipd.it Web: http://www.math.unipd.it/~frossi/ Program Committee: Stefano Bistarelli (University of Pescara, Italy --- C.N.R., Pisa, Italy) Simon De Givry (Institut National de La Recherche Agronomique, Castanet Tolosan Cedex, France) Boi Faltings (EPFL, Switzerland) Carmen Gervet (IC-Parc, Imperial College, UK) Ulrich Junker (ILOG, France) Javier Larrosa (Universitat Politècnica de Catalunya, UPC, Barcelona, Spain) Jimmy H. M. Lee (The Chinese University of Hong Kong, Hong Kong) Pedro Meseguer (IIIA-CSIC, Bellaterra, Spain) Barry O'Sullivan (University College Cork, Ireland) Francesca Rossi (University of Padova, Italy) Thomas Schiex (Institut National de La Recherche Agronomique, Castanet Tolosan Cedex, France) Toby Walsh (Cork Constraints Computation Centre, Ireland)

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Re: [Haskell] IO question
by b.i.mills＠massey.ac.nz 12 Jun '04

12 Jun '04

While what I want to say has implicitly been said I'd just like to suggest that there is a simple way to put the whole matter less cryptically. The "obvious" question that just about anyone starting with the IO monad asks is how can I write (IO a -> a)? The "simple" answer is, you don't have to because you can use (>>=) with something (a -> b) to write (IO a -> IO b). That is, if you are dealing with an IO operation, you must always think in terms of the complete read-modify-write sequence. <personal> This requires you to think explicitly about the temporal nature of your code, but I see the strength of the IO monad as being exactly that, it forces you to be explicit. For me writing (IO a -> a) is logically non-sensical, you have just broken the semantics of IO, that's all. I know what (IO a -> a) is, because I know that the Haskell language is not the whole universe. But, it is like *(int *)(&x) in C, the meaning depends on the implimentation of the language, outside the semantics of the language per se. </personal> Regards, Bruce.

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Errata to the Revised Haskell 98 Report
by Simon Peyton-Jones 11 Jun '04

11 Jun '04

Gentle Haskellers It's pleasing that there have been so few bug reports about the Revised Haskell 98 report, which was published in January 2003. But there have been some: see http://research.microsoft.com/~simonpj/haskell98-revised/haskell98-revis ed-bugs.html and every now and again another one comes up. Malcolm Wallace has kindly agreed to take over custody of this errata list. There's no current plan to do more than keep a list of errors, but if any genuine ambiguities are found, he'll start a discussion to try to nail them down. Thanks Malcolm! Simon

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IO question
by Tom Hofte 11 Jun '04

11 Jun '04

Hi, I have an question about the IO monad I want to have a function that unpack an IO. I should have the type: IO a -> a. Is this possible? Kind regards Tom Hofte

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topology in Haskell
by Martin Escardo 11 Jun '04

11 Jun '04

Dear Haskell-list members, This is to advertise the monograph Synthetic topology of data types and classical spaces, to appear in ENTCS 87, 150pp, three parts, 6+5+2 chapters. http://www.cs.bham.ac.uk/~mhe/papers/entcs87.pdf (or .dvi or .ps) Chapter 3 develops topology in Haskell, without assuming any previous knowledge of topology. Notions of topology such as space, continuous map, open set, closed set, discrete space, Hausdorff space, compact space are defined directly in the programming language. Theorems in topology are proved by writing programs. The development here is purely operational. This gives some surprising results, e.g. that the type ((Int->Bool)->Int) has decidable equality (for total elements). Chapter 12 has more sophisticated computational applications, which invoke classical topology with the aid of denotational semantics. We apply the Tychonoff theorem of classical topology to show that a certain (we hope surprising) Haskell program has the correct termination properties. Martin Escardo

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Implicit parameters
by Per Larsson 09 Jun '04

09 Jun '04

When using implicit parameters I have noticed (at least for me) a rather puzzling behaviour with GHC and Hugs. Given the declarations data Env = Env {numlines :: Int, numcols :: Int} initEnv = Env {numlines = 0, numcols = 1} withEnv :: ((?env :: Env) => IO a) -> IO a withEnv io = let ?env = initEnv in io I can write code like: main = withEnv (do let lines = numlines ?env putStrLn ("Initial number of lines is " ++ (show lines))) which works as expected, but the version below main = withEnv $ do let lines = numlines ?env putStrLn ("Initial number of lines is " ++ (show lines)) is not accepted by either GHC or Hugs! Is this a bug or have stumbled into a context where it actually matters if you use '$' or explicit grouping with parentheses? Per Larsson

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Re: how to write a list builder? fixpoint?
by oleg＠pobox.com 08 Jun '04

08 Jun '04

> Is it possible to write a function to build a list [a]? > so that I can write [a,b,c,d] as "getBuilt $ build a b c d"? Yes, in the format very close to desired. > {-# OPTIONS -fglasgow-exts #-} > {-# OPTIONS -fallow-undecidable-instances #-} > > module Foo where > > class BuildList a r | r-> a where > build' :: [a] -> a -> r > > instance BuildList a [a] where > build' l x = reverse$ x:l > > instance BuildList a r => BuildList a (a->r) where > build' l x y = build'(x:l) y That's it. It works both on GHC and Hugs. *Foo> build' [] True :: [Bool] [True] *Foo> build' [] True False :: [Bool] [True,False] *Foo> build' [] True False False :: [Bool] [True,False,False] *Foo> build' [] 'a' 'b' 'c' 'd' 'e' :: [Char] "abcde" *Foo> build' [] (1::Int) :: [Int] [1] *Foo> build' [] (1::Int) (2::Int) :: [Int] [1,2] *Foo> build' [] (1::Int) (2::Int) (3::Int) :: [Int] [1,2,3] Note that the type annotation [Bool] etc. at the end is required: it is the delimiter of the list. Who would have thought that the type annotation can play the role of Nil...

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