- Haskell - Haskell.org

Inferring from context declarations
by George Russell 21 Feb '01

21 Feb '01

I'm sorry if this is a FAQ, but I'm curious to know why Haskell (or at least, GHC and Hugs) doesn't seem able to use contexts on variables at the front of data declarations. For example the following data Eq x => Foo x = Foo x equal :: Foo x -> Foo x -> Bool equal (Foo x1) (Foo x2) = (x1==x2) provokes an error when I try to compile it with both GHC and Hugs. So if there is a reason, what is it? If not, is it on the list of things to fix? I can't say that it's a major inconvenience, it just means that when I use a type like Foo I have to put totally pointless extra context declarations in.

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making a Set
by G Murali 21 Feb '01

21 Feb '01

hi there, I'm tryng to get my concepts right here.. can you please help in defining a funtion like makeSet :: (a->Bool)->Set a I understand that we need a new type Set like data Set a = Set (a->Bool) what puzzles me is how to apply the funtion to all elements belonging to type a. [a] refers to a list of any type, similarly how to access all elements of type a. All help will be greatly appreciated.. _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.

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EOF marker
by Andre W B Furtado 21 Feb '01

21 Feb '01

[Sorry if you are receiving this message for the second time] Hello there. I used the code below to read and print the bitmap file attached (bitmap.bmp), which cointains the character ASCII 26 (or 1A-hexa), that is used as an end of file marker in MS-DOS: import IO import IOExts main :: IO () main = do bmFile <- openFileEx "bitmap.bmp" (BinaryMode ReadMode) charloop bmFile 0 charloop :: Handle -> Integer -> IO () charloop bmFile 70 = hClose bmFile charloop bmFile n = do hSeek bmFile AbsoluteSeek n a <- hGetChar bmFile putStrLn [a] charloop bmFile (n+1) This program did not stuck at the special character ASCII 26, but if you change putStrLn by putStr, a strange thing will happen: after reading the ASCII 26 char, the program won't show characters anymore. Does anyone know why this happens? Thanks, Andre Furtado

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Re: Synonym Type Constructors
by Ashley Yakeley 19 Feb '01

19 Feb '01

At 2001-02-19 06:34, Fergus Henderson wrote: >As the Haskell Report section 4.2.2 says: > > | Type constructor symbols T introduced by type synonym declarations > | cannot be partially applied; it is a static error to use T without the > | full number of arguments. Thanks, I missed that. How annoying. Is this really necessary? It would be nice if T were, as you say, a first-class type-constructor. -- Ashley Yakeley, Seattle WA

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putStr vs. putStrLn
by Andre W B Furtado 19 Feb '01

19 Feb '01

Hello there. I used the code below to read and print the bitmap file attached (bitmap.bmp), which contains the character ASCII 26 (or 1A-hexa). This special character is used by MSDOS as an end of file marker. import IO import IOExts main :: IO () main = do bmFile <- openFileEx "bitmap.bmp" (BinaryMode ReadMode) charloop bmFile 0 charloop :: Handle -> Integer -> IO () charloop bmFile 70 = hClose bmFile charloop bmFile n = do hSeek bmFile AbsoluteSeek n a <- hGetChar bmFile putStrLn [a] charloop bmFile (n+1) This program did not stuck after reading the special character, but if you change putStrLn by putStr you will notice that a strange thing happens: after reading the special character, the program won't show any more characters. Why does it happen? Thanks, Andre Furtado

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Small functional image toolkit
by andrew＠andrewcooke.free-online.co.uk 19 Feb '01

19 Feb '01

Hi, Just to show that the help people have given me over the last week or two hasn't been completely wasted, I've put the source for a small Haskell toolkit that can be used to generate images on the 'net. The links from this page also show the literate programming haskell.sty in use (I posted this yesterday, it's also available at the site): http://www.andrewcooke.free-online.co.uk/jara/pancito/index.html Cheers, Andrew -- http://www.andrewcooke.free-online.co.uk/index.html

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Synonym Type Constructors
by Ashley Yakeley 19 Feb '01

19 Feb '01

I don't know if this is a bug in Hugs 98, or whether it's a misunderstanding of mine. The Haskell 98 Report Sec. 4.2.2 claims that 'type' introduces a new type constructor. Yet it doesn't seem possible to declare the type constructor an instance of a class: -- class MyClass c where foo :: a -> c a type T m = IO m instance MyClass T where foo = return -- Hugs gives: (line 6): Not enough arguments for type synonym "T" So is T a real type constructor or not? -- Ashley Yakeley, Seattle WA

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Type class inference trouble
by Ken Shan 19 Feb '01

19 Feb '01

Hello all, I'm trying to implement some simple natural language semantics with Haskell (Hugs), and I'm running into trouble with type classes. Here's what I want to do: Suppose that x :: a -> b y :: a then I want to write apply x y = x y :: b Moreover, if x :: a y :: a -> b I also want to write apply x y = y x :: b So far I was able to implement what I want, as follows: class Applicable a b c | a b -> c where apply :: a -> b -> c instance Applicable (a -> b) a b where apply = ($) instance Applicable a (a -> b) b where apply = flip ($) The code above allows me to say int :: Int -> Int int = id test = apply (int 3) (apply ((+)::Int->Int->Int) (int 5)) which results in test == 8. Now, suppose that m is a Monad. If x :: m a y :: m (a -> b) I want to write apply x y = do x' <- x; y' <- y; return x y :: m b Similarly, if x :: m (a -> b) y :: m a I want to write apply x y = do x' <- x; y' <- y; return y x :: m b In general, if apply :: a -> b -> c works, I also want apply :: m a -> m b -> m c to work, by apply = liftM2 apply Thus I attempted: import Monad instance (Monad m, Applicable a b c) => Applicable (m a) (m b) (m c) where apply = liftM2 apply test2 = apply [int 3] (apply [(+)::Int->Int->Int] [int 5]) But Hugs said: ERROR M4.hs:23 - Unresolved top-level overloading *** Binding : test2 *** Outstanding context : Applicable Int (Int -> Int) b What's wrong? Given that class Applicable a b c | a b -> c, shouldn't Hugs be able to figure out automatically what type b would result in Applicable Int (Int -> Int) b? Thanks in advance... -- Edit this signature at http://rodimus.digitas.harvard.edu/cgi-bin/ken/sig "The day Microsoft makes something that doesn't suck is probably the day they start making vacuum cleaners" - Ernst Jan Plugge

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SAS'01 : Submission Deadline Extension
by Radhia Cousot 19 Feb '01

19 Feb '01

========My apologies for duplicates of this announcement========= ================================================================= = = = Call For Papers = = = = 8th INTERNATIONAL STATIC ANALYSIS SYMPOSIUM = = = = = = La Sorbonne, Paris = = 16-18 July, 2001 = = = = http://www.ens.fr/sas01/ = = = ================================================================= = Submissions deadline extension: February 23, 2001. = ================================================================= Static Analysis is increasingly recognized as a fundamental technique for high performance implementations and verification systems of high-level programming languages. The series of Static Analysis Symposia has served as the primary venue for presentation of theoretical, practical and applicative in the area. The Eigth International Static Analysis Symposium (SAS2'01) will be held at La Sorbonne in Paris and followed immediately at La Mutualite by the Thirteenth Conference on Computer Aided Verification (CAV'01, http://www.lsv.ens-cachan.fr/cav01/}. Previous symposia were held in Santa Barbara, Venice, Pisa, Paris, Aachen, Glasgow and Namur. The technical program for SAS'01 will consist of invited lectures, tutorials, panels, presentations of refereed papers, and software demonstrations. Contributions are welcome on all aspects of Static Analysis, including, but not limited to abstract interpretation data flow analysis verification systems optimizing compilers abstract domains program specialization theoretical frameworks type inference abstract model checking complexity analysis abstract testing security analysis Submissions can address any programming paradigm, including concurrent, constraint, functional, imperative, logic and object-oriented programming. Survey papers that present some aspect of the above topics with a new coherence are also welcome. Submitted papers must not substantially overlap with papers that have been published or that are simultaneously submitted to a journal or a conference with a refereed proceedings. Submitted papers should be at most 15 single space 11-point font pages excluding bibliography and well-marked appendices. Program committee members are not required to read any appendices, and so a paper should be intelligible without them. The papers should be submitted as PostScript documents that are interpretable by Ghostscript and/or in PDF format, and they must be printable on both US letter and A4 paper; to facilitate this, extensive use of special fonts and colors should be avoided. Submissions should arrive by February 23, 2001. All submissions must be done electronically at http://www.ens.fr/sas01/. Authors will be notified of the acceptance or rejection of their papers by April 2, 2001. Final versions of the accepted papers must be received in electronic form by April 30, 2001. Important Dates: Submission: February 15, 2001. Notification: April 2, 2001. Final Version: April 30, 2001. Program Chair: Patrick Cousot Ecole Normale Superieure Departement d'Informatique 45, Rue d'Ulm 75230 Paris Cedex 05, France Email: sas01(a)ens.fr Phone: & + 33 1 44 32 20 64 Fax: & + 33 1 44 32 21 52 Program Committee: M. Bruynooghe (KU, Leuven) P. Cousot (ENS, Paris) G. Fil'e (Padova) M. Hagiya (Tokyo) C. Hankin (IC, London) L. Hendren (McGill, Montreal) M. Hermenegildo (UPM, Madrid) N. Jones (DIKU, Denmark) J. Larus (Microsoft, Redmond) J. Palsberg (Purdue) S. Sagiv (Tel Aviv) D. Sands (Chalmers, Goteborg) D. Schmidt (Kansas S.) M.L. Soffa (Pittsburgh) H. Sondergaard (Melbourne) R. Wilhelm (Saarbruken) K. Yi (KAIST, Taejon) General Chair: Radhia Cousot CNRS & Ecole polytechnique rcousot(a)lix.polytechnique.fr

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SFP-WS: SFP'01 - Call for Papers
by Hans Wolfgang Loidl 19 Feb '01

19 Feb '01

[standard apologies for multiple copies of this CFP apply] CALL FOR PAPERS 3RD SCOTTISH FUNCTIONAL PROGRAMMING WORKSHOP AUGUST 22nd-24th 2001 UNIVERSITY OF STIRLING http://www.cee.hw.ac.uk/~greg/sfp3/ We are pleased to announce the 3rd Scottish Functional Programming Workshop, to be held this August at the University of Stirling. All participants are expected to present a paper at the Workshop on some contemporary research topic within the broad field of Functional Programming. Participants will be selected on the basis of a 2 page abstract of their paper. All presented papers will appear in the draft proceedings. After the Workshop, a refereed selection of papers will be published as a book 'Trends in Functional Programming 2001' by Intellect. Key Dates: 4th May: Submission of 2 page abstract 18th May: Notification of acceptance 20th July: Registration 10th Aug: Submission of paper for draft proceedings 22nd to 24th Aug: Workshop For further details, please see: http://www.cee.hw.ac.uk/~greg/sfp3/ Sharon Curtis - scu(a)cs.stir.ac.uk | Greg Michaelson - greg(a)cee.hw.ac.uk | Workshop Coordinators Phil Trinder - trinder(a)cee.hw.ac.uk |

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