February 2001 - Haskell

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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framework for composing monads?
by Elke Kasimir 18 Feb '01

18 Feb '01

Does someone like to comment on this? I'm planning a new cli/odbc-based database connectivity library for Haskell 98 and want to manage hidden state (various management information) on the Haskell side. Some libs. i.e. for gui, extend the IO monad for this using some "start" function: main :: IO () main = start prog prog :: GUI () ... I could do the same with my library, but then it is difficult to combine, for example, Gui operations with Db operations. I found a better solution in Jeuring/Meijer 1995 in Mark P. Jones' contribution which consists of using monad transformers and lifting monad operations to class level like this: type M a = DB (GUI IO) a startM = start . start main = startM prog prog :: (Io m, Gui m, Db m) => m () prog = ... I hope that this approach not only solves the problem mentioned above but also supports a more modular approach to handling state etc. in general and encourages the programmer to add his own monads (instead of packing all into IO or passing around state etc. explicitly). However, to be really useful, it would be best if there could be established a "standard framework" for such libraries. It would be nice if there were at least agreement on how to lift IO operations to class level: 'Io.putStr' (which provokes name clashes with the standard prelude),'Io.putStrC' (C for class level), or some other prefix/suffix. In addition to Mark P. Jones' examples in the mentioned article, - Class Io must be provided, - all specific monad operations should be encapsulated in ADTs, - it must be possible to map 'lift' into the ADT In the attachements I have worked out a scenario, where DB and GUI are state monad transformers and Io and Gui are defined as follows: class Monad m => Io m where io :: IO a -> m a class Monad m => Db m where db :: DBImpl m a DBImpl is an ADT which encapsulates a set of characteristic monad operations, in this case the 'modify' operation of a state transformer. For such ADTs, a special class Liftable l where mapLift :: (MonadT t, Monad m) => l m -> l (t m) has been introduced to supply a unified symbol for applying lift to the encapsulated monad transformation(s). (I don't like the class name either but didn't find a better up to now...) However, there remains a problem with the type system. Ideally, it would be possible to require: class (Monad m, Monad (t m)) => MonadT t where lift = ... such that MonadT applications become automatically members of class Monad, and declare instance (Db m, MonadT t) => Db (t m) where db = mapLift db (same for class Gui etc.) with the obvious effects. Both seems to be not even possible in the extended type systems of Hugs and ghc. I want to create a library for Haskell 98, so this is of minor importance, but if there was a way to make life easier for those who use type system extensions without complicating the situation for Haskell 98 users, I would of course prefer such a solution. Until then, again, I think that "standard framework" would make it easier for users to create those instance declarations by themselves needed to combine monad transformers from different sources. Elke. --- "If you have nothing to say, don't do it here..." Elke Kasimir Skalitzer Str. 79 10997 Berlin (Germany) fon: +49 (030) 612 852 16 mail: elke.kasimir(a)catmint.de> see: <http://www.catmint.de/elke> for pgp public key see: <http://www.catmint.de/elke/pgp_signature.html>

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Re: Literate Programming in Haskell?
by andrew＠andrewcooke.free-online.co.uk 18 Feb '01

18 Feb '01

Hi, I'm no expert, but I've found a solution that works for me. I use the \begin{code} ... \end{code} delimiters (rather than >). Since these aren't standard Latex, I threw together a style file that called the listings package to handle this. The result is a file that ghc and hugs can compile directly, but which latex can also process. No need for weaving and tangling, while line numbers in error messages are correct. On the other hand, I'm not doing anything very sophisticated - no labelling of blocks or anything like that. I needed to update to the version of tetex in the "testing" Debian release to get this to work - the listing package in the standard distribution doesn't support Haskell (not that I really use that support), so if you get an error message about being unable to load something, that may be the problem. (Warning: I wouldn't recommend moving to Debian testing at the moment; when I last tried the X support had changed significantly and failed to install, so don't upgraed anything apart from tetex). I've included the style file. I don't have a public example yet (hopefully later this week), but I can email you a postscript file directly if you want. Haskell is the first language where I've managed to work happily with literate programming - partly because of the direct integration and partly because the order isn't critical and the code is compact. Cheers, Andrew PS The style file includes lots of stuff unrelated to literate programming - delete everything but the first block, if you like. On Sun, Feb 18, 2001 at 01:53:41AM -0500, Tom Moertel wrote: > In the Haskell community is there a generally accepted best way to > approach Literate Programming? The language has support for literate > comments, but it seems that many common LP tools don't respect it. > > For example, in order to convert some .lhs code into LaTeX via the noweb > LP tools, I had to write a preprocessor to convert the ">" code blocks > into something that noweb would respect. (The preprocessor actually > does a bit more and, in conjunection with noweb, gives pretty good > results for little effort. For an example, see: > > http://www.ellium.com/~thor/hangman/cheating-hangman.lhs > http://www.ellium.com/~thor/hangman/cheating-hangman.pdf > ) > > Yet somehow, I don't think that my homebrew approach is optimal. Can > anybody recommend a particularly elegant LP setup for Haskell > programming? Or if you have an approach that works well for you, would > you mind sharing it? > > Cheers, > Tom > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- http://www.andrewcooke.free-online.co.uk/index.html

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Lifting to the limit
by Saswat Anand 17 Feb '01

17 Feb '01

Dear All, I have this following simplified problem. Please help. type Formula a b = a -> b type Env = (Integer,Char) fun :: Formula Integer Integer fun n = n+1 sun :: Formula Char Integer sun c = toInteger.ord c I want to be able to write (sun + fun) (2,'a') (fun + sun) (2,'a') both of which should be equal to (sun 'a' + fun 2) So I did this: lift2 :: (a -> b -> c) -> Formula d a -> Formula e b -> Formula Env c lift2 f a1 a2 = \env -> f ( (lift a1) n ) ( (lift a2) n ) -- Here hugs gives error class Lift a where lift :: Formula a b -> Formula Env b instance Lift Char where lift f = \(i,c) -> f c instance Lift Integer where lift f =\(i,c) -> f i With this, hugs (with extensions enabled) gives following error: Cannot justify constraints in explicitly typed binding ***Expression : lift2 ***Type : (a -> b -> c) -> Formula d a -> Formula e b -> Formula Env c ***Given Constraint: () ***Constraints : (Lift d, Lift e) Any help is very much appreciated. Thanks, Saswat

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