February 2006 - Haskell

Announcing Djinn, version 2004-12-11, a coding wizard
by Lennart Augustsson 07 Apr '11

07 Apr '11

Howdy, y'all! I've written a small program that takes a (Haskell) type and gives you back a function of that type if one exists. It's kind of fun, so I thought I'd share it. It's probably best explained with a sample session. calvin% djinn Welcome to Djinn version 2005-12-11. Type :h to get help. # Djinn is interactive if not given any arguments. # Let's see if it can find the identity function. Djinn> f ? a->a f :: a -> a f x1 = x1 # Yes, that was easy. Let's try some tuple munging. Djinn> sel ? ((a,b),(c,d)) -> (b,c) sel :: ((a, b), (c, d)) -> (b, c) sel ((_, v5), (v6, _)) = (v5, v6) # We can ask for the impossible, but then we get what we # deserve. Djinn> cast ? a->b -- cast cannot be realized. # OK, let's be bold and try some functions that are tricky to write: # return, bind, and callCC in the continuation monad Djinn> type C a = (a -> r) -> r Djinn> returnC ? a -> C a returnC :: a -> C a returnC x1 x2 = x2 x1 Djinn> bindC ? C a -> (a -> C b) -> C b bindC :: C a -> (a -> C b) -> C b bindC x1 x2 x3 = x1 (\ c15 -> x2 c15 (\ c17 -> x3 c17)) Djinn> callCC ? ((a -> C b) -> C a) -> C a callCC :: ((a -> C b) -> C a) -> C a callCC x1 x2 = x1 (\ c15 _ -> x2 c15) (\ c11 -> x2 c11) # Well, poor Djinn has a sweaty brow after deducing the code # for callCC so we had better quit. Djinn> :q Bye. To play with Djinn do a darcs get http://darcs.augustsson.net/Darcs/Djinn or get http://darcs.augustsson.net/Darcs/Djinn/Djinn.tar.gz Then just type make. (You need a Haskell 98 implementation and some libraries.) And then start djinn. For the curious, Djinn uses a decision procedure for intuitionistic propositional calculus due to Roy Dyckhoff. It's a variation of Gentzen's LJ system. This means that (in theory) Djinn will always find a function if one exists, and if one doesn't exist Djinn will terminate telling you so. This is the very first version, so expect bugs. :) Share and enjoy. -- Lennart

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Haskell-mode 2.0
by Stefan Monnier 02 Feb '09

02 Feb '09

I have recently taken over maintainership of Haskell-mode, and after making a bunch of changes, I figured it would be a good idea to make a new release. You can find this new release at: http://www-perso.iro.umontreal.ca/~monnier/elisp/ If you think it should also be found at http://www.haskell.org/haskell-mode/, then please tell John Peterson to get back to me (my email to him seem to go straight to the bitbucket. John?). This release has several changes that have all been only lightly tested, especially w.r.t compatibility with various Emacsen. See appended the NEWS file that describes some of the notewirthy changes. Stefan Changes since 1.45: * keybindings C-c <char> have been replaced by C-c C-<char> so as not to collide with minor modes. * The following modules are now automatically activated without having to add anything to haskell-mode-hook: haskell-font-lock (just turn on global-font-lock-mode). haskell-decl-scan (just bind `imenu' to some key). * In recent Emacsen, haskell-doc hooks into eldoc-mode. * haskell-hugs and haskell-ghci are superceded by inf-haskell. * Indentation rules have been improved when using layout inside parens/braces. * Symbols like -> and \ can be displayed as actual arrows and lambdas. See haskell-font-lock-symbols. * Tweaks to the font-lock settings. Among other things paren-matching with things like \(x,y) should work correctly now. * New maintainer <monnier(a)gnu.org>.

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Compilation of big, computed tables
by Stefan Karrmann 24 Oct '06

24 Oct '06

Dear all, can ghc compile huge tables into efficient code if they are constant at compile time? Two examples may clearify the question: big1 :: UArray Int Char big1 = array (0,1000) $! map (\i -> (i,toEnum i)) [0..1000] big2 = sum [0..10000]::Int -- == 50005000 == n*(n+1)/2 where n = 10000 Both values are constant at compile time. As they are given by pure functions, the compiler could evaluate them and write the *result* into the object file 'foo.o'. This would save code size and run time. I peeked into 'foo.hc' but I didn't found 0x2fb0408 nor an array {0, 1, 2, 3, ..., 1000} or similar things. The function in big2 should show that the computation of the value could be very time consuming. If the compiler does not compute it, the source file could be generated by a helper program (or template Haskell?). The function in big1 should show that the conversion of the data into an array could be run-time, code-size and heap-size consuming. (You need the list (maybe explicitly - think of 1000 fixed pseudo-random numbers) and convert it into an array.) If the compiler generates the unboxed array directly it could be rather efficient. PS: I compiled with: ghc6 -c -O2 -keep-tmp-files -keep-hc-files foo.hs Regards, -- Stefan Karrmann

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"strange" behavior of Implicit Parameters
by Eike Scholz 08 Aug '06

08 Aug '06

Hi, I have just a short question, about the semantics of Implicit parameters on GHC 6.4. Given the following code: > (???) x f > = let ?foo = 1337 > in f x > fun :: (?foo::Int,Show x) => x -> String > fun x = "x = "++(show x)++"; ?foo = "++(show ?foo) > test = let ?foo = 23 > in 42 ??? fun I would expect: test == "x = 42; ?foo = 1337" Because I thought, that it gets reduced like: let ?foo = 23 in 42 ??? fun ~> let ?foo = 23 in (let ?foo = 1337 in fun 42) ~> ... ~> "x = 42; ?foo = 1337" however I get "x = 42; ?foo = 23" in ghci. Is there a special reason, why the implicit parameter is reduced first? Could it be a bug? Reducing it my way would be very useful for combinator-libraries, since it could be used passing around state info, or simple debug text, that tells where a Runtime-error occured? an odd example: > undef = error ?errstr > (<?>) x s = let ?errstr = ?errstr++"\n"++s in x -- annotate error tags so error annotations will accumulate and it would be easy to resolve runtime errors. best regards Eike Scholz

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Visual Haskell: Could not find module `Control.Monad.Writer'
by Bernd Holzmüller 02 Mar '06

02 Mar '06

Hi all, I am playing around with Visual Haskell, V0.1 (which really is great stuff!). In the main module I wrote the following import: import Control.Monad.Writer for which I get the error message: Could not find module `Control.Monad.Writer': use -v to see a list of the files searched for I use the GHCOptions: -fglasgow-exts -fallow-overlapping-instances. Is this a bug in Visual Haskell or what's wrong here? Thanks, Bernd

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-fno-monomorphism-restriction makes type-inference ambiguous?
by Eike Scholz 28 Feb '06

28 Feb '06

Hi, thanks for the last help and hints. I have encountered an other problem, and again I don't quite understand the reason why I get the results I get. ghci seems to infer different types for the same expression. Consider that I have disabled the monomorphism restriction in module AGC.lhs (which is attached). and I have a toplevel definition of: > mylength = synAttr listLength loding the module in ghci (6.4) gives (beside some correct warnings): $ Ok, modules loaded: Main. $ *Main> :type synAttr $ synAttr :: (Data b) => ((?stack::[Dyn]) => b -> a) -> Attr a $ *Main> :type listLength $ listLength :: (?stack::[Dyn]) => List -> Float $ *Main> :type (synAttr listLength) $ (synAttr listLength) :: Attr Float $ *Main> :type mylength $ mylength :: (?stack::[Dyn]) => Dyn -> Dyn -> [Dyn] -> Maybe Float $ *Main> let mylength = synAttr listLength $ *Main> :type mylength $ mylength :: Dyn -> Dyn -> [Dyn] -> Maybe Float where > type Attr a = Dyn -> Dyn -> [Dyn]-> Maybe a the problem I have is that inferred types for the toplevel declaration mylength differ from the verbatim equal definition in the Let experssion. for the toplevel it infers: mylength :: (?stack::[Dyn]) => Dyn -> Dyn -> [Dyn] -> Maybe Float for the let-Binding mylength :: Dyn -> Dyn -> [Dyn] -> Maybe Float and this is what I expected. Has anyone an Idea, why this happens? best regards, Eike Scholz PS: Beware of the comments in the attached file. This file is under heavy development. I am dyslexic and don't correct the comments while continuously rewriting code and comments. I hope that the comments are useful anyway. The (+>) (~>) (#>) operators are broken at the moment and don't work the way intended.

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RE: [Haskell] "strange" behavior of Implicit Parameters
by Simon Peyton-Jones 27 Feb '06

27 Feb '06

Look at the type of (???) (???) :: a -> (a->b) -> b Nothing about ?foo in there. You're passing a *monomorphic* function to (???), so fun is partially applied to ?foo before calling (???). Simon | -----Original Message----- | From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Eike Scholz | Sent: 27 February 2006 00:14 | To: haskell(a)haskell.org | Subject: [Haskell] "strange" behavior of Implicit Parameters | | Hi, | | I have just a short question, about the semantics of Implicit parameters | on GHC 6.4. | | Given the following code: | | > (???) x f | > = let ?foo = 1337 | > in f x | | > fun :: (?foo::Int,Show x) => x -> String | > fun x = "x = "++(show x)++"; ?foo = "++(show ?foo) | | > test = let ?foo = 23 | > in 42 ??? fun | | I would expect: | | test == "x = 42; ?foo = 1337" | | Because I thought, that it gets reduced like: | | let ?foo = 23 in 42 ??? fun | ~> let ?foo = 23 in (let ?foo = 1337 in fun 42) | ~> ... | ~> "x = 42; ?foo = 1337" | | however I get | | "x = 42; ?foo = 23" | | in ghci. | | Is there a special reason, why the implicit parameter is reduced first? | Could it be a bug? | | Reducing it my way would be very useful for combinator-libraries, since | it could be used passing around state info, or simple debug text, that | tells where a Runtime-error occured? | | an odd example: | | | > undef = error ?errstr | | > (<?>) x s = let ?errstr = ?errstr++"\n"++s in x -- annotate error tags | | | so error annotations will accumulate and it would be easy to resolve | runtime errors. | | | best regards | | Eike Scholz | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell

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Haskell Weekly News: February 27, 2006
by dons＠cse.unsw.edu.au 27 Feb '06

27 Feb '06

Haskell Weekly News: February 27, 2006 Greetings, and thanks for reading issue 26 of HWN, a weekly newsletter covering developments in the Haskell community. Each Monday, new editions are posted to [1]the Haskell mailing list and to [2]The Haskell Sequence. [3]RSS is also available. 1. http://www.haskell.org/mailman/listinfo/haskell 2. http://sequence.complete.org/ 3. http://sequence.complete.org/node/feed A fairly quiet week this week. Announcements * Long Live Edison. Robert Dockins [4]announced he had revived the Edison data structure code, and is maintaining a darcs repository, with a view to modernising the codebase. 4. http://article.gmane.org/gmane.comp.lang.haskell.general/13295 Haskell' This section covers activity on [5]Haskell' this week. * [6]The hierarchical module system * [7]Refactoring the array interface * [8]The worst syntax in Haskell * [9]Module export lists * Public/private sections [10]part 1 and [11]part 2 5. http://hackage.haskell.org/trac/haskell-prime 6. http://article.gmane.org/gmane.comp.lang.haskell.prime/619 7. http://article.gmane.org/gmane.comp.lang.haskell.prime/629 8. http://article.gmane.org/gmane.comp.lang.haskell.prime/632 9. http://article.gmane.org/gmane.comp.lang.haskell.prime/721 10. http://article.gmane.org/gmane.comp.lang.haskell.prime/741 11. http://article.gmane.org/gmane.comp.lang.haskell.prime/754 Darcs Corner * darcsweb 0.15-rc1. Alberto Bertogli [12]announced that a new version of darcsweb is available. 12. http://article.gmane.org/gmane.comp.version-control.darcs.user/9535 Contributing to HWN You can help us create new editions of this newsletter. Please see the [13]contributing information, send stories to dons -at- cse.unsw.edu.au. The darcs repository is available at: darcs get http://www.cse.unsw.edu.au/~dons/code/hwn 13. http://haskell.org/haskellwiki/HWN

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how to write an interactive program ? gui library to use ?
by minh thu 25 Feb '06

25 Feb '06

Hi all, 1/ I'd like to know how can I implement an interactive program (an editor) in haskell where some "things" have to be updated. The "things" can be text in a word processor, or a pixel array in a 2d graphics editor, and so on. Have I to pass the state (the "things") explicitely in the arguments of the main loop (a recursive function) ? Sorry if it's stupid... 2/ Another one concerning gui library: what's the best choice ? I need it to work both on linux and windows. I also need it can handle an OpenGL context. thanks a lot, minh thu

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CFP: Mathematically Structured Functional Programming
by Conor McBride 23 Feb '06

23 Feb '06

CALL FOR PAPERS Workshop on Mathematically Structured Functional Programming MSFP 2006 Kuressaare, Estonia, 2 July 2006 http://cs.ioc.ee/mpc-amast06/msfp/ a satellite workshop of MPC 2006 a "small workshop" of the TYPES project Background Something wonderful happened when monads arrived in Haskell: some human mathematics explaining the structure of certain computational phenomena became a mechanical means to implement those phenomena. It's a good way to go about functional programming - to dig out the mathematical structure underlying a problem and set it to work! There's more where monads came from, and we want it. This new workshop is about getting it. In recent years, a diverse array of mathematical structures has appeared in our programs: monads dualise to comonads and generalise to Freyd categories aka 'arrows'; 'container' types have a generalised polynomial structure supporting generic programming, not to mention a differential calculus; isomorphisms from 'high school algebra' are used to search libraries and repair type errors; coalgebras give structure to recursion; the list goes on... MSFP is broad in scope, covering the extraction of functionality from structure wherever it can be found. It complements the remit of its host conference, Mathematics of Program Construction, by seeking to enrich the language and toolset available for specifications and programs alike. It is also a "small workshop" of the FP6 IST coordination action TYPES. Invited speakers Andrzej Filinski, Københavns Universitet John Power, University of Edinburgh Important dates * Submission of papers: 10 April 2006 * Notification of authors: 8 May 2006 * Camera-ready version: 5 June 2006 Topics Submissions are welcome on, but by no means restricted to, topics from the following partially computed coinductive list: * structured effectful computation * structured recursion * structured tree and graph operations * structured syntax with variable binding * structures for datatype-genericity * structures for search * structured representations of functions * structured manipulation of mathematical structure * structured <thunk> in functional programming Authors concerned about the suitability of a topic are very welcome to contact Conor McBride, ctm(at)cs.nott.ac.uk. Submission and publication Papers in pdf not exceeding 15 pages and adhering to the eWiC style must be submitted by 10 April 2006 via an online submission webpage. Papers must report previously unpublished work and not be submitted concurrently to another conference with refereed proceedings. Accepted papers must be presented at the workshop by one of the authors. The proceedings of MSFP 2006 will be published in the Electronic Workshops in Computing (eWiC) series of the British Computer Society, http://ewic.bcs.org/. After the workshop, the authors of the best papers will be invited to submit revised and expanded versions to a special issue of the Journal of Functional Programming from Cambridge University Press. Programme committee Yves Bertot, INRIA Sophia Antipolis Marcelo Fiore, University of Cambridge Masahito Hasegawa, Kyoto University Graham Hutton, University of Nottingham Paul Levy, University of Birmingham Andres Löh, Universitt Bonn Christoph Lüth, Universitt Bremen Conor McBride, University of Nottingham (co-chair) Marino Miculan, Università degli Studi di Udine Randy Pollack, University of Edinburgh Amr Sabry, Indiana University Tarmo Uustalu, Institute of Cybernetics (co-chair) Main conferences MSFP 2006 is a satellite workshop of the 8th International Conference on Mathematics on Program Construction, MPC 2006, to take place 3-5 July. Co-located with MPC 2006, the 11th International Conference on Algebraic Methodology and Software Technology, AMAST 2006, will follow 5-8 July. Venue Kuressaare (pop. 16000) is the main town on Saaremaa, the second-largest island of the Baltic Sea. Kuressaare is a charming seaside resort on the shores of the Gulf of Riga highly popular with Estonians as well as visitors to Estonia. The scientific sessions of MPC/AMAST 2006 will take place at Saaremaa Spa Hotel Meri, one among the several new spa hotels in the town. The social events will involve a number of sites, including the 14th-century episcopal castle. Accommodation will be at Saaremaa Spa Hotels Meri and Rüütli. To get to Kuressaare and away, one must pass through Tallinn (pop. 402000), Estonia's capital city. Tallinn is famous for its picturesque medieval Old Town, inscribed on UNESCO's World Heritage List. TYPES support MSFP 2006 is an official workshop of the EU FP6 IST coordination action TYPES. Participants from TYPES sites/subsites may use project funds to cover their travel and participation. Local organizers MPC/AMAST 2006 is organized by Institute of Cybernetics, a research institute of Tallinn University of Technology. The local organizers are Tarmo Uustalu (chair), Monika Perkmann, Juhan Ernits, Ando Saabas, Olha Shkaravska, Kristi Uustalu. Contact email address of the local organizers: mpc06(at)cs.ioc.ee.

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