- Haskell-Cafe - Haskell.org

symbol $
by Maverick 08 May '07

08 May '07

Hi Do you mind tell what is mean by the symbol $ in this piece of code? Thanks a lot. > mcompile :: Prog -> State Int Code > mcompile (Assign n e) = return $ compileExpr e ++ [POP n] > mcompile (If e p1 p2) = do cur <- fresh > pp1 <- mcompile p1 > pp2 <- mcompile p2 > return $ (compileExpr e ++ (JUMPZ cur):pp1 ++ > JUMP (cur+1):(LABEL cur):pp2 ++ [LABEL (cur+1)]) > mcompile (While e p) = do cur <- fresh > mp <- mcompile p > return $ [LABEL cur] ++ compileExpr e ++ [JUMPZ (cur+1)] ++ mp ++ [JUMP cur,LABEL (cur+1)] -- View this message in context: http://www.nabble.com/symbol-%24-tf3701618.html#a10351407 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Weekly News: May 07, 2007
by dons＠cse.unsw.edu.au 07 May '07

07 May '07

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20070507 Issue 62 - May 07, 2007 --------------------------------------------------------------------------- Welcome to issue 62 of HWN, a weekly newsletter covering developments in the [1]Haskell community. This week sees the release of Atom, a hardware description language embedded in Haskell, along with the usual suite of new libraries and tools. In addition, The Monad.Reader Issue 7 was released, and the hackage upload festival continues unabated. 1. http://haskell.org/ Announcements Atom: Hardware Description in Haskell. Tom Hawkins [2]announced the release of [3]Atom, a high-level hardware description language embedded in Haskell, compiles conditional term rewriting systems into conventional HDL. 2. http://article.gmane.org/gmane.comp.lang.haskell.general/15209 3. http://www.funhdl.org/ The Monad.Reader: Issue 7. Wouter Swierstra [4]announced the latest issue of [5]The Monad.Reader. The Monad.Reader is a quarterly magazine about functional programming. It is less-formal than journal, but somehow more enduring than a wiki page or blog post. 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/22038 5. http://www.haskell.org/haskellwiki/The_Monad.Reader HDBC: Haskell Database Connectivity. John Goerzon [6]announced that [7]HDBC 1.1.2 is now released. HDBC provides an abstraction layer between Haskell programs and SQL relational databases. This lets you write database code once, in Haskell, and have it work with any number of backend SQL databases. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/15227 7. http://software.complete.org/hdbc FileManip: Expressive Filesystem Manipulation. Bryan O'Sullivan [8]announced the [9]FileManip package provides expressive functions and combinators for searching, matching, and manipulating files. 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/22090 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/FileManip-0.1 photoname: manipulate photos using EXIF data. Dino Morelli [10]announced the release of [11]photoname, a command-line utility for renaming and moving photo image files. The new folder location and naming are determined by two things: the photo shoot date information contained within the file's EXIF tags and the usually-camera-assigned serial number, often appearing in the filename. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/15187 11. http://ui3.info/d/proj/photoname.html RSA-Haskell: Command-line Cryptography. David Sankel [12]announced the release of [13]RSA-Haskell, a collection of command-line cryptography tools and a cryptography library written in Haskell. It is intended to be useful to anyone who wants to secure files or communications or who wants to incorporate cryptography in their Haskell application. 12. http://article.gmane.org/gmane.comp.lang.haskell.general/15207 13. http://www.netsuperbrain.com/rsa-haskell.html Haskell modes for Vim. Claus Reinke [14]summarised the various Haskell/Vim support currently available 14. http://article.gmane.org/gmane.comp.lang.haskell.general/15180 French Translation of Gentle Introduction to H98. The haskell-fr team [15]announced a completed a [16]translation into French of the 'Gentle Introduction to Haskell'. 15. http://article.gmane.org/gmane.comp.lang.haskell.general/15193 16. http://gorgonite.developpez.com/livres/traductions/haskell/gentle-haskell/ Haskell' This section covers the [17]Haskell' standardisation process. * [18]Polymorphic strict fields 17. http://hackage.haskell.org/trac/haskell-prime 18. http://thread.gmane.org/gmane.comp.lang.haskell.prime/2192 Hackage This week's new libraries in [19]the Hackage library database. 19. http://hackage.haskell.org/ * BitSyntax-0.2. Adam Langley. [20]A simple function for the construction of binary data. 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/BitSyntax-0.2 * filepath-1.0. Neil Mitchell. [21]Library for manipulating FilePath's in a cross platform way. 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/filepath-1.0 * Chart-2007.3.5. Tim Docker [22]A library for generating 2D Charts and Plots. 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Chart-2007.3.5 * FileManip-0.1. Bryan O'Sullivan [23]A Haskell library for working with files and directories. 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/FileManip-0.1 * hsns-0.5.2. Austin Seipp [24]A network sniffer written in a purely fun language. 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsns-0.5.2 * template-0.1. Johan Tibell [25]Simple string substitution library that supports dollar-based substitution. 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/template-0.1 * ASN1-0.0.1. Dominic Steinitz [26]ASN.1 suppport for X.509 identity and attribute certificates, PKCS8, PKCS1v15. 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ASN1-0.0.1 Discussion The Proper Definition of (evaluate :: a -> IO a). Isaac Dupree [27]described a variant of evaluate with modified semantics to the current implementation. 27. http://article.gmane.org/gmane.comp.lang.haskell.libraries/6858 Why is Data.Set not a monad?. Dan Doel [28]documented the reasons why Data.Set is not currently an instance of Monad. 28. http://article.gmane.org/gmane.comp.lang.haskell.libraries/6861 Chaos. Andrew Coppin [29]announced chaos, a fun image generating mystery program. 29. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/22177 The Functional Pearls. Don Stewart [30]collected the functional pearls known to be available online, on to a single page on the Haskell wiki. 30. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/22171 Blog noise [31]Haskell news from the blogosphere. * [32]HUG: Nested Data Parallelism in Haskell * [33]HUG: London Haskell User Group * [34]New York Functional Programmers meeting: roundup * [35]Python-style string split in Haskell * [36]Thinking in types * [37]Using the Haskell package system * [38]Fixed precision, an update * [39]Use of Text.XHtml.Strict for Outputting XHTML * [40]Idiom: Plan for Currying * [41]Bowling in Haskell * [42]The Trivial Monad * [43]Homeland Security Threat Level Monad * [44]Monads as universe helpers * [45]Understanding comonads * [46]Coding in Haskell: conciseness * [47]Functional programming in Wall Street * [48]Phantom Types for Real Problems * [49]Haskell rocks * [50]Advertising the ICFP Programming Contest * [51]Haskell wikibook * [52]Roll Your Own Window Manager: Part 1: Defining and Testing a Model * [53]Haskell and the Type Calculus: or, the Good -> Bad -> Ugliness of Types * [54]FileManip, an expressive Haskell library for manipulating files * [55]Programming in Haskell * [56]Parsing JSON in Haskell * [57]Namespace confusion * [58]A Scheme parser in Haskell * [59]Simple performance analysis * [60]Playing with Haskell unsafely * [61]repeat and sequence * [62]Haskell and C: functions returning more than one value 31. http://planet.haskell.org/ 32. http://www.londonhug.net/2007/05/01/hug-update-nested-data-parallelism-in-h… 33. http://www.phildawes.net/blog/2007/05/02/london-haskell-user-group/ 34. http://privosoft.blogspot.com/2007/05/new-york-functional-programmers-meeti… 35. http://gimbo.org.uk/blog/2007/04/20/splitting-a-string-in-haskell/ 36. http://totherme.livejournal.com/3845.html 37. http://gimbo.org.uk/blog/2007/04/27/haskell-packages-gotcha-global-vs-per-u… 38. http://augustss.blogspot.com/2007/04/fixed-precision-update-so-i-was-bit.ht… 39. http://www.movethemarkets.com/richard/2007/04/14/use-of-textxhtmlstrict-for… 40. http://www.movethemarkets.com/richard/2007/04/28/functional-programming-idi… 41. http://www.randomhacks.net/articles/2007/04/28/bowling-in-haskell 42. http://sigfpe.blogspot.com/2007/04/trivial-monad.html 43. http://sigfpe.blogspot.com/2007/04/homeland-security-threat-level-monad.html 44. http://gelisam.blogspot.com/2006/10/monads-as-universe-helpers.html 45. http://gelisam.blogspot.com/2007/04/i-understand-comonads.html 46. http://defowl.blogspot.com/2007/04/coding-in-haskell.html 47. http://codemiscellany.blogspot.com/2007/04/functional-programming-in-wall-s… 48. http://neilmitchell.blogspot.com/2007/04/phantom-types-for-real-problems.ht… 49. http://errantember.livejournal.com/50627.html 50. http://johanjeuring.blogspot.com/2007/05/advertising-icfp-programming-conte… 51. http://koweycode.blogspot.com/2007/05/haskell-wikibook-now-featured.html 52. http://cgi.cse.unsw.edu.au/~dons/blog/2007/05/01#xmonad_part1_model 53. http://disparatemathematician.blogspot.com/2007/05/haskell-and-type-calculu… 54. http://www.serpentine.com/blog/2007/05/01/filemanip-an-expressive-haskell-l… 55. http://www.regdeveloper.co.uk/2007/05/03/programming_haskell/ 56. http://notes-on-haskell.blogspot.com/2007/05/parsing-json.html 57. http://notes-on-haskell.blogspot.com/2007/05/namespaces-confusion.html 58. http://sami.samhuri.net/articles/2007/05/03/a-scheme-parser-in-haskell-part… 59. http://syntaxfree.wordpress.com/2007/05/04/lra/ 60. http://therning.org/magnus/archives/249 61. http://therning.org/magnus/archives/269 62. http://therning.org/magnus/archives/280 Quotes of the Week * Oleg K: So, `bind' is `let' and monadic programming is equivalent to programming in the A-normal form. That is indeed all there is to monads * kc5tja: Premature evil is the root of all optimization * Tommah: Remember, kids: if you program in a language with side effects, the terrorists win. * ndm: Comments are for people who can't sense what their code does from the indentation * jcreigh: GHC has lots of interesting features above Haskell98, I've noticed. 'You can take the red pill or the blue pill...' 'Hmm. What's the green pill?' 'What? Oh. That's GHC.' * schluehk: It's about a variant of the other big Haskell credo: once it compiles it works. Once you have written a prototype you have also a spec. If this is not agile I don't know what? It is a quite remarkable inversion. Formerly people wanted tools that are so versatile that they let them express almost everything with great ease and where they didn't care a lot about speed optimizations and corner cases in the early iterations. Now people want tools that restricts intentionally their expressivity to let them do big upfront design as source code. They want to be guided to initial perfection. Let's face it: Haskell has quite some momentum in the dialectic move. Code Watch Notable new features and bug fixes to the Haskell compilers. Thu May 3 06:19:55 PDT 2007. Simon Marlow. [63]Add history/trace functionality to the GHCi debugger. The debugger can now log each step of the evaluation without actually stopping, keeping a history of the recent steps (currently 50). When a (real) breakpoint is hit, you can examine previous steps in the history (and their free variables) using the :history, :back and :forward commands. 63. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/20551 Wed May 2 09:34:57 PDT 2007. Simon Peyton-Jones. [64]Make records work properly with type families. This fixes Trac #1204. There's quite a delicate interaction of GADTs, type families, records, and in particular record updates. 64. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/20522 About the Haskell Weekly News Each week, new editions are posted to [65]the Haskell mailing list as well as to [66]the Haskell Sequence and [67]Planet Haskell. [68]RSS is also available, and headlines appear on [69]haskell.org. Headlines are available as [70]PDF. To help create new editions of this newsletter, please see the [71]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [72]http://www.cse.unsw.edu.au/~dons/code/hwn 65. http://www.haskell.org/mailman/listinfo/haskell 66. http://sequence.complete.org/ 67. http://planet.haskell.org/ 68. http://sequence.complete.org/node/feed 69. http://haskell.org/ 70. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20070507.pdf 71. http://haskell.org/haskellwiki/HWN 72. http://www.cse.unsw.edu.au/~dons/code/hwn

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Re: [Haskell-cafe] Re: (Chaos) [An interesting toy]
by Andrew Coppin 07 May '07

07 May '07

>> So... if I'm not actually using lazyness for anything, would it do >> any harm to change Double to Double# in the Haskell source code? Or >> is there some reason why this would be undesirable? (The source is >> carefully constructed > > Well, you'd need -fglasgow-exts for it to even compile. Other than > that, you'd have to use all primitive operations and you might well be > preventing the compiler from doing useful optimizations. But this is > really a question for the Simons or other folks who actualy understand > the GHC internals. Hmm... that doesn't sound like such a hot idea...

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Monad and Monad transformer ???
by Maverick 07 May '07

07 May '07

hi I am a newbee to haskell so please forgive me if the question is not appropreate. I am trying to get the idea of using Monad and have to understand the following code. Can you please tell me is how different Monad and Monad transformer is? And what is mean by > newtype State s a = State { runState :: s -> (a, s) } --------------------------------- Define (State s) as a State monad > instance Monad (State s) where > return a = State $ \s -> (a,s) > m >>= k = State $ \s -> let (a,s') = runState m s in runState (k a) s' Thanks lot for your help I really do appreciate that Best regard -- View this message in context: http://www.nabble.com/Monad-and-Monad-transformer-----tf3699943.html#a10346… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Intermediate Haskell Books?
by Adrian Neumann 07 May '07

07 May '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 Are there any good books about intermediate to advanced Haskell? The descriptions here http://haskell.org/haskellwiki/Books_and_tutorials aren't very helpful. Adrian -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGPZCo11V8mqIQMRsRA8nVAJ9fs4c013qKOuEjX//uLW3hLTYtxACfTS/t 5D6I+/OGifs3ltYhx6rwFZA= =sNJP -----END PGP SIGNATURE-----

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c2hs errors when compiling hsGnuTls
by David House 07 May '07

07 May '07

Hey there, I'm getting the following errors when I try to compile hsGnuTls [1]: ~/hs/sandbox/hsgnutls $ c2hs --version C->Haskell Compiler, version 0.14.5 Travelling Lightly, 12 Dec 2005 build platform is "i486-pc-linux-gnu" <1, True, True, 1> ~/hs/sandbox/hsgnutls $ runhaskell Setup.lhs build Setup.lhs: Warning: The field "hs-source-dir" is deprecated, please use hs-source-dirs. Preprocessing library hsgnutls-0.2.3... c2hs: Error in C header file. /usr/include/bits/pthreadtypes.h:69: (column 6) [FATAL] >>> Syntax error! The symbol `;' does not fit here. Setup.lhs: got error code while preprocessing: Network.GnuTLS.GnuTLS c2hs version: I've attached the file it references in case that's relevant. Any tips on how I might address this? Thanks in advance, -David House, dmhouse(a)gmail.com

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The Monad.Reader - Call for copy
by Wouter Swierstra 07 May '07

07 May '07

Call for Copy The Monad.Reader - Issue 8 I would like to welcome articles for the next issue of The Monad.Reader. * The Monad.Reader * The Monad.Reader is a electronic magazine about all things Haskell. It is less-formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles, including: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. * Submission Details * Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue by e-mail (wss at cs.nott.ac.uk) to me before ** July 27, 2007 ** Articles should be written according to the guidelines available from http://www.haskell.org/haskellwiki/The_Monad.Reader Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out. Looking forward to your submission, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Kleene star in types
by Matthew Sackman 07 May '07

07 May '07

Apologies for sending this twice. Mutt obviously outsmarted mailman and put a useful mime type header in. Not so this one! This email is a .lhs. > {-# OPTIONS -fglasgow-exts #-} > {-# OPTIONS -fallow-undecidable-instances #-} > {-# OPTIONS -fallow-overlapping-instances #-} Let's implement the Kleene * in the type system. First some phantom types: > data At n > data Bt n > data Ct n > data Endt Now a chain to hold it together: > data Chain :: * -> * where > Ac :: (Chain n) -> Chain (At n) > Bc :: (Chain n) -> Chain (Bt n) > Cc :: (Chain n) -> Chain (Ct n) > Endc :: Chain Endt > > instance Show (Chain n) where > show (Ac n) = "Ac ~> " ++ (show n) > show (Bc n) = "Bc ~> " ++ (show n) > show (Cc n) = "Cc ~> " ++ (show n) > show (Endc) = "Endc" Now, first things first, implement identity. I don't really care about the actual functions right now, but let's put it in anyway. > class IdOp a b | a -> b, b -> a where > idOp :: a -> b > instance IdOp (Chain Endt) (Chain Endt) where > idOp x = x > instance (IdOp (Chain l) (Chain r)) => > IdOp (Chain (At l)) (Chain (At r)) where > idOp (Ac n) = Ac (idOp n) > instance (IdOp (Chain l) (Chain r)) => > IdOp (Chain (Bt l)) (Chain (Bt r)) where > idOp (Bc n) = Bc (idOp n) > instance (IdOp (Chain l) (Chain r)) => > IdOp (Chain (Ct l)) (Chain (Ct r)) where > idOp (Cc n) = Cc (idOp n) Now, I'm sure many of you are wondering why I didn't just write: instance IdOp a a where idOp = id I'll get back to that. Now, if I go zero steps, how hard is it to go zero or one steps? > class ZeroOneStep a b where > step :: a -> b > instance ZeroOneStep (Chain (At l)) (Chain l) where > step (Ac n) = n > instance ZeroOneStep (Chain (Bt l)) (Chain l) where > step (Bc n) = n > instance ZeroOneStep (Chain (Ct l)) (Chain l) where > step (Cc n) = n > instance (IdOp (Chain l) (Chain r)) => > ZeroOneStep (Chain l) (Chain r) where > step = idOp That's quite nifty. I now need the allow-overlapping-instances, but it works fine: > fooZeroOne :: (ZeroOneStep a b) => a -> b -> Bool > fooZeroOne _ _ = True $ Main> fooZeroOne (Ac $ Bc Endc) (Ac $ Bc Endc) $ True $ Main> fooZeroOne (Ac $ Bc Endc) (Bc Endc) $ True $ Main> fooZeroOne (Ac $ Bc Endc) (Endc) $ ...big exposion... $ Main> fooZeroOne (Endc) (Bc Endc) $ ...big exposion... $ Main> fooZeroOne (Endc) (Endc) $ True So now I want zero or more steps, i.e. the Kleene *: > class ZeroMoreSteps a b where > stepN :: a -> b > instance (ZeroMoreSteps (Chain l) (Chain r)) => > ZeroMoreSteps (Chain (At l)) (Chain r) where > stepN (Ac n) = stepN n > instance (ZeroMoreSteps (Chain l) (Chain r)) => > ZeroMoreSteps (Chain (Bt l)) (Chain r) where > stepN (Bc n) = stepN n > instance (ZeroMoreSteps (Chain l) (Chain r)) => > ZeroMoreSteps (Chain (Ct l)) (Chain r) where > stepN (Cc n) = stepN n I.e. if we can already get from l to r then it's not very hard to go one step further. Now I need to bring in single step case: > instance (ZeroOneStep (Chain l) (Chain r)) => > ZeroMoreSteps (Chain l) (Chain r) where > stepN = step So now it should be done right? Because ZeroOneStep allows for zero steps (via IdOp) so that should be it right? > fooZeroMore :: (ZeroMoreSteps a b) => a -> b -> Bool > fooZeroMore _ _ = True $ Main > fooZeroMore Endc Endc $ True $ Main > fooZeroMore (Ac Endc) Endc $ True $ Main > fooZeroMore (Ac Endc) (Ac Endc) $ ...big exposion... Now what I think is happening is this: the rules for overlapping instances say that the instance which is chosen is the one that specifies the most information matching the particular context. In this case, I think it's choosing and trying to use the "wrong" instances because of this rule. Even if I try: > instance ZeroMoreSteps (Chain l) (Chain l) where > stepN = id It still blows up. So I have to do it really manually: > instance ZeroMoreSteps (Chain Endt) (Chain Endt) where > stepN = id > instance ZeroMoreSteps (Chain (At l)) (Chain (At l)) where > stepN = id > instance ZeroMoreSteps (Chain (Bt l)) (Chain (Bt l)) where > stepN = id > instance ZeroMoreSteps (Chain (Ct l)) (Chain (Ct l)) where > stepN = id $ Main > fooZeroMore (Ac Endc) (Ac Endc) $ True Now it works. So am I right - is the reason for this the selection of the instance to use given overlapping instances? If so, can anything be done to make it "better"? I can think of two ideas, which I suspect should both be laughed out of town asap: i) backtrack - if you can't make the instance work then go back to where you last had a choice and try the next instance; ii) whilst selecting the instance which is most specific seems like the "sensible" choice, there are cases, eg guards, where the vertical order in the program determines the ordering of things to try. Maybe allow the programmer to specify an order to try things in? Note however how cool this now is: > data Cell :: * -> * where > Cell :: x -> Cell a -- muh ha Ha HAA! > data Section :: * -> * -> * where > SectionTo :: (ZeroMoreSteps a b) => > (Cell a) -> b -> Section a b > SectionFrom :: (ZeroMoreSteps a b) => > a -> (Cell b) -> Section a b > fooCoolness :: (ZeroMoreSteps a b) => > a -> Section a b -> Section a b -> b > fooCoolness chain _ _ = stepN chain $ Main > fooCoolness (Bc . Ac $ Endc) (SectionTo (Cell True) (Ac $ Endc)) (SectionFrom (Bc . Ac $ Endc) (Cell 1)) $ Ac ~> Endc And stepN just goes the "right" number of steps. The point is that I don't need to use pattern matching to decompose the various constructorsof Section in order to find (and perhaps fail) the unused section of the chain. Cheers, Matthew

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Kleene star in types
by Matthew Sackman 07 May '07

07 May '07

This email is a .lhs. > {-# OPTIONS -fglasgow-exts #-} > {-# OPTIONS -fallow-undecidable-instances #-} > {-# OPTIONS -fallow-overlapping-instances #-} Let's implement the Kleene * in the type system. First some phantom types: > data At n > data Bt n > data Ct n > data Endt Now a chain to hold it together: > data Chain :: * -> * where > Ac :: (Chain n) -> Chain (At n) > Bc :: (Chain n) -> Chain (Bt n) > Cc :: (Chain n) -> Chain (Ct n) > Endc :: Chain Endt > > instance Show (Chain n) where > show (Ac n) = "Ac ~> " ++ (show n) > show (Bc n) = "Bc ~> " ++ (show n) > show (Cc n) = "Cc ~> " ++ (show n) > show (Endc) = "Endc" Now, first things first, implement identity. I don't really care about the actual functions right now, but let's put it in anyway. > class IdOp a b | a -> b, b -> a where > idOp :: a -> b > instance IdOp (Chain Endt) (Chain Endt) where > idOp x = x > instance (IdOp (Chain l) (Chain r)) => > IdOp (Chain (At l)) (Chain (At r)) where > idOp (Ac n) = Ac (idOp n) > instance (IdOp (Chain l) (Chain r)) => > IdOp (Chain (Bt l)) (Chain (Bt r)) where > idOp (Bc n) = Bc (idOp n) > instance (IdOp (Chain l) (Chain r)) => > IdOp (Chain (Ct l)) (Chain (Ct r)) where > idOp (Cc n) = Cc (idOp n) Now, I'm sure many of you are wondering why I didn't just write: instance IdOp a a where idOp = id I'll get back to that. Now, if I go zero steps, how hard is it to go zero or one steps? > class ZeroOneStep a b where > step :: a -> b > instance ZeroOneStep (Chain (At l)) (Chain l) where > step (Ac n) = n > instance ZeroOneStep (Chain (Bt l)) (Chain l) where > step (Bc n) = n > instance ZeroOneStep (Chain (Ct l)) (Chain l) where > step (Cc n) = n > instance (IdOp (Chain l) (Chain r)) => > ZeroOneStep (Chain l) (Chain r) where > step = idOp That's quite nifty. I now need the allow-overlapping-instances, but it works fine: > fooZeroOne :: (ZeroOneStep a b) => a -> b -> Bool > fooZeroOne _ _ = True $ Main> fooZeroOne (Ac $ Bc Endc) (Ac $ Bc Endc) $ True $ Main> fooZeroOne (Ac $ Bc Endc) (Bc Endc) $ True $ Main> fooZeroOne (Ac $ Bc Endc) (Endc) $ ...big exposion... $ Main> fooZeroOne (Endc) (Bc Endc) $ ...big exposion... $ Main> fooZeroOne (Endc) (Endc) $ True So now I want zero or more steps, i.e. the Kleene *: > class ZeroMoreSteps a b where > stepN :: a -> b > instance (ZeroMoreSteps (Chain l) (Chain r)) => > ZeroMoreSteps (Chain (At l)) (Chain r) where > stepN (Ac n) = stepN n > instance (ZeroMoreSteps (Chain l) (Chain r)) => > ZeroMoreSteps (Chain (Bt l)) (Chain r) where > stepN (Bc n) = stepN n > instance (ZeroMoreSteps (Chain l) (Chain r)) => > ZeroMoreSteps (Chain (Ct l)) (Chain r) where > stepN (Cc n) = stepN n I.e. if we can already get from l to r then it's not very hard to go one step further. Now I need to bring in single step case: > instance (ZeroOneStep (Chain l) (Chain r)) => > ZeroMoreSteps (Chain l) (Chain r) where > stepN = step So now it should be done right? Because ZeroOneStep allows for zero steps (via IdOp) so that should be it right? > fooZeroMore :: (ZeroMoreSteps a b) => a -> b -> Bool > fooZeroMore _ _ = True $ Main > fooZeroMore Endc Endc $ True $ Main > fooZeroMore (Ac Endc) Endc $ True $ Main > fooZeroMore (Ac Endc) (Ac Endc) $ ...big exposion... Now what I think is happening is this: the rules for overlapping instances say that the instance which is chosen is the one that specifies the most information matching the particular context. In this case, I think it's choosing and trying to use the "wrong" instances because of this rule. Even if I try: > instance ZeroMoreSteps (Chain l) (Chain l) where > stepN = id It still blows up. So I have to do it really manually: > instance ZeroMoreSteps (Chain Endt) (Chain Endt) where > stepN = id > instance ZeroMoreSteps (Chain (At l)) (Chain (At l)) where > stepN = id > instance ZeroMoreSteps (Chain (Bt l)) (Chain (Bt l)) where > stepN = id > instance ZeroMoreSteps (Chain (Ct l)) (Chain (Ct l)) where > stepN = id $ Main > fooZeroMore (Ac Endc) (Ac Endc) $ True Now it works. So am I right - is the reason for this the selection of the instance to use given overlapping instances? If so, can anything be done to make it "better"? I can think of two ideas, which I suspect should both be laughed out of town asap: i) backtrack - if you can't make the instance work then go back to where you last had a choice and try the next instance; ii) whilst selecting the instance which is most specific seems like the "sensible" choice, there are cases, eg guards, where the vertical order in the program determines the ordering of things to try. Maybe allow the programmer to specify an order to try things in? Note however how cool this now is: > data Cell :: * -> * where > Cell :: x -> Cell a -- muh ha Ha HAA! > data Section :: * -> * -> * where > SectionTo :: (ZeroMoreSteps a b) => > (Cell a) -> b -> Section a b > SectionFrom :: (ZeroMoreSteps a b) => > a -> (Cell b) -> Section a b > fooCoolness :: (ZeroMoreSteps a b) => > a -> Section a b -> Section a b -> b > fooCoolness chain _ _ = stepN chain $ Main > fooCoolness (Bc . Ac $ Endc) (SectionTo (Cell True) (Ac $ Endc)) (SectionFrom (Bc . Ac $ Endc) (Cell 1)) $ Ac ~> Endc And stepN just goes the "right" number of steps. The point is that I don't need to use pattern matching to decompose the various constructorsof Section in order to find (and perhaps fail) the unused section of the chain. Cheers, Matthew

1 0

RE: [Haskell] Monad and Monad transformer???
by Simon Peyton-Jones 07 May '07

07 May '07

As someone mentioned, don't worry about being new to Haskell, but DO send your messages to haskell-cafe(a)haskell.org, not the main Haskell list. I'm redirecting this so that the Helpful Denizens of the Haskell Cafe can help you. Simon | -----Original Message----- | From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Duong Thai Ha | Sent: 06 May 2007 17:03 | To: haskell(a)haskell.org | Subject: [Haskell] Monad and Monad transformer??? | | hi | I am a newbee to haskell so please forgive me if the question is not | appropreate. | | I am trying to get the idea of using Monad and have to understand the | following code. | | Can you please tell me is how different Monad and Monad transformer is? | And what is mean by | | > newtype State s a = State { runState :: s -> (a, s) } | | --------------------------------- | | Define (State s) as a State monad | | > instance Monad (State s) where | > return a = State $ \s -> (a,s) | > m >>= k = State $ \s -> let (a,s') = runState m s in runState (k | a) s' | | Thanks lot for your help | I really do appreciate that | | Best regard | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell

1 0

GADTs, type classes, existentials
by Mike Hamburg 07 May '07

07 May '07

Hello all, I'm trying to build a variation on Maps which supports a fast "concat-like" operation, for a library I'm writing. I'd rather not re-implement Data.Map, so I'm having a try with GADTs. The relevant part of my source file: ------------------------------------------------------------------------- data MetaMap k v where Map :: Ord k => Map.Map k v -> MetaMap k v Cat :: MetaMap k1 (MetaMap k2 v) -> MetaMap (k1,k2) v {- other, similar constructors -} lookup :: Monad m => k -> MetaMap k v -> m v lookup k (Map m) = Map.lookup k m lookup k (Cat m) = case k of (k1,k2) -> lookup k1 m >>= lookup k2 ------------------------------------------------------------------------- Unfortunately, this doesn't work: Data/MetaMap.hs:45:20: Could not deduce (Ord k) from the context (Monad m) arising from use of `Map.lookup' at Data/MetaMap.hs:45:20-33 Possible fix: add (Ord k) to the type signature(s) for `lookup' In the expression: Map.lookup k m In the definition of `lookup': lookup k (Map m) = Map.lookup k m This error happens because the Ord constraint isn't propagated into the function body. Of course, I can't add Ord k to the type signature for lookup, because one can't deduce Ord k from Ord (k1,k2). Is there a clean way around this error? Thanks for your time, Mike Hamburg

4 4

Mounting haskell.org wiki under WikipediaFS?
by dons＠cse.unsw.edu.au 07 May '07

07 May '07

Anyone tried editing haskell.org's wiki as text, using: http://wikipediafs.sourceforge.net/ "WikipediaFS is a mountable Linux virtual file system that enables you to deal with Wikipedia (or any Mediawiki-based site) articles as if they were real files. It is thus possible to use a real text editor to view and edit articles." Seems like this might make some of the larger edit passes a lot easier. -- Don

2 1

Is Excel a FP language?
by Tony Morris 07 May '07

07 May '07

In a debate I proposed "Excel is a functional language". It was refuted and I'd like to know what some of you clever Haskellers might think :) My opposition proposed (after some "weeding out") that there is a distinction between Excel, the application, the GUI and Excel, the language (which we eventually agreed (I think) manifested itself as a .xls file). Similarly, VB is both a language and a development environment and referring to VB is a potential ambiguity. I disagree with this analogy on the grounds that the very definition of Excel (proposed by Microsoft) makes no distinction. Further, it is impossible to draw a boundary around one and not the other. I also pointed to the paper by Simon Peyton-Jones titled, "Improving the world's most popular functional language: user-defined functions in Excel", which quite clearly refers to Excel as a [popular] functional language. The debate started when I referred to the fact that financial institutions change their functional language from Excel to something like OCaml or Haskell. Of course, there is no doubting that these companies can replace their entire use of Excel with a functional language, which I think is almost enough to fully support my position (emphasis on "almost"). -- Tony Morris http://tmorris.net/

14 15

Silly mail client
by Andrew Coppin 07 May '07

07 May '07

OK, this is hacking me off now... Does ANYBODY know how I can convince Thunderbird to send replies to Haskell Cafe rather than sending them to the original poster? This is really becoming tiresome...

7 7

Topoi, the categorical Analysis of Logic
by Philippe de Rochambeau 07 May '07

07 May '07

Hello, has anybody on this list read Goldblatt's "Topoi, the categorical Analysis of Logic"? Did reading that book make you a better Haskell programmer? If so, how? Cheers, phiroc

2 1

Any Haskellers in St Louis, MO?
by Aditya Siram 06 May '07

06 May '07

Hi there.. Are there any Haskellers in St Louis , MO? Is there an active group here? Want to start one? Thanks... Deech _________________________________________________________________ Exercise your brain! Try Flexicon. http://games.msn.com/en/flexicon/default.htm?icid=flexicon_hmemailtaglineap…

7 8

Re: [Haskell-cafe] Chaos
by Andrew Coppin 06 May '07

06 May '07

Just so people know... The version in Darcs now has strict Colour and Vector constructors. (Makes it run a few percent faster.)

1 0

[IO Int] -> IO [Int]
by Phlex 06 May '07

06 May '07

Hello all, I'm trying to learn haskell, so here's is my first newbie question. I hope this list is appropriate for such help requests. I'm trying to write a function with the signature [IO Int] -> IO [Int] Here is my first attempt : conv :: [IO Int] -> IO [Int] conv l = do val <- (head l) return (val : (conv (tail l))) This does not work as I'm consing an Int to an (IO [Int]). So I tried this : conv2 :: [IO Int] -> IO [Int] conv2 l = do val <- (head l) rest <- (conv2 (tail l)) return (val : rest) That works, but it won't work for infinite lists. How could I achieve the desired result ? Thanks in advance, Sacha

6 5

The Functional Pearls
by dons＠cse.unsw.edu.au 06 May '07

06 May '07

I've created a wiki page collecting the 'functional pearl' papers that have appeared in JFP and ICFP and other places over the last 20 odd years. http://haskell.org/haskellwiki/Research_papers/Functional_pearls Lots of lovely functional programs there. There's also a list on that page of pearls that don't appear to be online. If you know where they live, please add the links! Cheers, Don

4 5

Displaying infered type signature of 'offside' functions
by Georg Sauthoff 06 May '07

06 May '07

Hi, I like the strong static type system of Haskell for various reasons. One reason is, that it makes easier to understand new code. I.e. when I read code I type ':t foo' in ghci/hugs from time to time, to check my own idea of the type signature, if it is not included in the source code. Well, ':t ' doesn't work withwith non-top-level/local functions (or does it?). Thus I searched for a way to get this information for functions, which are defined offside (of the main indentation level). So, what tools do you use to get the inferred type signature of local functions? I first looked at the GHC-API, but I found this comment in http://darcs.haskell.org/ghc/compiler/main/GHC.hs : - NOTE: -- - things that aren't in the output of the typechecker right now: [..] -- - type signatures Hm, is this comment in sync with the current GHC source? Then I looked at the ghc options. '-ddump-types' only dumps the signatures of global functions ... But '-ddump-tc' includes type signatures of offside defined functions. I.e. something like ghc -ddump-tc Foo.lhs 2>&1 \ | awk '/^[A-Z].+lhs/, ! // { print $0; }' \ | awk '/ :: / , /\[\]/ { if (!($0 ~ /\[\]/)) print $0; }' \ | less does the trick. But as the haskell/ghc wiki mentions, the -ddump-* output is optimized for human reading and not for some kind of 'automatic use'. For automatic use one should look at the GHC-API ... Well, I mention this, because I would like to integrate some lookup feature (for type signatures) into vim (if it doesn't exist yet). Best regards Georg Sauthoff

9 14

Problems with Hs-Plugins
by Philipp Volgger 06 May '07

06 May '07

I tried following very simple program: > module Main where > import System.Eval.Haskell > > main = do i <- eval "1 + 6 :: Int" [] :: IO (Maybe Int) > if isJust i then putStrLn (show (fromJust i)) else return () I compile it with "ghc -c Main.hs" and everything seems fine. When I run it with "ghc Main.o" I get following message: > Main.o(.text+0x48):fake: undefined reference to > `AltDataziTypeable_zdfTypeableInt_closure' > Main.o(.text+0x4d):fake: undefined reference to > `SystemziEvalziHaskell_eval_closure' > Main.o(.text+0x36f):fake: undefined reference to > `__stginit_SystemziEvalziHaskell_' > Main.o(.rodata+0x0):fake: undefined reference to > `SystemziEvalziHaskell_eval_closure' > Main.o(.rodata+0x4):fake: undefined reference to > `AltDataziTypeable_zdfTypeableInt_closure' > collect2: ld returned 1 exit status When I run it with "runhaskell Main" I get: > > GHCi runtime linker: fatal error: I found a duplicate definition for > symbol > _GHCziWord_fromEnum1_closure > whilst processing object file > c:/ghc/ghc-6-4-2/ghc-6.4.2/HSbase1.o > This could be caused by: > * Loading two different object files which export the same symbol > * Specifying the same object file twice on the GHCi command line > * An incorrect `package.conf' entry, causing some object to be > loaded twice. > GHCi cannot safely continue in this situation. Exiting now. Sorry. So I did probably something totally wrong, but what?

2 1

a question concerning type constructors
by Eric 06 May '07

06 May '07

Hi all, In Haskell, is it possible to declare a type constructor with a variable number of type variables e.g. data Tuple * .... allowing the following declarations: t: Tuple u: Tuple Bool v: Tuple Bool Int w: Tuple Bool Int Char ? E.

3 2

Detect Either Windows or Linux environment
by SevenThunders 05 May '07

05 May '07

Is there a simple way to detect what operating system a given Haskell program is running under? It would help to make some programs that might have to interact with the operating system more portable. I haven't been able to quite figure out a simple way to do this. -- View this message in context: http://www.nabble.com/Detect-Either-Windows-or-Linux-environment-tf3695277.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

5 5

Re: Monad definition question
by oleg＠pobox.com 05 May '07

05 May '07

Ilya Tsindlekht wrote > > It may be useful to relate to imperative programming: > > m1 >>= (\x -> m2) > > is > > let x = m1 in m2 > The analogy is not always straight-forward - try the list monad. This equivalence holds even for the List Monad. Here is an example of non-determinism: http://caml.inria.fr/pub/ml-archives/caml-list/2007/02/1d4df471019050b89e20… and here's an excerpt from that message, showing lets (aka bind): let numbers = List.map (fun n -> (fun () -> n)) [1;2;3;4;5];; let pyth () = let (v1,v2,v3) = let i = amb numbers in let j = amb numbers in let k = amb numbers in if i*i + j*j = k*k then (i,j,k) else failwith "too bad" in Printf.printf "got the result (%d,%d,%d)\n" v1 v2 v3;; Here, 'amb' is like 'msum' and any error is mzero. If we replace 'let' with 'do', and '=' with '<-' in some places, it looks almost like Haskell. Although the scheduler (the `run' function of the `monad') described in the message accumulates all possible worlds, it returns as soon as one `thread' made it successfully to the end. It is trivial to get the scheduler to run the remaining `treads' searching for more answers (by threads I certainly don't mean OS threads). The user code above stays as it is. The implementation of amb has been extended to annotate choices (and so possible worlds) with probabilities, which propagate in due course. The result not merely lists the answers but also their computed probabilities. > > Filinski then showed that the latter seemingly missing aspect > > indeed only appears to be missing. > Would this require some kind of macros doing extensive pre-processing > of the code? Hopefully the code above showed that no macros and no preprocessing required. In that code, addition, multiplication, if-then-else and all other OCaml operations remain as they have always been, with no modifications or redefinitions whatsoever. Delimited continuations suffice for everything, as Filinski proved in his paper.

1 0

Re: Monad definition question
by oleg＠pobox.com 05 May '07

05 May '07

Ilya Tsindlekht wrote: > Does the definition of monad silently assume that if f and f' are equal > in the sense that they return the same value for any argument o correct > type then m >>= f = m >>= f' Of course NOT! Here's an example, in a State monad f x = put True f' x = put False Clearly, _by the definition above_, f and f' are the same -- for any argument of correct type, they return the same value, namely, (). However, if we perform the observation execState (return 'a' >>= f) True execState (return 'a' >>= f') True we quite clearly see the difference. Robin Green wrote: > How could it be otherwise? How are you going to distinguish between f > and f' if they are indistinguishable functions, in Haskell? Because f and f' are NOT referentially transparent functions. They are NOT pure functions, their application may have _an effect_. And comparing effectful computations is quite difficult. It's possible: I believe (bi)simulation is the best approach; there are other approaches. It may be useful to relate to imperative programming: m1 >>= (\x -> m2) is let x = m1 in m2 Indeed, monadic 'bind' is *exactly* equivalent to 'let' of impure eager languages such as ML. The first monadic law return x >>= f === f x is trivial because in eager languages, any value (which is an effectful-free computation) is by default injected into the world of possibly effectful expressions: Any value is an expression. The second law m >>= (\x -> return x) === m becomes let x = e in x === e and the third law (m1 >>= (\x -> m2)) >>= (\y -> m3) === m1 >>= (\x -> m2 >>= \y -> m3) provided x is not free in m3 becomes let y = (let x = m1 in m2) in m3 === let x = m1 in let y = m2 in m3 So, `bind' is `let' and monadic programming is equivalent to programming in the A-normal form. That is indeed all there is to monads. Here's the paragraph from the first page of Filinski's `Representing Monads' (POPL94) "It is somewhat remarkable that monads have had no comparable impact on ``impure'' functional programming. Perhaps the main reason is that -- as clearly observed by Moggi, but perhaps not as widely appreciated in the ``purely functional'' community -- the monadic framework is already built into the semantic core of eager functional languages with effects, and need not be expressed explicitly. ``Impure'' constructs, both linguistic (e.g., updatable state, exceptions, or first-class continuations) and external to the language (I/O, OS interface, etc.), all obey a monadic discipline. The only aspect that would seem missing is the ability for programmers to use their own, application-specific monadic abstractions -- such as nondeterminism or parsers [31] -- with the same ease and naturality as built-in effects." Filinski then showed that the latter seemingly missing aspect indeed only appears to be missing. It is important to understand that once we come to monads, we lost referential transparency. Monadic code is more difficult to reason about -- as any imperative code. One often sees the slogan that Haskell is the best imperative language. And with monad, it is. One often forgets that 'best' here has the down-side. Haskell amplifies both advantages _and_ disadvantages of imperative programming. At least imperative programmers don't have to think about placing seq at the right place to make sure a file is read from before it is closed, and don't have to think about unsafeInterleaveIO. It seems the latter has become so indispensable that it is recommended to Haskell novices without a second thought. One may wonder if functional programming still matters.

2 1

Re: [Haskell-cafe] Debuggingy
by Monang Setyawan 05 May '07

05 May '07

On 5/5/07, Stefan O'Rear <stefanor(a)cox.net> wrote: > On Sat, May 05, 2007 at 11:36:16AM +0700, Monang Setyawan wrote: > > Hi, I'm a beginner Haskell user. > > > > Is there any way to trace/debug the function application in GHC? > > Absolutely! > > stefan@stefans:/tmp$ ghci X.hs > ___ ___ _ > / _ \ /\ /\/ __(_) > / /_\// /_/ / / | | GHC Interactive, version 6.7.20070502, for Haskell 98. > / /_\\/ __ / /___| | http://www.haskell.org/ghc/ > \____/\/ /_/\____/|_| Type :? for help. > > Loading package base ... linking ... done. > [1 of 1] Compiling Main ( X.hs, interpreted ) > Ok, modules loaded: Main. > *Main> :break fac Great!! Thanks, it really helps. I should update my GHC to the newest version (I use the old 6.4.2 with no break command) Is there any editor/IDE supporting this break command? It should be cooler if we can debug functions just by placing mark in the line. > > Stefan > -- Demi masa.. -- Demi masa..

1 0

Debugging
by Monang Setyawan 05 May '07

05 May '07

Hi, I'm a beginner Haskell user. Is there any way to trace/debug the function application in GHC? -- Demi masa..

2 1

Re: [Haskell-cafe] [IO Int] -> IO [Int]
by Daniel Fischer 05 May '07

05 May '07

> -----Ursprüngliche Nachricht----- > Von: "Rich Neswold" <rich.neswold(a)gmail.com> > Gesendet: 04.05.07 19:31:53 > An: Phlex <Phlex(a)telenet.be> > CC: haskell-cafe(a)haskell.org > Betreff: Re: [Haskell-cafe] [IO Int] -> IO [Int] On 5/4/07, Phlex <Phlex(a)telenet.be> wrote: > Hello all, > > I'm trying to learn haskell, so here's is my first newbie question. > I hope this list is appropriate for such help requests. > > I'm trying to write a function with the signature [IO Int] -> IO [Int] > > > Control.Monad has a function (called "sequence") that does this for you. In fact, "sequence" is a more generic solution since its signature is ( > Monad m => [m a] -> m [a]). Unfortunately, this won't work for infinite lists either, for those you'd need an 'unsafe' action, namely 'unsafeInterleaveIO', like import System.IO.Unsafe conv :: [IO a] -> IO [a] conv [] = return [] conv (x:xs) = do a <- x as <- unsafeInterleaveIO (conv xs) return (a:as) *Test> :set -fno-print-bind-result *Test> xs <- conv $ map return [1 .. ] *Test> take 20 xs [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] > > As a newbie, I found it educational to peruse the various modules in "http://haskell.org/ghc/docs/latest/html/libraries/ > ". Just start looking at modules that sound interesting and see what has already been defined. Some modules at first may be too advanced, but if you go back to them in a few days (weeks?), they'll start making more sense, too. > Very good advice, I think, and it's also very instructive to read the source code, you can learn a lot from that. > > -- > Rich HTH, Daniel

2 1

Poor first impression
by Fernando Cassia 05 May '07

05 May '07

I admit in shame never having heard about Haskell before. I know about PHP, Python, IBM' s REXX, TCL, TCL/TK, perl... but Haskell, never. So, here's how I landed in Haskell-land: I was looking for a simple ncurses-based text mode mp3 player with some sort of basic GUI and found "HMP3" written in, you guessed it, Haskell. So I follow the directions and download the huge 30MB+ ghc-6.6.1-i386-unknown-linux.tar.bz2. Bunzip2 it. tar xvf it. ./configure and "make install". So far, so good. and I get the following message, supposedly telling me that the haskell compiler was installed OK... ======================================================================= Installation of ghc-6.6.1 was successful. To use, add /usr/local/bin to your PATH. For documentation, see /usr/local/share/ghc-6.6.1/html/index.html ======================================================================= (/usr/local/bin is already in my path) So I decide to call the ghc compiler with no arguments to see if it was indeed installed, and I get this: [root@localhost ghc-6.6.1]# ghc /usr/local/lib/ghc-6.6.1/ghc-6.6.1: error while loading shared libraries: libreadline.so.4: cannot open shared object file: No such file or directory # So, I conclude that Haskell is not ready for prime time, if it cannot install itself correclty including shared libs in a standard Fedora Core 6 system. Goodbye Haskell, I just wanted to compile a MP3 player, and perhaps if the compiler installed OK with no issues, I'd have taken a look at the language. But as of right now, I don't have time to waste with broken compiler installers. Byebye FC

19 29

outputing .hs file without comments
by José Miguel Vilaça 05 May '07

05 May '07

Hi all, Is there a simple tool or command to remove all comments from a Haskell file, i.e. something that outputs the input file but without any comments on it? Best Miguel Vilaça

4 3

Hugs dropped Text.Regex?!
by John Goerzen 04 May '07

04 May '07

I learned today, as I tried to run my ftphs unit tests under Hugs, that Hugs dropped Text.Regex between 200503.08 and 200609.21. The Hugs download page claims that they did this because the new GHC implementation of Text.Regex is not compatible with Hugs. But why can't Hugs keep the old implementation? This is going to render Hugs just about useless for me. I like regexps. -- John

2 1

Monad definition question
by Ilya Tsindlekht 04 May '07

04 May '07

Does the definition of monad silently assume that if f and f' are equal in the sense that they return the same value for any argument o correct type then m >>= f = m >>= f' More specifically, the definition says x >>= return = x. How does one justify from this that x >>= (return . id) = x? Are values of type a -> b in general assumed to be maps from the set of values of type a into the set ov values of type b? (What bothers me is that the problem whether two lambda-expressions define the same map is clearly undecidable.) More generally, is some kind of logic without equality more appropriate for formalisation of Haskell then the usual kind(s) of logic with equality?

2 1

"Programming in Haskell" made it to RegDeveloper
by dons＠cse.unsw.edu.au 04 May '07

04 May '07

http://www.regdeveloper.co.uk/2007/05/03/programming_haskell/ Mmm... mainstream exposure. -- Don

1 0

Any Haskellers in Chicagoland?
by Joshua Ball 04 May '07

04 May '07

> I'd love to post an ANN: Chicago Haskell user group, but i want to > make sure there's more than one of me. > > -johnnnnnnn I live in the Northwest Suburbs of Chicago (specifically Wheaton), and I would LOVE to join a Chicago Haskell user group.

1 0

Any Haskellers in Chicagoland?
by John Melesky 03 May '07

03 May '07

I'd love to post an ANN: Chicago Haskell user group, but i want to make sure there's more than one of me. -johnnnnnnn

2 1

'Proper' use of the State monad
by Denis Volk 03 May '07

03 May '07

Hello all, I am trying to make a (turn-based) game in Haskell and need to pass around quite a bit of information, so using the State monad seems most appropriate. My question is, which is a better idea: 1) Using State GameState r and then call execState for each game event (i.e. user input) so I can do IO 2) Using StateT GameState IO () and have the entire game live in one big execStateT call. (I note XMonad does something similar.) There are difficulties with the first option, including keeping even more state about what we're doing (for instance, are we in a menu?), and adding stuff later would possibly require substantial rewrites. Other than the fact that I would have IO in places where it perhaps shouldn't be, the second option seems better, but I am new to all this and it may be that I'm missing something.

8 8

Bloom Filter
by tom 03 May '07

03 May '07

Hi all, I'm pretty new to Haskell, I've been working on a Bloom filter[1] implementation as a learning exercise. I'd really appreciate it if someone more experienced would comment on the code. I'm sure there's plenty of places where I'm doing things in silly or overly complex ways. I've packaged it all with Cabal because I wanted to see how that all worked, I don't think it's ready for release yet! I would like to at some point get it polished up and release it if anyone thinks it might be useful. You can download it at: http://www.almostobsolete.net/bloom-0.0.tar.gz I've also put the Haddock docs for it online at: http://www.almostobsolete.net/doc/html/Data-yBloom.html All comments will be very much appreciated :p Thanks Tom [1] There's a nice description of what a Bloom filter is here: http://www.cs.wisc.edu/~cao/papers/summary-cache/node8.html

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FW: [Haskell-cafe] RE:Cross-over from Haskell.org [Haskell] Re: Newbie: what are the advantages of Haskell?
by Taillefer, Troy (EXP) 03 May '07

03 May '07

These Haskell lists seems to have a problem of having to many elitist pricks on it admittedly I am probably in this category as well so I will help you guys and by eliminating one prick and remove myself from the list good luck with the rest of them Troy -----Original Message----- From: u.stenzel(a)web.de [mailto:u.stenzel@web.de] Sent: Tuesday, May 01, 2007 5:35 PM To: Taillefer, Troy (EXP) Subject: Re: [Haskell-cafe] RE:Cross-over from Haskell.org [Haskell] Re: Newbie: what are the advantages of Haskell? Taillefer, Troy (EXP) wrote: > I really dislike Perl as a programming language but I have to strongly > disagree about your statements about CPAN and the quality of its > contents. Consider Sturgeon's Law: 90% of everything is crap. It's true of CPAN, too, which is only a stand in for any large collection of libraries. We don't gain anything from wrapping or reimplementing the crap, therefore, if you like some particular library, you should ask for *that* library, nor for more libraries in general. no need to get all touchy-feely about this. > Perl is popular so it must have some merit. So is Crack. I still won't smoke it, though. > I don't subscribe to the > flawed reasoning that Perl Hackers just don't know any better or that > they are dumb, or intellectual inferior in some way. Well, I didn't make that point in the mail you're replying to, but I subscribe to it. Perl *is* unadultered crap, it *is* a bad idea carried to perfection, a shoddy language that teaches bad habits, rewards stupidity and encourages you to attack every problem with the same blunt old tool, the regex, and most Perl Hackers, those who voluntarily use it to solve actual problem, *are* in fact dumb, don't want to know any better and are resistant to education. Perl is in every way the opposite of Haskell, and I happen to like Haskell. Imitating Perl is even worse than imitating C++. ...but I don't even wan't to discuss this, neither in private nor on a mailing list. If you want to argue, ask Google for Erik Naggum; nothing more needs to be said about it. > ... I am pro choice. And I *choose* to belittle Perl Hackers as much as I want. If that stops anyone from using Perl, it's their *choice*. And you can *choose* to hate me for belittling you, jackass. Political Correctness is even more misguided than Perl.

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Cabal, lib and exe in one
by Magnus Therning 03 May '07

03 May '07

I'm trying to create a single cabal file containing specs for both a library and an executable using that library. I'm not having much luck though :( This is what I have so far: name: foo version: 0.1 exposed-modules: Foo.Bar other-modules: Foo.Qux Foo.C2HS hs-source-dirs: src include-dirs: csrc c-sources: csrc/qux.c extensions: ForeignFunctionInterface build-depends: base, haskell98 executable: foo hs-source-dirs: test-src main-is: foo.hs other-modules: Foo.Bar When built this is the message I get: test-src/foo.hs:5:7: Could not find module `Foo.Bar': Use -v to see a list of the files searched for. How do I specify that Foo.Bar is found where cabal puts it (./dist/build/)? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. Finagle's Fifth Law: Always draw your curves, then plot your readings.

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Hugs/nhc getting progressively slower
by Neil Mitchell 02 May '07

02 May '07

Hi, I like to develop on Hugs, because its a nice platform to work with, and provides WinHugs, auto-reloading, sub-second compilation etc. Unfortunately some of the newer libraries (ByteString/Binary in particular) have been optimised to within an inch of their lives on GHC, at the cost of being really really slow on Hugs. Taking the example of Yhc Core files, which are stored in binary. Using a very basic hPutChar sequence is miles faster (10x at least) than all the fancy ByteString/Binary trickery. Taking the example of nobench, Malcolm told me he reimplemented ByteString in terms of [Char] and gained a massive performance increase (6000x springs to mind, but that seems way too high to be true) using nhc. Could we have a collective thought, and decide whether we wish to either kill off all compilers that don't start with a G, or could people at least do minimal benchmarking on Hugs? I'm not quite sure what the solution is, but it probably needs some discussion. Thanks Neil

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Is Hs-Plugins dead?
by Philipp Volgger 02 May '07

02 May '07

Is Hs-Plugins still under develeopment; is there still somebody who is updating it?

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Type-level programming problem
by oleg＠pobox.com 02 May '07

02 May '07

Thomas Schilling wrote: > data T > class Foo ns a b c | ns -> a, ns -> b, ns -> c where > mkFoo :: ns > defaultA :: a > defaultB :: c -> IO b > defaultC :: [T] -> c > f :: c -> b -> a -> (b, Int) > data DefaultA > instance Foo ns a b c => Apply DefaultA ns a where > apply _ _ = defaultA > > but I get: > > Could not deduce (Foo ns1 a b1 c1) > from the context (Apply MakeAs ns a, Foo ns a b c) > arising from use of `defaultA' Indeed. Let us examine the inferred type of defaultA: *Main> :t defaultA defaultA :: (Foo ns a b c) => a We see it is a value polymorphic over four type variables: ns, a, b, and c. The type variable 'a' is also the type of the value, so we have a way to instantiate it. There is no direct way to instantiate the remaining three. If there were a functional dependency a -> ns, a->b, a->c, we could have instantiated the remaining variables. But there are no such dependencies. So, there is really no way we can ever instantiate the type variables ns, b and c -- and so the typechecker will complain. So, we need either a functional dependency a -> ns in the definition of Foo, or defaultA should have a signature defaultA :: ns -> a (and ditto for other defaults). As I understand, the function 'defaultA' can be present in different components, identified by ns. When we write 'defaultA' however, how can we say that we mean defaultA of component X rather than of component Y? There isn't any way to name the desired component... Incidentally, if we represent components by records data XRec = XRec { defaultA :: XA } then the type of defaultA is Xref -> XA. It is the function from the type of the `namespace'. This seems to suggest the signature of defaultA should be ns -> a ... BTW, there are other ways to add the name of the namespace to the signature of defaultA. For example: newtype TaggedT ns a = TaggedT a class Foo ns a b c | ... defaultA :: TaggedT ns a or class Foo ns a b c | ... defaultA :: ns a etc.

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Getting the number of seconds since epoch from System.Time.ClockTime
by Martin Percossi 02 May '07

02 May '07

Hello haskell-cafe, In System.Time, data ClockTime = TOD Integer Integer , where the first integer represents the number of seconds since epoch, and the other represents the number of picoseconds. Is there a way of retrieving the first part? (In Haskell 98, the ClockTime type is abstract). TIA Martin My music: http://www.youtube.com/profile?user=thetonegrove

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what exactly does "deriving (Functor, Monad, MonadIO)" do?
by Thomas Hartman 01 May '07

01 May '07

I was trying to follow the reasoning in Don's article on using haskell for shell scripting http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10 In the source listing at the end we is newtype Shell a = Shell { runShell :: ErrorT String IO a } deriving (Functor, Monad, MonadIO) and I don't understand it what "deriving" is doing here, nor have I been able to find documentation on it. http://en.wikibooks.org/wiki/Haskell/Class_declarations claims: "You can only use deriving with a limited set of built-in classes. They are: Eq Ord Enum Bounded Show Read " But, here we are deriving classes not in that list. So, is this a recently added feature? Or something that came in from {-# OPTIONS -fglasgow-exts #-} ? I would just like to understand this, and I can't figure out how to begin. Thanks for any help! thomas.

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FIT for Haskell
by Philipp Volgger 01 May '07

01 May '07

Who wrote FIT for Haskell on http://darcs.haskell.org/FIT/? Does anybody know if the version is stable?

2 1

Bay Area Haskell users' group, anyone?
by Tim Chevalier 01 May '07

01 May '07

Hello all, With all the user groups that seem to have been forming lately, I figure it's high time to start a Bay Area group for people interested in Haskell (on any level). I interpret "Bay Area" broadly, since I'm in Monterey -- perhaps encompassing all of Northern California. If you're potentially interested, reply to me off-list, and I'll see what kind of response I get. I imagine that the first meeting, at least, would consist of people gathering informally over food, drink, or such. Cheers, Tim -- Tim Chevalier * chevalier(a)alum.wellesley.edu * Often in error, never in doubt Confused? See http://catamorphism.org/transition.html

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Creating pseudo terminals
by Tom Hawkins 01 May '07

01 May '07

How can I create a pseudo terminal in Linux with Haskell? Nothing is jumping out at me in the standard library. I haven't done this before in any language, so any tips would be appreciated. From what I gather, a call to posix_openpt or openpty returns a master and a slave, or alternatively opening /dev/ptmx followed by calls to grantpt and unlockpt. I'm building a load balancing and sharing system similar to Platform's LSF. The pseudo terminal is for interactive jobs running on a remote machine. Thanks for any help! -Tom

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Type-level programming problem
by Thomas Schilling 01 May '07

01 May '07

Hi, I have a type class similar to this one. data T class Foo ns a b c | ns -> a, ns -> b, ns -> c where mkFoo :: ns defaultA :: a defaultB :: c -> IO b defaultC :: [T] -> c f :: c -> b -> a -> (b, Int) The idea is, that I define classes of components where the data types of the methods are component-specific. I therefore use a namespace to define the component types (associated types would be nicer, but are not widely supported). Given a few sample instances, I can define a configuration: data X; data XA = XA; data XB = XB; data XC = XC instance Foo X XA XB XC where mkFoo = undefined defaultA = XA defaultB XC = return XB defaultC _ = XC f _ b _ = (b,0) mkX = mkFoo :: X data Y; data YA = YA; data YB = YB; data YC = YC instance Foo Y YA YB YC where mkFoo = undefined defaultA = YA defaultB YC = return YB defaultC _ = YC f _ b _ = (b,1) mkY = mkFoo :: Y config = mkX .*. mkY .*. HNil Using this configuration, I now want to define various functions that work on As, Bs or Cs, respectively. For example, I'd like to have a :: XA :*: XB :*: HNil a = hMap DefaultA config but I fail to figure out how to define DefaultA. I tried with this one: data DefaultA instance Foo ns a b c => Apply DefaultA ns a where apply _ _ = defaultA but I get: Could not deduce (Foo ns1 a b1 c1) from the context (Apply MakeAs ns a, Foo ns a b c) arising from use of `defaultA' at /Users/nominolo/Devel/Haskell/testcase1.hs:126:16-23 Possible fix: add (Foo ns1 a b1 c1) to the class or instance method `apply' In the expression: defaultA In the definition of `apply': apply _ _ = defaultA In the definition for method `apply' I guess I need some type class that also binds b and c. Or maybe I need to do some things completely different at all. Ideally, I'd hope to keep the work for adding new Foo instances as low as possible, though. I attached the relevant parts of the HList implementation and the code. Any suggestions? /Thomas

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RE:Cross-over from Haskell.org [Haskell] Re: Newbie: what are the advantages of Haskell?
by Taillefer, Troy (EXP) 30 Apr '07

30 Apr '07

Udo, I am cross posted/migrated this to Haskell-cafe per Simons request sorry for the late response I don't check this email on weekends I really dislike Perl as a programming language but I have to strongly disagree about your statements about CPAN and the quality of its contents. I have worked professional in Perl (Feel free to feel sorry for me) for over a year and I have always been impressed with what I could find in CPAN. Perl obviously has a niche and is "well supported by its community". Perl is popular so it must have some merit. I don't subscribe to the flawed reasoning that Perl Hackers just don't know any better or that they are dumb, or intellectual inferior in some way. Programmers have very good and valid reason for choosing their tools and I will have words for those who irrational state otherwise especially in a belittling manner I am pro choice. I am going to borrow some Ebonics "Don't language Hate Appreciate" Troy -----Original Message----- From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Udo Stenzel Sent: Saturday, April 28, 2007 7:06 AM To: haskell(a)haskell.org Subject: Re: [Haskell] Re: Newbie: what are the advantages of Haskell? Michael T. Richter wrote: > I wish I knew the language better so I could start working on those > libraries. Which ones? "those libraries" cannot come into existence until someone says what's actually missing. (The bulk of CPAN is crap and is certainly not worth being reimplemented.) -Udo -- "Object-oriented programming is an exceptionally bad idea which could only have originated in California." -- E. W. Dijkstra

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Indexing of content at haskell.org by search engines
by Henning Thielemann 30 Apr '07

30 Apr '07

I have noticed that only parts haskell.org are indexed by Google. E.g. mail archives of Haskell-Cafe and HaskellWiki are not indexed, but cvs.haskell.org, darcs.haskell.org and hackage.haskell.org are.

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