December 2007 - Haskell-Cafe

Waiting for thread to finish
by Maurício 11 Dec '07

11 Dec '07

Hi, After I have spawned a thread with 'forkIO', how can I check if that thread work has finished already? Or wait for it? Thanks, Maurício

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Need help please.
by Ryan Bloor 11 Dec '07

11 Dec '07

hi I am writing a basic Parser from scratch. So far I have functions;# removeSpaces# match - which checks if a string is a substring of another# orParser which combines two parser's abilities# Basic pasrers like... parseInt, parseTrue, parseFalse, parseBoolusing the orParser on True and False.What I want to do now is have a parseBinaryOp that recognises:parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")]So I think that I have to split the initial string into four parts."+" becomes op'(' becomes tokenF')' becomes tokenB"5" becomes e1"2" becomes e2parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) =I am not sure how to go about separating the string for how I need itusing my other functiuons. Ryan _________________________________________________________________ Celeb spotting – Play CelebMashup and win cool prizes https://www.celebmashup.com

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Quickcheck generator for arrays of integers
by Paulo J. Matos 11 Dec '07

11 Dec '07

Hello all, I using an IArray to represent a matrix. I'm trying to write some properties checks with Quickcheck. Quickcheck lacks instance generators for arrays of ints for example, is there anything I can use out there or should I define it myself? (I'm asking because it seems to be something that other people already did) Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

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Literate HTML
by Neil Mitchell 11 Dec '07

11 Dec '07

Hi I want literate Haskell, but where the literate bit forming a document is actually HTML, not latex. Does anyone have any idea how to go about this? For a start, how do I persuade GHC to run the file: C:\Documents\Uni\tagsoup>runhaskell index.html Warning: ignoring unrecognised input `index.html' <interactive>:1:108: attempting to use module `Main' (Main.hs) which is not loaded And leading on to that, how Cabal might be used to compile the file. Secondly, what tricks can people think of for placing the code inside - I have come up with: <code> > foo = 1 </code> And: <code>  foo = 1 \end{code} </code> I can then hide the trailing \end{code} with a margin-bottm: -2em. Does anyone have any other ideas? Thanks Neil

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[OT] A nice organized collection of threads in Haskell-Cafe
by Vimal 11 Dec '07

11 Dec '07

Hi, I am working on a product to analyze posts made in Forums, Usenet and discussion mailing lists like Haskell-Cafe. For this, I require the messages to be accessible in this format: <forum> (* example: Haskell-cafe *) [ list of - <thread> [ list of - <post> </post> ] </thread> ] </forum> as XML. However, I find that that the messages (in haskell-cafe/usenet) themselves aren't organized in this fashion. I would like to know if there is any way in which I can get the archives in this fashion. Thanks, -- ~Vimal RLE :) encode = map (length &&& head) . group decode = concatMap (uncurry replicate)

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RE: [Haskell] Nested guards?
by Simon Peyton-Jones 11 Dec '07

11 Dec '07

[redirecting to Haskell Cafe] | > It is clear that this situation must not stay this way. Bit by bit, | > disciples of Perl and Python discover Haskell and demand that Haskell will | > be plastered with syntactic sugar until the simplicity of the functional | > approach isn’t visible anymore. Sadly, they seem to be successful with this | > as syntax extensions like parallel list comprehensions show. I think it is helpful to distinguish "superficial" complexity from "deep" complexity. All of Haskell's syntactic sugar is just that: it can be translated into a small purely-functional language using straightforward translation schemes. Even something relatively complicated like the "order by/group by" extension that Phil and I proposed at this year's Haskell workshop, has an easy translation that takes a dozen or two lines to give in full detail. In contrast, something like higher order functions, or mutable state, is deep complexity. Both have a pervasive effect on the language semantics and on its implementation. (The effect of mutable state is much, much worse, but they are both deep.) Concerning Haskell, I'm quite relaxed about superficial complexity, as you'll have seen from what happens in GHC. Section 3.6 of the History of Haskell paper addresses this point specifically [1]. You may disagree with the choice made by the Haskell committee at the time, and with subsequent developments, but it was quite a conscious choice, and one not without its advantages. Simon [1] http://research.microsoft.com/%7Esimonpj/papers/history-of-haskell/history.…

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What is the role of $!?
by PR Stanley 10 Dec '07

10 Dec '07

Hi What is the role of $! ? As far as I can gather it's something to do with strict application. Could someone explain what it is meant by the term strict application please? Thanks, Paul

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Software Tools in Haskell
by Tommy McGuire 10 Dec '07

10 Dec '07

In the "if anyone is interested,..." department.... For reasons that remain unclear, early this fall I started translating Brian W. Kernighan and P.J. Plaugher's classic _Software Tools in Pascal_ into Haskell. I have completed most of it, up to the second part of chapter 8 which presents a proto-m4 preprocessor. I have the code online including notes, comments, descriptions, and a few alternate approaches. Attractions include: * A fair gamut of the usual Unix suspects: proto-cat, proto-wc, proto-tr, proto-compress, proto-ar, proto-grep, etc. * A usable editor, if you consider a de-featured ed-alike to be usable. * A simple monadic regular expression engine. * Zippers, Parsec, the State monad, the StateT monad transformer, and other attempts to sully Computing Science's brightest jewels. * Lots and lots of really bad Haskell, including a fair bit that is a direct translation of 30-year old Pascal (see xindex in translit, Ch. 2, if you need to skip lunch). Programming really has advanced, you know. Anyway, the URL is: http://www.crsr.net/Programming_Languages/SoftwareTools Questions and comments would be appreciated, especially suggestions for how to make the code cleaner and more understandable. Flames and mockery are welcome, too, but only if they're funny---remember, I've been staring at Haskell, Pascal (plus my job-related Perl, CORBA, and C++) for a while; there's no telling what my mental state is like. [I had intended to wait until I had the whole thing done to make this announcement, but I recently moved and have not made much forward progress since, other than putting what I had done online.] -- Tommy M. McGuire mcguire(a)crsr.net

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Re: type class question
by Peter Padawitz 10 Dec '07

10 Dec '07

Jules Bean wrote: > Peter Padawitz wrote: > >>>> So the fundep would solve the problem. >>> >> But, actually, it doesn't :-( > > But actually, it does! Indeed... Sorry, I think I left intE out of the cycle. This might be the reason why it did not work before. > Ben Franksen's answer from yesterday compiles fine for me if I add the > missing fundep, block -> command. > > Your original code compiles without error, given the fundep. Exact > code I compiled attached at the bottom of this document. You may have > to repair long lines! > > Incidentally, I question why the "compFoo" are methods. Why not just > make them polymorphic functions? They don't look like you expect > instances to change them. The code continues to compile if I make them > functions and amend their signatures as required. I put compFoo into the class for the same reason why /= is part of the class Eq: both functions are unique as soon as the others have been instantiated. > > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} > > type Block = [Command] > data Command = Skip | Assign String IntE | Cond BoolE Block Block | > Loop BoolE Block > data IntE = IntE Int | Var String | Sub IntE IntE | Sum [IntE] | > Prod [IntE] > data BoolE = BoolE Bool | Greater IntE IntE | Not BoolE > > class Java block command intE boolE | block -> command, command -> > intE, intE -> boolE, boolE -> block > where block_ :: [command] -> block > skip :: command > assign :: String -> intE -> command > cond :: boolE -> block -> block -> command > loop :: boolE -> block -> command > intE_ :: Int -> intE > var :: String -> intE > sub :: intE -> intE -> intE > sum_ :: [intE] -> intE > prod :: [intE] -> intE > boolE_ :: Bool -> boolE > greater :: intE -> intE -> boolE > not_ :: boolE -> boolE > compBlock :: Block -> block > compBlock = block_ . map compCommand > compCommand :: Command -> command > compCommand Skip = skip > compCommand (Assign x e) = assign x (compIntE e) > compCommand (Cond be cs cs') = cond (compBoolE be) (compBlock > cs) (compBlock cs') > compCommand (Loop be cs) = loop (compBoolE be) (compBlock cs) > compIntE :: IntE -> intE > compIntE (IntE i) = intE_ i > compIntE (Var x) = var x > compIntE (Sub e e') = sub (compIntE e) (compIntE e') > compIntE (Sum es) = sum_ (map compIntE es) > compIntE (Prod es) = prod (map compIntE es) > compBoolE :: BoolE -> boolE > compBoolE (BoolE b) = boolE_ b > compBoolE (Greater e e') = greater (compIntE e) (compIntE e') > compBoolE (Not be) = not_ (compBoolE be)

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help
by Ryan Bloor 10 Dec '07

10 Dec '07

hi I am writing a basic Parser from scratch. So far I have functions;# removeSpaces# match - which checks if a string is a substring of another# orParser which combines two parser's abilities# Basic pasrers like... parseInt, parseTrue, parseFalse, parseBoolusing the orParser on True and False.What I want to do now is have a parseBinaryOp that recognises:parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")]So I think that I have to split the initial string into four parts."+" becomes op'(' becomes tokenF')' becomes tokenB"5" becomes e1"2" becomes e2parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) =I am not sure how to go about separating the string for how I need itusing my other functiuons. Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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