- Haskell-Cafe - Haskell.org

Re: Why is type 'b' forced to be type 'm a' and not possibly 'm a -> m a'
by Vivian McPhail 20 Sep '06

20 Sep '06

> Vivian McPhail wrote: > > > > class Forkable a where > > > > fork :: String -> a -> a -> a > .... > > What I would like to be able to do is > > differentiate between Forkable (m a -> > > b) and Forkable (<function type> -> b). > > Have you tried this combination of instances? > > instance Forkable (IO a) where ... > -- and similarly for all the concrete > -- monad types you will use fork with > > instance (Forkable a, Forkable b) => > Forkable (a -> b) where ... > > Alternatively, since the fork function seems to be all about > propagating a value (the String), would Control.Monad.Reader > serve your purpose? The value that gets 'forked' is not actually the string, it is the result of a monadic computation. > > Regards, > Tom > I have tried: > instance Forkable (USM NRef) where... Which is my Monad The problem lies with > instance (Forkable a, Forkable b) => Forkable (a -> b) where > fork n a1 a2 a = fork n (a1 a) (a2 a) because I need the arg a to be evaluated before it gets passed to a1 and a2. This definition does the right thing when type 'a' is a function type, because it is not a value, but with something like 'm a -> (m a -> m a) -> m a' with Forkable (a -> b) the first arg gets evaluated twice, to be more concrete: With (and golden white) eggs I want the 'eggs' that is passed to 'golden' to be the same as the 'eggs' that is passed to 'white', i.e. -> and1 (golden2 eggs3) (white4 eggs3) and not -> and1 (golden2 eggs3) (white4 eggs5) So to do this I need to be able to recognise the case where the 'a' of (a -> b) is of type 'm a' so that I can evaluate it -- doesn't typecheck instance (Monad m, Forkable (m a), Forkable b) => Forkable (m a -> b) where fork n a1 a2 a = do a' <- a fork n (a1 $ return a') (a2 $ return a') Tom suggested that I might be able to use the Reader monad, but I'm not clear as to how I could do this. Cheers, Vivian -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.405 / Virus Database: 268.12.5/451 - Release Date: 19/09/2006

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sections for record settors
by Tim Newsham 20 Sep '06

20 Sep '06

Hi, When you create a record in haskell, it automatically generates gettor functions for accessing the record fields. It also provides you with a record update syntax: rec {field=val} but there is no settor function. It would be nice if there was some sort of section-like syntax to access the settor function, like: {field=} -- val rec {field=val} -- rec or even if there was a name for that function (such as field_settor). Without such a syntax, one often has to write lambdas like: \r v -> r {field=v} \r -> r {field=3} Consider for example an cmd line parser that collects flags in a record. Instead of writing: opt "l" (NoArg {c_loop=False}) "dont loop", opt "o" (optStr {c_out=} "dir") "output dir", opt "t" (optInt {c_maxtime=} "mins") "maximum time" we would have to write it out as: options = [ opt "l" (NoArg (\c -> c {c_loop=False})) "dont loop", opt "o" (optStr (\v c -> c {c_out=v}) "dir") "output dir", opt "t" (optInt (\v c -> c {c_maxtime=v}) "mins") "maximum time" ] where opt cs d s = Option cs [] d s optStr f name = ReqArg f name optInt f name = ReqArg (f . read) name getOptions :: String -> [String] -> IO Config getOptions prog argv = let header = "Usage: " ++ prog ++ " [OPTIONS...] files..." in let usage s = ioError (userError (s ++ "\n" ++ usageInfo header options)) in let (updates, files, errs) = getOpt2 options argv in let config = updates defConfig {c_files=files} in case () of _ | not (null errs) -> usage (concat errs) | null (c_files config) -> usage "no files specified" | otherwise -> return config Tim Newsham http://www.thenewsh.com/~newsham/

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Re: Why is type 'b' forced to be type 'm a' and not possibly 'm a -> m a'
by tpledger＠ihug.co.nz 20 Sep '06

20 Sep '06

Vivian McPhail wrote: > > > class Forkable a where > > > fork :: String -> a -> a -> a .... > What I would like to be able to do is > differentiate between Forkable (m a -> > b) and Forkable (<function type> -> b). Have you tried this combination of instances? instance Forkable (IO a) where ... -- and similarly for all the concrete -- monad types you will use fork with instance (Forkable a, Forkable b) => Forkable (a -> b) where ... Alternatively, since the fork function seems to be all about propagating a value (the String), would Control.Monad.Reader serve your purpose? Regards, Tom

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[OT] Yet Another program written in Haskell: a language tag parser
by Stephane Bortzmeyer 20 Sep '06

20 Sep '06

The program is written in Haskell, hence my shameless ad here: http://www.bortzmeyer.org/gabuzomeu-parsing-language-tags.html GaBuZoMeu is a set of programs to parse and check language tags (see the RFC 4646 produced by the IETF Working Group LTRU - Language Tag Registry Update). Language tags are used by several protocols (like HTTP) or languages (like XML) to indicate the human language you want or have. Examples of language tags are "fr" (French), "apa" (Apache), "se-FI" (Swedish, as practiced in Finland) or "uk-Latn" (ukrainian in the latin script). An IANA registry stores the registered values (http://www.iana.org/assignments/language-subtag-registry) A language tag can be well-formed (syntactically correct) or not. This can be tested without access to the registry. A langauge tag can be valid or not (all its subtags registered). This depends on the copy of the registry you use (since the registry changes). GaBuZoMeu includes the following programs: * check-wf checks the well-formedness of tag(s). Example of use: "check-wf fr en-AU". * check-valid checks the validity of tag(s). Same use. GaBuZoMeu is written in Haskell (http://www.haskell.org/) so you'll need an Haskell compiler such as ghc (http://haskell.org/ghc/) To compile, just type "make". To check that everything is OK, you can type "make test". You should get zero "Errors" and zero "Failures". GaBuZoMeu is distributed as free software, under the GNU General Public Licence. Remarks, patches and bug reports are welcome. Written and maintained by Stephane Bortzmeyer <bortzmeyer(a)nic.fr>.

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map (-2) [1..5]
by Tamas K Papp 20 Sep '06

20 Sep '06

The code in the subject generates an error. I understand why this is (- is treated as part of the number), but I don't know how to solve it, ie how to tell Haskell that - is a function/binary operator? Thanks, Tamas

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Wiki contact
by Neil Mitchell 19 Sep '06

19 Sep '06

Hi, Two questions: 1) Is there any way to get ... to be syntax hilighted as dots (or just leaft as is) on the Haskell wiki in <haskell> tags, having it as _._ looks ugly - see http://haskell.org/haskellwiki/Keywords#where - note that the first and last dots are underlined hyperlinks. 2) How do you go about contacting the person responsible for the wiki? Is there a wiki page to drop messages off on? An email address? An email list? - it confused me! Thanks Neil

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Traversing a graph in STM
by Einar Karttunen 19 Sep '06

19 Sep '06

Hello Is there an elegant way of traversing a directed graph in STM? type Node nt et = TVar (NodeT nt et) type Edge et = TVar et data NodeT nt et = NodeT nt [(Node nt et, Edge et)] type MyGraph = Node String Int When implementing a simple depth first search we need a way to mark nodes (= TVars) as visited. In addition multiple concurrent searches should be possible. Is it possible to avoid passing around an explicit Set of visited nodes? And is there a better way of getting TVar identity than StableNames? - Einar Karttunen

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foreach
by Tim Newsham 19 Sep '06

19 Sep '06

I was rewriting some non-haskell code in haskell and came up with this construct: foreach l f = mapM_ f l main = do args <- getArgs foreach args (\arg -> do foreach [1..3] (\n -> do putStrLn ((show n) ++ ") " ++ arg) ) ) which is reminiscent of foreach in other languages. Seems fairly useful and I was wondering how hard it would be to add some syntactic sugar to the "do" construct to make it a little prettier (ie. not require the parenthesis, binding and nested do, as: main = do args <- getArgs foreach args arg foreach [1..3] n putStrLn ((show n) ++ ") " ++ arg) would this type of transformation be possible with template haskell or does this need stronger support from the parser to pull off? Tim Newsham http://www.thenewsh.com/~newsham/

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darcs get-together tuesday evening (2006-09-19 18:00)
by Eric Y. Kow 18 Sep '06

18 Sep '06

Dear darcs users, hackers and observers (*), We're getting together around 18:00 this Tuesday to discuss and/or hack on darcs. This will be after the ICFP sessions for that day, in Portland, Oregon, specifically the Marriott downtown waterfront LL1. David Roundy will be there, Ian Lynagh and me too. Hope to see you there. (*) Sorry for the spam, Haskellers. I figured we might as well try to catch ICFP bystanders. -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

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Re: [Haskell-cafe] Problems interpreting
by Andrea Rossato 18 Sep '06

18 Sep '06

On Mon, Sep 18, 2006 at 12:54:34PM +0200, Albert Crespi wrote: > Thank you very much for your reply! > As I said, it is my first experience with Haskell, I have been programming > in Java and C for some years, and I find this language very different from > them. Anyway I'll try to fix the function with the information that you gave > me. > Thanks again! > You're welcome. By the way, this is what the comments say you are trying to do: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a => a -> [a] -> [a] -> [a] substitute e l1 l2= [c | c <- check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else check_elem xs This is the result: *Main> substitute 1 [1,2,3] [] [2,3] *Main> substitute 1 [1,2,3] [7,8,9] [7,8,9,2,3] *Main> Have fun with Haskell. Ciao Andrea

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