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Either e Monad
by Deokhwan Kim 20 Sep '06

20 Sep '06

Dear all, Where is the Monad instance declaration of Either e? >From the description of Control.Monad.Error, I deduce that Either e is an instance of Monad. http://haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-Error.h… class Monad m => MonadError e m | m -> e where ... Error e => MonadError e (Either e) But, I cannot find the Monad instance declaration of Either anywhere. Thanks. -- Deokhwan Kim

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MonadList?
by Michael Shulman 20 Sep '06

20 Sep '06

In another thread, I wrote: > main = (>> return ()) $ runListT $ do > arg <- ListT getArgs > n <- ListT $ return [1..3] > liftIO $ putStrLn ((show n) ++ ") " ++ arg) The frequent occurence of "ListT $ return" in my code when I use the ListT monad transformer has made me wonder why there isn't a standard typeclass `MonadList', like those for the other monad transformers, encapsulating the essence of being a "list-like" monad -- in this case, the ability to select from a list of things. I quickly wrote one for myself: class MonadList m where option :: [a] -> m a instance MonadList [] where option = id instance (Monad m) => MonadList (ListT m) where option = ListT . return Has anyone else thought about or done something like this? Mike

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Re: Numeric type classes
by Henning Thielemann 20 Sep '06

20 Sep '06

On Mon, 11 Sep 2006, Ross Paterson wrote: > On Mon, Sep 11, 2006 at 04:26:30PM +0200, Henning Thielemann wrote: > > On Sat, 9 Sep 2006, Ross Paterson wrote: > > > I think that a finer grain numeric hierarchy, while retaining Num, etc, > > > is feasible without changing the language: unlike the case of monads, > > > the people who will be defining instances of numeric classes are the > > > very ones who are inconvenienced by the current hierarchy. The main > > > impact on clients of the classes is that some functions would have > > > more general types. > > > > There are many Num instances around in libraries where people wrap to > > external libraries: functionalMetapost, CSound wrapper in Haskore, > > SuperCollider (GSL too?). What about Num (algebraically Ring) instances of > > polynomials, residue classes and other such advanced mathematical objects? > > And what do abs and signum mean for Haskore's orchestra expressions, > polynomials, residue classes, vectors, matrices, functions, etc? For clarification: Haskore does not define any arithmetic for music, but CSound provides some arithmetic and Haskell wraps it with Num instances. > The people who define those wish they were defining Ring, but they must > define Num. It seems we are at a point, where we have to define what is a 'number'. More precisely: Can you tell me the difference between numbers and "more complex mathematical objects"? Is a complex number a number? Is a quaternion a number? Is a residue class a number? We can calculate with integers modulo some other integer like with integers - is that considered computation with numbers? Shall we distinguish between matrices of numbers and matrices of more complex mathematical objects? In signal theory matrices of polynomials are very common.

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Java or C to Haskell
by Carajillu 20 Sep '06

20 Sep '06

I'm trying to write in Haskell a function that in Java would be something like this: char find_match (char[] l1, char[] l2, char e){ //l1 and l2 are not empty int i = 0; while (l2){ char aux = l2[i]; char[n] laux = l2; while(laux){ int j = 0; if(laux[j] = aux) laux[j] = e; j++; } if compare (l1, laux) return aux; else i++; } return ''; } compare function just compares the two lists and return true if they are equal, or false if they are not. it is really a simple function, but I've been thinking about it a lot of time and I can't get the goal. It works like this: find_match "4*h&a" "4*5&a" 'h' ----> returns '5' (5 matches with the h) find_match "4*n&s" "4dhnn" "k" ----> returns '' (no match at all - lists are different anyway) I'm trying to use map for scanning the list l2, then using some function for substitute the char, and finally something like zipWith x == y laux l1 for comparing the lists, but I can't find a way for putting all this together!! -- View this message in context: http://www.nabble.com/Java-or-C-to-Haskell-tf2303820.html#a6403589 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ICFP programming contest 2006 results: video stream
by dons＠cse.unsw.edu.au 20 Sep '06

20 Sep '06

Malcolm Wallace has recorded the ICFP programming contest results announcement as video, straight from the ICFP conference in Portland. He's posted it to Google Video, and it's available to download (120M) or stream from Google video, here: http://video.google.com/videoplay?docid=6419094369756184531 Thanks Malcolm! -- Don

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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 19 Sep '06

19 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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