November 2002 - Haskell-Cafe

Why no findM ? simple Cat revisited
by Ahn Ki-yung 11 Nov '03

11 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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splatterhunks
by William Lee Irwin III 22 Apr '03

22 Apr '03

-- This program takes patch filenames as arguments and splits the patch -- files up on a hunk-by-hunk basis. I have found this useful for Linux -- kernel development, and perhaps others might find it useful in -- similar contexts where patches are the primary method of communication. module Main where import System import List main = getArgs >>= mapM_ splatterHunkFile splatterHunkFile file = mapM_ (\(f,ss) -> writeFile f (unlines ss)) . zip (map (((reverse . takeWhile (/= '/') . reverse $ file)++).("-"++).show) [1..]) . splatterHunks =<< readFile file splatterHunks ss = map (map unlines) $ zipWith (\s -> map (s:)) (map head . (splitList "diff") $ lines ss) (map (splitList "@@") . (splitList "diff") $ lines ss) splitList prefix ss = map ($ss) $ zipWith (\m n -> (if n >= 0 then take (n - m) else id) . drop m) (map fst . filter (\(n,s) -> isPrefixOf prefix s) $ zip [0..] ss) (tail (map fst . filter (\(n,s) -> isPrefixOf prefix s) $ zip [0..] ss) ++ [-1])

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Sets
by Mark T.B. Carroll 02 Jan '03

02 Jan '03

Am I right in thinking that sets of sets don't really work with GHC's Data.Set because "Set a" things aren't instances of Ord? How should I do sets of sets with it? (e.g. Set.mkSet [Set.unitSet 1] gives "No instance for (Ord (Set a))") -- Mark

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RE: Record of STRefs better than STRef to a Record?
by Simon Marlow 27 Dec '02

27 Dec '02

> If I use an STRef to a record, will a new record be created > each time I want > to update a single field? Or can I expect GHC to optimize it > and have the field of the record updated in place? You'll get a new record for each update. This might not be so bad though, depending on the number of fields in your record. > Right now I'm using a record of STRefs, like: > data E s = E{ > refi :: STRef s Int, > refc :: STRef s Char > } > > but it can get a little messy, since thread s propagates to > every datatype that uses the record type in it's definition. Here's another trick if you use this route: data E s = E{ refi :: !STRef s Int, refc :: !STRef s Char } and compile with -funbox-strict-fields. This will eliminate the boxing of the STRefs. Cheers, Simon

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someone help me
by Nuno Silva 30 Nov '02

30 Nov '02

I'm relatively new to haskell and have some questions: 1- I want to convert an integer to a char( if I say prelude> convert 4 ... it should return prelude> '4') 2- and vice versa? 3- I want to define a function that stores in a variable the following... pretended: (Int,Int,Int,Int) give :: IO Int give = randomRIO (0,9) main = do k <- (give,give,give,give) actualy this isn't possible. but can someone please tell how to do this??? thank you very much --------------------------------- Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now

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RE: Implementing forward refs in monadic assembler and interprete r
by Erkok, Levent 29 Nov '02

29 Nov '02

> For the moment I've chosen to do the mdo desugaring manually instead > of moving to the CVS ghc (or exclusively using Hugs.) Will mdo be in > the next release? (That's 5.04.2, I think -- though I've recently > been confused about GHC releases.) The relevant revision to Lex.lhs > is pretty recent and doesn't seem to be on ghc-5-04-branch. The current CVS-head for GHC contains the mdo implementation (thanks to Simon PJ). I'm not sure what they're going to call it (i.e. 5.04.2, or 5.05, etc.), but the next release will have mdo for sure. I don't know when they'll make a new release though, maybe one of the Simon's can comment on that? -Levent.

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Re: Implementing forward refs in monadic assembler and interprete r
by Mike Gunter 29 Nov '02

29 Nov '02

Thank to all who replied. After giving up on continuations (which make everything but forward branches seductively easy), I settled on something with the following interface: > class MonadGoto lbl gm | gm -> lbl where > liftG :: Monad m => m () -> gm m () > label :: Monad m => gm m lbl > gotoIf :: Monad m => m Bool -> lbl -> gm m () Note that, unlike a monad transformer, only ()-producing computations can be embedded. This limitation reflects the problem of def-use across basic-block boundaries. Of course, computations in the embedded monad can be done there, then lifted, e.g.: > ;top <- label > ; ... > ; liftG (do v <- m1 > ; ... > ; m2 v > ; ...) > ; brPositive r3 top > ; ... I've implemented MonadGoto as > newtype GotoT m a = GotoT { unGotoT :: WriterT [GotoLbldT m ()] LblSupply a } > > data GotoLbldT m a = Label !Lbl > | GotoIf (m Bool) !Lbl > | Inner (m a) > > wrL = GotoT . tell . (:[]) > instance MonadGoto Lbl GotoT where > liftG = wrL . Inner > label = GotoT $ lift fromSupply >>= \l -> tell [Label l] >> return l > gotoIf p l = wrL $ GotoIf p l where GotoLbldT gets converted to > data GotoLbldRunT m a = ThenLabel { before :: m (), next :: GotoLbldRunT m a, labelLbl :: !Lbl } > | ThenGotoIf { before :: m (), next :: GotoLbldRunT m a > , gotoPred :: m Bool, gotoLbl :: !Lbl } > | Tail (m a) which can then be run > type JumpArray m v = [(Lbl, m v)] > runGM :: Monad m => JumpArray m v -> GotoLbldRunT m v -> (JumpArray m v, m v) . My Verilog has delayed branches (because that's the easy thing to do.) So, I implemented a DelayedGotoT which provides for delayed branches by shuffling the "GotoLbldT"s and using a StateT to save the result of the gotoPred evaluation. "Erkok, Levent" <levent.erkok(a)intel.com> writes: > You need a fairly recent version of Hugs to run this > example, November 2002 release would do (try with -98). For ghc, you need > the CVS version, or wait till the next release. As far as I know, no other > implementation supports mdo, nor there are any plans for it. For the moment I've chosen to do the mdo desugaring manually instead of moving to the CVS ghc (or exclusively using Hugs.) Will mdo be in the next release? (That's 5.04.2, I think -- though I've recently been confused about GHC releases.) The relevant revision to Lex.lhs is pretty recent and doesn't seem to be on ghc-5-04-branch. thanks, mike

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Merhaba;
by www.erotikmagaza.com 26 Nov '02

26 Nov '02

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Random Color
by Mike T. Machenry 21 Nov '02

21 Nov '02

I am trying to construct an infinate list of pairs of random colors. I am hung up on getting a random color. I have: data Color = Blue | Red | Green deriving (Eq, Ord, Show) am I supposed to instantiate a Random class instance from color? I am not sure exactly how the random number generator works. Is this the best way to go about it? Should I instead get a random Integer index of an array of Colors? thanks, -mike

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Re: Sets
by Christopher Milton 19 Nov '02

19 Nov '02

This might be of some help, circa Haskell 1.2, SetMap.hs - sets and maps in Haskell Author: Nick North ftp://ftp.cs.chalmers.se/pub/haskell/library/SetMap.hs Chris ===== Christopher Milton cmiltonperl(a)yahoo.com __________________________________________________ Do you Yahoo!? Yahoo! Web Hosting - Let the expert host your site http://webhosting.yahoo.com

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