Re: [Haskell-cafe] IO in lists
Thanks for all the excellent answers to my original question. Somehow it feels like I advanced and got one level closer to a black belt in Haskell due to this; I've now legitimately used a function from System.IO.Unsafe :-) I tried to document it all: http://therning.org/magnus/archives/249 /M http://liw.iki.fi/liw/log/2007-01.html#20070116b -- Magnus Therning (OpenPGP: 0xAB4DFBA4) [EMAIL PROTECTED] Jabber: [EMAIL PROTECTED] http://therning.org/magnus pgpZG9AiJbjF8.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
G'day all. Quoting Yitzchak Gale <[EMAIL PROTECTED]>: > What can be done to get an improved list transformer > into MTL? Not sure. But a lot of people use mine: http://sigcomp.srmr.co.uk/~rjp/Nondet.hs (My darcs repository is down at the moment, unfortunately.) Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
I wrote: But the list monad [] is not a transformer, so you can't lift in it, even if the contained type happens also to be a monad. Andrew Bromage wrote: ListT is also not a transformer. True, unfortunately. But it does provide MonadTrans and MonadIO instances that solve problems like this in practice. What can be done to get an improved list transformer into MTL? See here for details: http://www.haskell.org/hawiki/ListTDoneRight Can someone with permissions on the new wiki please get this important page moved over? Thanks, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
G'day all. On Tue, Jan 16, 2007 at 14:06:08 +0200, Yitzchak Gale wrote: > >But the list monad [] is not a transformer, so you can't lift in it, > >even if the contained type happens also to be a monad. Quoting Magnus Therning <[EMAIL PROTECTED]>: > Yeah, I had some vague thought of that being a problem, which lead me to > ListT. ListT is also not a transformer. See here for details: http://www.haskell.org/hawiki/ListTDoneRight Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
Try the code below. It is a fairly structured way to get exactly the behavior
you asked for. The lazy and unsafeLazy versions are the ones you are
interested in.
module Main where
import Data.Char
import System.IO
import System.IO.Unsafe
newtype Stream a = Stream {next:: (IO (Maybe (a,Stream a)))}
-- Run this "main" (e.g. in GHCI) and type several lines of text.
-- The program ends when a line of text contains 'q' for the second time
--
main = do
hSetBuffering stdin NoBuffering
hSetBuffering stdout NoBuffering
print "Test of strict"
opWith =<< strict untilQ
print "Test of unsafeStrict"
opWith $ unsafeStrict untilQ
print "Test of lazy"
opWith =<< lazy untilQ
print "Test of unsafeLazy"
opWith $ unsafeLazy untilQ
-- Shorthand for test above. Processing the input through toUpper
opWith = mapM_ print . lines . map toUpper
untilQ :: Stream Char
untilQ = Stream $ do
c <- getChar
if c == 'q'
then return Nothing
else return (Just (c,untilQ))
strict :: Stream a -> IO [a]
strict s = do
mc <- next s
case mc of
Nothing -> return []
Just (c,s') -> do rest <- strict s'
return (c:rest)
lazy :: Stream a -> IO [a]
lazy s = unsafeInterleaveIO $ do
mc <- next s
case mc of
Nothing -> return []
Just (c,s') -> do rest <- lazy s'
return (c:rest)
unsafeStrict :: Stream a -> [a]
unsafeStrict s = unsafePerformIO (strict s)
unsafeLazy :: Stream a -> [a]
unsafeLazy s = unsafePerformIO (lazy s)
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Re: [Haskell-cafe] IO in lists
Hi, Am Dienstag, den 16.01.2007, 19:19 + schrieb David House: > On 16/01/07, Yitzchak Gale <[EMAIL PROTECTED]> wrote: > > listChars2 :: ListT IO Char > > listChars2 = do > > c <- lift getChar > > if c == 'q' > > then return c > > else return c `mplus` listChars2 > > It's probably eaiser to work with normal lists: > > listChars :: IO [Char] > listChars = do > c <- getChar > if c == 'q' > then return c > else liftM2 (:) (return c) listChars But that is not lazy any more, is it? The idea of the OT was, I think, that he can use the first elements of the list even before the last one was entered. Greetings, Joachim -- Joachim Breitner e-Mail: [EMAIL PROTECTED] Homepage: http://www.joachim-breitner.de ICQ#: 74513189 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
On 16/01/07, Yitzchak Gale <[EMAIL PROTECTED]> wrote: listChars2 :: ListT IO Char listChars2 = do c <- lift getChar if c == 'q' then return c else return c `mplus` listChars2 It's probably eaiser to work with normal lists: listChars :: IO [Char] listChars = do c <- getChar if c == 'q' then return c else liftM2 (:) (return c) listChars -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
Hi again,
if sequence' should work with empty lists, then better write it like
this:
sequence' ms = foldr k (return []) ms
where
k m m' = do { x <- m; xs <- unsafeInterleaveIO m'; return (x:xs) }
Joachim
--
Joachim "nomeata" Breitner
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JID: [EMAIL PROTECTED] | http://www.joachim-breitner.de/
Debian Developer: [EMAIL PROTECTED]
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Re: [Haskell-cafe] IO in lists
Hi, you can also try with unsafeInterleaveIO, works like a charm, and you really feel the laziness: *Main> main -- now entering “testq” t"teessttq" import System.IO.Unsafe sequence' :: [IO a] -> IO [a] sequence' (x:xs) = do r <- x; rs <- unsafeInterleaveIO (sequence' xs) return (r:rs) main = do allChars <- sequence' $ repeat getChar let getChars = takeWhile (/= 'q') allChars print getChars Unfortunately, this did not work: allChars <- sequence $ repeat $ unsafeInterleaveIO getChar Probably because of something sequence is doing. Greetings, Joachim -- Joachim "nomeata" Breitner mail: [EMAIL PROTECTED] | ICQ# 74513189 | GPG-Key: 4743206C JID: [EMAIL PROTECTED] | http://www.joachim-breitner.de/ Debian Developer: [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
Hello Magnus, Tuesday, January 16, 2007, 2:34:51 PM, you wrote: > I can take getChar and create an infinate list: > listChars = getChar : listChars > but how do I go about creating a finite list, e.g. a list that ends as > soon as 'q' is pressed? you definitely should look into http://haskell.org/haskellwiki/IO_inside -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
On Tue, Jan 16, 2007 at 14:06:08 +0200, Yitzchak Gale wrote: [..] >But the list monad [] is not a transformer, so you can't lift in it, >even if the contained type happens also to be a monad. Yeah, I had some vague thought of that being a problem, which lead me to ListT. Your statement put some good words to that thought. >Perhaps you are looking for something like this, using the monad >transformer version of []: > >listChars2 :: ListT IO Char >listChars2 = do > c <- lift getChar > if c == 'q' > then return [c] > else return [c] `mplus` listChars2 > >GHC finds this much more tasty, and then > >runListT listChars2 > >does what I think you may want. Yes, that is fairly close to what I want to do. Now, it'd be really good if I could apply a function to each item in the ListT, with the constraint that it should be done lazily. I.e. the following will not do runListT listChars2 >>= (mapM_ putChar) because it first reads the input until 'q' is pressed. I.e. this produces the interaction: abcqabcq What I want is the interaction: aabbccqq Hope you understand what I mean. /M P.S. I'm aware a reqursive solution is possible and I've already coded one that works. I'm just curious if a "map solution" is possible. -- Magnus Therning (OpenPGP: 0xAB4DFBA4) [EMAIL PROTECTED] Jabber: [EMAIL PROTECTED] http://therning.org/magnus pgpkBnPyR98zZ.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
Oops, sorry, that should be: listChars2 :: ListT IO Char listChars2 = do c <- lift getChar if c == 'q' then return c else return c `mplus` listChars2 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO in lists
Magnus Therning wrote: Not a very descriptive subject, I know, but here's what I'd like to do. I can take getChar and create an infinate list: listChars = getChar : listChars but how do I go about creating a finite list, e.g. a list that ends as soon as 'q' is pressed? I was thinking of something like listChars1 = do c <- lift getChar if c == 'q' then [return c] else [return c] ++ listChars1 However, that triggers an interesting behaviour in ghci: You are trying to define something of type [IO Char]. But the list monad [] is not a transformer, so you can't lift in it, even if the contained type happens also to be a monad. Perhaps you are looking for something like this, using the monad transformer version of []: listChars2 :: ListT IO Char listChars2 = do c <- lift getChar if c == 'q' then return [c] else return [c] `mplus` listChars2 GHC finds this much more tasty, and then runListT listChars2 does what I think you may want. Regards, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
