Re: library of monadic functions [was: Why no findM ? simple Cat revisited]
Andrew J Bromage wrote: I can also make a case for: findM'' :: (Monad m) = (a - Bool) - [m a] - m a findM'' p [] = fail findM'': not found findM'' p (x:xs) = p x = \b - if b then return x else findM'' p xs The last line doesn't seem to compile. Don't you mean findM'' p (x:xs) = x = \b - if p b then return b else findM'' p xs instead? Sam ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: library of monadic functions [was: Why no findM ? simple Cat revisited]
G'day all. Quoting Samuel Tardieu [EMAIL PROTECTED]: The last line doesn't seem to compile. Don't you mean [...] Err... yes. Cheers, Andrew Bromage ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
library of monadic functions [was: Why no findM ? simple Cat revisited]
I appreciate your comment.
I agree that the type of findM should be the one you suggested,
and it still fits my original purpose. It's no more than a step arout.
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { b - f x; if b then return (Just x) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (=return.isLeft) (hCat stdin)
where hCat h = try (hGetLine h=putStrLn) : hCat h
\end{code}
Yes, you are right.
It was enough because, you don't really care about what you found, you just
want to search and stop when you do find something. You are returning the
action that returned an element that satisfied your
condition, not the actual element like before.
I expetct the next Haskell Library Report includes findM.
It's obviously useful.
I think both versions can be very useful:
findM :: (Monad m) = (a - m Bool) - [a] - m (Maybe a)
findM' :: (Monad m) = (a - Bool) - [m a] - m (Maybe a)
Same can be said for,
takeWhileM :: (Monad m) = (a - m Bool) - [a] - m [a]
takeWhileM' :: (Monad m) = (a - Bool) - [m a] - m [a]
both would be usefull for different purposes.
Oh and since we're on it I also miss,
iterateM :: (Monad m) = (a - m a) - a - m [a]
untilM :: (Monad m) = (a - m a) - a - m [a]
etc etc...
I've just been coding them as I need them, like many of you I suppose.
J.A.
___
Haskell-Cafe mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: library of monadic functions [was: Why no findM ? simple Cat revisited]
G'day all. On Wed, Nov 20, 2002 at 08:25:46PM +, Jorge Adriano wrote: I think both versions can be very useful: findM :: (Monad m) = (a - m Bool) - [a] - m (Maybe a) findM' :: (Monad m) = (a - Bool) - [m a] - m (Maybe a) I can also make a case for: findM'' :: (Monad m) = (a - Bool) - [m a] - m a findM'' p [] = fail findM'': not found findM'' p (x:xs) = p x = \b - if b then return x else findM'' p xs This goes with the philosophy that library functions shouldn't just return Maybe. Somewhere, somehow, there is a most general version of findM to be found. :-) Cheers, Andrew Bromage ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
