Jorge Adriano wrote:
Seems to me like the name findM could be misleading mapM :: (Monad m) => (a -> m b) -> [a] -> m [b] filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
These take a monadic function and a list of elements. Yours works the other way around (takes a function and a list of 'monadic elements'). I'd expect the definition of findM to be:
findM' :: (Monad m) => (a -> m Bool) -> [a] -> m (Maybe a) findM' f [] = return Nothing findM' f (x:xs) = do { b <- f x; if b then return (Just x) else findM' f xs }
This one doesn't serve your purpose though. J.A.
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} I expetct the next Haskell Library Report includes findM. It's obviously useful.