On Thu, Oct 18, 2007 at 22:58:48 +1000, Matthew Brecknell wrote:
>Magnus Therning:
>> Still no cigar :(
>
>Yes, this is a little more subtle than I first thought. Look at liftM
>and filterM:
>
>liftM f m1 = do { x1 <- m1; return (f x1) }
>
>filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
>filterM _ [] = return []
>filterM p (x:xs) = do
> flg <- p x
> ys <- filterM p xs
> return (if flg then x:ys else ys)
>
>In liftM, the result of (f x1) is not forced, and in filterM, flg is
>not tested until after xs is traversed. The result is that when filterM
>runs the (p x) action, a file is opened, but hasEmpty (and thus
>readFile) is not forced until all other files have likewise been
>opened.
>
>It should suffice to use a more strict version of liftM:
>
>liftM' f m1 = do { x1 <- m1; return $! f x1 }That did indeed fix it. Just out of curiosity, how would I go about finding this myself? (Ideally it'd be an answer other than "read the source for the libraries you are using". :-) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus
pgpJO22GUwQ7M.pgp
Description: PGP signature
_______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
