Simon Raahauge DeSantis wrote:
> [...] Here's the snippet in particular (and the results from
> evaluation in hbi):
> > getDirectoryContents "." >>= filter (\(x:_) -> x /= '.');
> [65] Cannot unify types:
> Prelude.IO
> and (Prelude.[])
> in (>>=) (getDirectoryContents ".") (filter (\I ->
> case I of {
> Prelude.[] -> Pfail "No match in Pinteractive"
> | (:) x I4625 -> (/=) x '.'
> }
> ))
> [...] Is this because the function being used with filter would
> break when given an empty list (or is that Pfail bit there to
> handle that breaking?)
The reason is that getDirectoryContents is an IO *action*,
filter (\...) is a *function*, and the >>= operator sequences two
actions, as can be seen from their types:
getDirectoryContents :: FilePath -> IO [FilePath]
filter (\(x : _) -> x /= '.') :: [FilePath] -> [FilePath]
(>>=) :: Monad a => a b -> (b -> a c) -> a c
So you have to lift your filtering into the IO monad:
getDirectoryContents "." >>= (\xs -> return (filter (\(x:_) -> x /= '.') xs))
or to impress your friends with your higher-order programming skills:
;-)
liftM (filter $ (/= '.') . head) $ getDirectoryContents "."
The funny Pfail in the error message stems from the fact that hbc
typechecks a desugared version of the source program, which can be
confusing.
Hope that helps,
Sven
--
Sven Panne Tel.: +49/89/2178-2235
LMU, Institut fuer Informatik FAX : +49/89/2178-2211
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