The result of the filtering to the right of >>= must be of monadic
type, you can use `return' for this. In hbi:
> getDirectoryContents "." >>= (return . filter (\(x:_) -> x /= '.'));
["GoodNews","BadNews","OldNews","mbox"]
>
Or, by using `fmap', `head', and a section:
> filter ((/='.') . head) `fmap` getDirectoryContents ".";
["GoodNews","BadNews","OldNews","mbox","NewNews"]
>
(Use `map' instead of `fmap' if you don't have Haskell98.)
/M
Simon Raahauge DeSantis writes:
> I'm working on a little toy program in hbc (now that I have a haskell
> compiler running I decided to give it a try ;), but I seem to have run into
> a monad problem. 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 '.'
> }
> ))
>
> (The idea here is to filter out all the dot files from the directory
> listing)
> Unfortunately I can't make hide or hair of this. 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?)
> TIA
> --
> -Simon Raahauge DeSantis
