[Haskell-cafe] printing a list of directories which don't exist

[Daniel Fischer]

Am Mittwoch, 14. Mai 2008 17:47 schrieb Mike Jarmy:
> Newbie question:  Given a list of type '[FilePath]', how do I create a list
> of all those directories which do not actually exist, and then print the
> list?  I've figured out how to extract the ones which *do* exist, like so:
>
> module Main where
>
> import Control.Monad (filterM)
> import System.Directory (doesDirectoryExist)
> import System.Environment (getArgs)
>
> main :: IO ()
> main = do
>     dirs <- getArgs
>     let existing = filterM doesDirectoryExist dirs
>     ......
>
> which gives me a list of type 'IO [FilePath]'.  However, because of the
> 'IO' tag, I cannot figure out how to do any of the following 3 things
> (noted in comments):

What you want is 'fmap' (from the Functor class) or 'liftM' (from the Monad 
class).

	bogusDirs <- filterM (fmap not . doesDirectoryExist) dirs

should work, same with liftM in place of fmap.

>
>     -- filter via composition
>     let bogusDirs = filterM (not . doesDirectoryExist) dirs
>
>     -- test for emptiness
>     if bogusDirs /= []
>        -- print the list
>        then putStrLn $ "bogus: " ++ show bogusDirs
>        else putStrLn "OK"
>
> Can anyone set me straight?  How do I make the IO tag go away, or am I
> going about this all wrong?  E.g. the 'filterM (not . doesDirectoryExist)
> dirs' expression gives the following compilation error:
>
> ~/code/haskell$ ghc -o newbie newbie.hs
>
> newbie.hs:16:35:
>     Couldn't match expected type `Bool' against inferred type `IO Bool'
>       Expected type: FilePath -> Bool
>       Inferred type: FilePath -> IO Bool
>     In the second argument of `(.)', namely `doesDirectoryExist'
>     In the first argument of `filterM', namely
>         `(not . doesDirectoryExist)'