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

14 May 2008

      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):

    -- 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)'

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

Mike Jarmy