[Haskell-cafe] printing a list of directories which don't exist
mjarmy at gmail.com
Wed May 14 11:47:25 EDT 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
-- 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
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)'
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