printing a list of directories which don't exist
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)'
On Wed, 14 May 2008, Mike Jarmy wrote:
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
should be
existing <- filterM doesDirectoryExist dirs
I think.
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)'
participants (3)
-
Daniel Fischer -
Henning Thielemann -
Mike Jarmy