On Thu, May 6, 2010 at 11:51 AM, Bill Atkins
Almost - "liftM modificationTime" has type Status -> IO EpochTime. Like other IO functions (getLine, putStrLn), it returns an IO action but accepts a pure value (the modification time)
ghci> :m +Control.Monad System.Posix.Files ghci> :t liftM modificationTime liftM modificationTime :: (Monad m) => m FileStatus -> m System.Posix.Types.EpochTime where m = IO in this case.
Also, I like this style: import Control.Applicative ((<$>)) blah = do times <- mapM (PF.modificationTime <$> PF.getFileStatus) filenames ... The <$> operator evaluates to fmap so it's a cleaner way to apply a pure function to an IO value.
Usually I'd agree but in fact PF.getFileStatus is not an IO value, but an IO function, so you need to map over its result: mapM ((PF.modificationTime <$>) . PF.getFileStatus) filenames but then you lose the convenience of the <$> as an infix operator, so mapM (liftM PF.modificationTime . PF.getFileStatus) filenames is probably clearer in this case. Or, if you're feeling particularly silly: mapM (fmap fmap fmap modificationTime getFileStatus) filenames