On Thu, 2010-05-06 at 12:09 +0100, Ben Millwood wrote: > On Thu, May 6, 2010 at 11:51 AM, Bill Atkins <watk...@alum.rpi.edu> wrote: > > 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 <$> as an infix operator, so > > mapM (liftM PF.modificationTime . PF.getFileStatus) filenames > > is probably clearer in this case. Or, if you're feeling particularly silly: > > mapM (fmap fmap fmap modificationTime getFileStatus) filenames
I usually use <.> (IMHO should be in Control.Applicative next to <$>) which is the same to <$> as . is to $: g <.> f = fmap g . f :: Functor f => (b -> c) -> (a -> f b) -> a -> f c times <- mapM (PF.modificationTime <.> PF.getFileStatus) filenames Regards
signature.asc
Description: This is a digitally signed message part
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
