[Haskell-cafe] Re: mixing map and mapM ?

On Thu, 2010-05-06 at 12:09 +0100, Ben Millwood wrote:

On Thu, May 6, 2010 at 11:51 AM, Bill Atkins 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