Yep, you and Ben are both correct. Mea culpa and sorry for the bad answer.
Just curious: why does getModificationTime take an IO FileStatus rather than a FileStatus? On Thu, May 6, 2010 at 7:00 AM, Neil Brown <nc...@kent.ac.uk> wrote: > 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) > > Also, I like this style: > > import Control.Applicative ((<$>)) > > blah = do > times <- mapM (PF.modificationTime <$> PF.getFileStatus) filenames > ... > > The <$> operator evaluates to fmap so it's a cleaner way to apply a pure > function to an IO value. > > That won't type-check (unless I've missed some crafty trick with the > types?!); you have two functions you want to compose, but the <$> operator > (i.e. fmap) applies a function on the left to a functor-value on the right. > You would instead need: > > > times <- mapM ((PF.modificationTime <$>) . PF.getFileStatus) filenames > > At which point I prefer Ivan's liftM version rather than the above section > (or worse: using (<$>) prefix). The original request is a relatively common > thing to want to do, so I was slightly surprised that hoogling for: > > (b -> c) -> (a -> f b) -> a -> f c > > didn't turn up any relevant results. This function is a lot like (<=<) but > with a pure rather than side-effecting function on the left-hand side. > > Thanks, > > Neil. >
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