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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