[Haskell-cafe] mixing map and mapM ?

[Bill Atkins]

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 <nccb2 at 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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