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

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. On Thu, May 6, 2010 at 1:20 AM, wrote:

On Thu, 6 May 2010 15:07:30 +1000 Ivan Miljenovic wrote:

...

On 6 May 2010 15:01, wrote:

...

I was doing the following:

do status <- mapM PF.getFileStatus filenames let times = map PF.modificationTime status let sorted = sortBy (\(_, t1) (_,t2) -> compare t1 t2) (zip filenames times)

times <- mapM (liftM PF.modificationTime . PF.getFileStatus) filenames

However, I'd be tempted to leave it as is (and hope/assume that fusion does its magic).

well now it's obvious :-) I did have liftM in there, but just couldn't quite figure out how to tie things together.

to be completely clear : liftM takes modificationTime from

Status -> EpochTime

to

IO Status -> IO EpochTime

so now it can operate on the results of getFileStatus, which returns `IO Status`.

mapM gathers the [IO EpochTime] into `IO [EpochTime]` and then <- gives [EpochTime].

It's a little more clear in the verbose form, isn't it ?

Thanks !

Brian _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe