[Haskell-beginners] Help! Trapped in the IO Monad!

[Alexander Dunlap]

On Wed, Jan 28, 2009 at 5:18 PM, Erik de Castro Lopo
<mle+cl at mega-nerd.com> wrote:
> Alexander Dunlap wrote:
>
>> You can do (something like; this is untested)
>>
>> splitDirFile :: [FilePath] -> IO ([FilePath],[FilePath])
>> splitDirFile [] = return ([],[])
>> splitDirFile (f:fs) = do
>>   (yess,nos) <- splitDirFile fs
>>   exists <- doesDirectoryExist f
>>   return $ if exists
>>     then (f:yess,nos)
>>     else (yess,f:nos)
>
> Untested, but seems to work perfectly :-). Thanks.
>
> However, that brings me to the next stage where again I'm trapped.
> I would like to do a foldl' on a function that returns IO [FilePath].
>
> I tried using Control.Monad.foldM, but then I end up with a function
> taht return :
>
>   IO (IO ([FilePath]))
>
> which doesn't work :-).
>
> It seems pretty obvious that I could implement a recursion like
> Alexander did above for splitDirFile but I was wondering if there
> was a more general solution.
>
> Cheers,
> Erik

It seems like foldM ought to do what you want. Could you post some
more details please?

Alex