[Haskell-cafe] Use forM_ with Maybe, it's Foldable!

[Felipe Almeida Lessa]

Herbert Valerio actually pointed out to me in PVT that funnily enough
I've actually *forgotten* and *rediscovered* this idiom! =)

  http://www.haskell.org/pipermail/libraries/2011-November/017257.html

Cheers!

On Sat, Jan 26, 2013 at 5:43 PM, Felipe Almeida Lessa
<felipe.lessa at gmail.com> wrote:
> Hey,
>
> Just wanted to share this little insight I had that I don't think is a
> common idiom.  Often times I need to define:
>
>   whenJust :: Monad m => Maybe a -> (a -> m b) -> m ()
>   whenJust (Just x) f = f x
>   whenJust Nothing _ = return ()
>
> It always bugged me that you can't find this function anywhere.
> Actually, you can find it on at least a couple of packages [1], but
> you can't find it on a standard place.  Or can you?
>
> A few days ago I decided to hoogle the type of whenJust [2] and what I
> discovered is that
>
>   import Data.Foldable (forM_)
>   whenJust = forM_
>
> For example, instead of:
>
>   mfoo <- doSomething
>   case mfoo of
>     Just foo -> ...
>     Nothing -> return ()
>
> You may just write:
>
>   forM_ mfoo $ \foo ->
>     ...
>
> There you go. I hope this is useful to someone =).
>
> Cheers,
>
> [1] http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=whenJust
> [2] http://www.haskell.org/hoogle/?hoogle=Maybe%20a%20-%3E%20(a%20-%3E%20m%20b)%20-%3E%20m%20()
>
> --
> Felipe.



-- 
Felipe.