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 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)%...)

-- Felipe.

-- Felipe.