Control.Monad proposal: Add whenJust

[Roman Cheplyaka]

* Ben Millwood <haskell at benmachine.co.uk> [2013-05-12 10:11:01+0100]
> You can mostly minimise harm by only hiding specific things, but
> that's still more effort than I feel like I should have to go to. I
> think if we decide that the Foldable approach is useful enough to go
> in base, we should not make it a second-class citizen.

... except it is in base already :)

Roman