darcs patch: Add justIf to Data.Maybe

[Jon Fairbairn]

Joachim Breitner <mail at joachim-breitner.de> writes:
> are you proposing that, if check is available, no justIf is needed in
> the libraries? Or are you just proposing a different implementation?

For the record, I am opposed to justIf on the grounds that it is a
specialisation of something that should be typed at MonadPlus (and which
would consequently have to have a different name).

-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk