darcs patch: Add justIf to Data.Maybe

[Johan Tibell]

Hi,

Inspired by a discussion we've been having around the network package
and its API I would like to ask: Are all these small little functions
really worth it?

They're trivially defined using the current API and they make the
current API bigger and hence more complicated. Is there a way to
figure out which are the most general functions expose them and let
developers use composition to define the rest?

Just a thought,

Johan