darcs patch: Add justIf to Data.Maybe

[Yitzchak Gale]

Johan Tibell wrote:
> Are all these small little functions
> really worth it?
> They're trivially defined using the current API...
> let developers use composition to define the rest?

I completely agree with you. But in this case, even
though these functions really are trivial, they come
up *all* the time, and there is just something awkward
about them.

We certainly won't add more than one of the two:

justIf x b = guard b >> return x
check p x = guard (p x) >> return x

justIf = flip $ check . const
check = ap justIf

Perhaps the following is a bit of an explanation
of why they feel awkward:

<yitz> @pl \ p x -> guard (p x) >> return x
<lambdabot> (`ap` return) . (((>>) . guard) .)

<yitz> @pl \ p x -> if p x then return x else mzero
<lambdabot> flip flip mzero . (`ap` return) . (if' .)

Please note that if we choose to add justIf, it should
be spelled with an uppercase 'I' to be consistent with
Haskell conventions.

Thanks,
Yitz