guard

[Ollie Charles]

Any interest in something like:

guard : a -> (a -> Bool) -> Maybe a
guard a pred = if pred a then Just a else Nothing

I'm actually using it in the sense of guard someHtmlView (always someCondition), so I don't strictly need a predicate function, a single bool would do. But the more general case seems like it might be the better addition.

[Dave Keen]

I also use the very similar when all the time:

when : Bool -> a -> Maybe a
when test value =
    if test then
        Just value
    else
        Nothing

I would love to see one or both of these two functions added to the library.

[Ian Phillips]

@ccapndave I have the very same function, also unless as the inverse of it (mainly because it reads better).

[Sky Rose]

Decided not to include this after discussion in #49