Library enhancement request: Data.Maybe.justIf

Hi,

I found myself often writing the idiom "if a then Just v else Nothing". It can be nicely abbreviated by the function:

justIf :: a -> Bool -> Just a
justIf x True = Just x
justIf x False = Nothing

Which would allow me to write the nicely write- and readable v justIf a.

Asking on #haskell, I got positive feedback about this addition:

<endojelly> nomeata, I'm all for it. my last project was *full* of that idiom

Trac metadata

Trac field	Value
Version	6.10.4
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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