haskell - Lift to Maybe using a predicate

Question

I'm searching for something like

liftPredMaybe :: (a -> Bool) -> a -> Maybe a
liftPredMaybe p a
  | p a = Just a
  | otherwise = Nothing

Is there such a function in Haskell already?

Answer 1

Not quite a ready-made solution, but with guard (from Control.Monad) and (<$) (from Data.Functor) we can write:

ensure :: Alternative f => (a -> Bool) -> a -> f a
ensure p a = a <$ guard (p a)

(Thanks to Daniel Wagner for suggesting a nice name for this function.)

A more pointfree spelling of dubious taste is p -> (<$) <*> guard . p.