使用谓词提升到Maybe

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 -> (<$) <*> guard . p \\p -> (<$) <*> guard . p .

Answer 2

One way to compose it is like this, using Control.Monad :

liftPredM :: MonadPlus m => (a -> Bool) -> a -> m a
liftPredM p = mfilter p . return

Another alternative is to use Data.Foldable :

liftPredF :: (a -> Bool) -> a -> Maybe a
liftPredF f = find f . pure

This is, however, less general, so I'd lean towards favouring the MonadPlus -based implementation.

In both cases, though, the idea is to first lift a 'naked' value into a container using either pure or return , and then apply a filter. Often, you don't need to declare an actual function for this; instead, you can just inline the composition where you need it.

Examples:

Prelude Control.Monad> liftPredMaybe even 42 :: Maybe Integer
Just 42
Prelude Control.Monad> liftPredMaybe (even . length) "foo" :: Maybe String
Nothing
Prelude Control.Monad> liftPredMaybe (odd . length) "foo" :: Maybe String
Just "foo"