Just package:explainable-predicates

just :: Predicate a -> Predicate (Maybe a)
explainable-predicates Test.Predicates
A Predicate that accepts Maybe values of Just x, where x matches the given child Predicate. 
>>> accept (just (eq "value")) Nothing
False

>>> accept (just (eq "value")) (Just "value")
True

>>> accept (just (eq "value")) (Just "wrong value")
False