Data.Generics.Is

Generate predicates from constructor names or from quoted patterns.

You must enable the TemplateHaskell extension to use this module

From constructors

Given a constructor for type T, is generates a function of type T -> Bool.

The function evaluates its argument to WHNF, and returns True if the head constructor matches the given one, False otherwise.

$(isNot 'Con) ≡ not . $(is 'Con)

>>> $(is 'Just) (Just 5)
True
>>> $(isNot 'Left) (Left "a")
False

is :: Name -> Q Exp

isNot :: Name -> Q Exp

From patterns

Given a pattern for type T, isP generates a function of type T -> Bool.

The function returns True if the expression matches the pattern; a and False otherwise.

$(isNot patQ) ≡ not . $(isP patQ)

$(isP [p| Con{} |]) ≡ $(is 'Con)

>>> $(isP [p| Just _ |]) Nothing
False
>>> $(isNotP [_,_,_]) [2,1]
True

isP :: Q Pat -> Q Exp

isNotP :: Q Pat -> Q Exp