Generate predicates from constructor names or from quoted patterns.

You must enable the TemplateHaskell extension to use this module.

Given a constructor (or pattern synonym) 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 '(:)) [1,2,3]
False

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

Given a type T, for each constructor C, we can declare predicates isC, isNotC : T → Bool.

Type T can be a newtype, data, or GADT declaration.

Constructors with non-alphanumeric names (e.g. :+:) are ignored silently. As a workaround, we suggest giving the constructors alphanumeric names, and creating pattern synonyms with the desired symbolic names.

data E a where
   Plus     :: E Int -> E Int -> E Int
   And      :: E Bool -> E Bool -> E Bool 
   Lit      :: a -> E a
   (:*:)    :: (Num a) => E a -> E a -> E a
   Showable :: (Show a) => a -> E String

pattern a :+: b = Plus a b