Skip to content

GitLab

GHC
GHC
Open
Opened by cactus@trac-cactus

Associated pattern synonyms

The PatternSynonyms wiki page has a section on (typeclass-)associated pattern synonyms:

class ListLike l where
      pattern Nil :: l a
      pattern Cons :: a -> l a -> a
      isNil :: l a -> Bool
      isNil Nil = True
      isNil (Cons _ _) = False
      append :: l a -> l a -> l a

instance ListLike [] where
      pattern Nil = []
      pattern Cons x xs = x:xs
      append = (++)

headOf :: (ListLike l) => l a -> Maybe a
headOf Nil = Nothing
headOf (Cons x _) = Just x
Edited by Ben Gamari