n-arity.hs

-- | How do I write inductive instances for n-arity functions? Does this have a
-- name? How can I add functional dependencies to 'MapLast'?
class MapLast s t a b where
  mapLast :: (a -> b) -> (s -> t)
instance MapLast a b a b where
  mapLast = ($)
instance MapLast (z -> a) (z -> b) a b where
  mapLast = (.)
instance MapLast (y -> z -> a) (y -> z -> b) a b where
  mapLast = (.) . (.)
instance MapLast (x -> y -> z -> a) (x -> y -> z -> b) a b where
  mapLast = (.) . (.) . (.)
instance MapLast (w -> x -> y -> z -> a) (w -> x -> y -> z -> b) a b where
  mapLast = (.) . (.) . (.) . (.)

--  Example usage:
--
--  > mapLast not True :: Bool
--  False
--  > mapLast not (\() -> True) () :: Bool
--  False
--  > mapLast not (\() () -> True) () () :: Bool
--  False
--  > mapLast not (\() () () -> True) () () () :: Bool
--  False
--  > mapLast not (\() () () () -> True) () () () () :: Bool
--  False