bilist.hs

{-# LANGUAGE GADTs, StandaloneDeriving #-}

data List a b where
  Nil   :: List a b
  ConsI :: Int -> List Bool Int -> List Int Bool
  ConsB :: Bool -> List Int Bool -> List Bool Int

deriving instance (Show a, Show b) => Show (List a b)

gistfile1.txt

*Main> :t ConsI 1 (ConsB True Nil)
ConsI 1 (ConsB True Nil) :: List Int Bool

*Main> :t ConsI 1 (ConsI True Nil)

<interactive>:1:10:
    Couldn't match expected type `Bool' with actual type `Int'
    Expected type: List Bool Int
      Actual type: List Int Bool
    In the return type of a call of `ConsI'
    In the second argument of `ConsI', namely `(ConsI True Nil)'
*Main> :t ConsI 1 (ConsI 2 Nil)

<interactive>:1:10:
    Couldn't match expected type `Bool' with actual type `Int'
    Expected type: List Bool Int
      Actual type: List Int Bool
    In the return type of a call of `ConsI'
    In the second argument of `ConsI', namely `(ConsI 2 Nil)'