module ABC where

newtype A = A ()
newtype B = B ()
newtype C = C ()

data T = TA A | TB B | TC C

class TClass a where
  f1 :: a -> a -> a
  f2 :: a -> a -> a
  f3 :: a -> a -> a
instance TClass A where
  f1 (A _) (A _) = A ()
  f2 (A _) (A _) = A ()
  f3 (A _) (A _) = A ()
instance TClass B where
  f1 (B _) (B _) = B ()
  f2 (B _) (B _) = B ()
  f3 (B _) (B _) = B ()
instance TClass C where
  f1 (C _) (C _) = C ()
  f2 (C _) (C _) = C ()
  f3 (C _) (C _) = C ()

apply :: TClass a => (a -> a -> a) -> T -> T -> T
apply fn x y =
  case (x, y) of
    (TA x, TA y) -> TA (fn x y)
    (TB x, TB y) -> TB (fn x y)
    (TC x, TC y) -> TC (fn x y)
    _            -> error "T constructors do not match"

{---------------------------------------------------------------------------------------------------
[1 of 1] Compiling ABC              ( ABC.hs, ABC.o )

ABC.hs:29:25:
    Couldn't match type `A' with `C'
    In the return type of a call of `fn'
    In the first argument of `TA', namely `(fn x y)'
    In the expression: TA (fn x y)

ABC.hs:29:28:
    Couldn't match type `A' with `C'
    In the first argument of `fn', namely `x'
    In the first argument of `TA', namely `(fn x y)'
    In the expression: TA (fn x y)

ABC.hs:29:30:
    Couldn't match type `A' with `C'
    In the second argument of `fn', namely `y'
    In the first argument of `TA', namely `(fn x y)'
    In the expression: TA (fn x y)

ABC.hs:30:25:
    Couldn't match type `B' with `C'
    In the return type of a call of `fn'
    In the first argument of `TB', namely `(fn x y)'
    In the expression: TB (fn x y)

ABC.hs:30:28:
    Couldn't match type `B' with `C'
    In the first argument of `fn', namely `x'
    In the first argument of `TB', namely `(fn x y)'
    In the expression: TB (fn x y)

ABC.hs:30:30:
    Couldn't match type `B' with `C'
    In the second argument of `fn', namely `y'
    In the first argument of `TB', namely `(fn x y)'
    In the expression: TB (fn x y)
---------------------------------------------------------------------------------------------------}