Question

Code:

data Exp a = Const a | Eq (Exp a) (Exp a)

I want the Const a to contain a value of type show so that i can print it later. So in C# i would write:

class Const : Exp { IShow X; }
class Eq : Exp { Exp X, Y; }

How can i do that in Haskell?

Answer 1

{-# LANGUAGE GADTs #-}

data Exp a where
    Const :: Show a => a -> Exp a
    Eq :: Exp a -> Exp a -> Exp a

If you want to allow for varying data types in different branches of Eq that’s fine too.

data Exp where
    Const :: Show a => a -> Exp
    Eq :: Exp -> Exp -> Exp