gistfile1.hs

{-# LANGUAGE GADTs, EmptyDataDecls#-}

module Calculator where

type Stack = [Double]

data Arity = Unary | Binary
data Associativity = Left | Right

data Operator = Operator Arity Associativity

data Expr a where
    Add :: Expr (Operator Binary Left)
    Sub :: Expr (Operator Binary Left)