gistfile1.hs

{-# LANGUAGE GADTs, EmptyDataDecls#-}

module Calculator where

type Stack = [Double]

-- Empty data declarations to use as 'tags'
data Unary
data Binary
data Left
data Right
data Operator a b
data Paren a

-- Just for fun, encode EVERYTHING into the type
data Expr a where
    Add :: Expr (Operator Binary Left)
    Sub :: Expr (Operator Binary Left)
    Mul :: Expr (Operator Binary Left)
    Div :: Expr (Operator Binary Left)
    Exp :: Expr (Operator Binary Right)
    Lpr :: Expr (Paren Left)
    Rpr :: Expr (Paren Right)
    Val :: Expr (Double)