asdf.hs

{-# LANGUAGE TypeFamilies, DeriveFunctor #-}
module Expr where

import Data.Functor.Foldable

data ExprF a
 = ConstF Double
 | AddF a a
 | SubF a a
 | MulF a a
 | DivF a a
 | NegF a
 deriving Functor

data Expr = Const Double
          | Add Expr Expr
          | Sub Expr Expr
          | Mul Expr Expr
          | Div Expr Expr
          | Neg Expr

type instance Base Expr = ExprF
instance Recursive Expr where
    project (Const d) = ConstF d
    project (Add e1 e2) = AddF e1 e2
    project (Sub e1 e2) = SubF e1 e2
    project (Mul e1 e2) = MulF e1 e2
    project (Div e1 e2) = DivF e1 e2
    project (Neg e) = NegF e