Eval.hs

-- Copyright 2018 Google LLC.
-- SPDX-License-Identifier: Apache-2.0

{-# LANGUAGE FlexibleContexts #-}

import Control.Applicative
import Control.Monad.Writer

data Expr = Lit Int | Plus Expr Expr | Times Expr Expr
  deriving (Eq, Show)

ev ev e = ev' e
  where ev' (Lit i) = pure i
        ev' (Plus a b) = liftA2 (+) (ev a) (ev b)
        ev' (Times a b) = liftA2 (*) (ev a) (ev b)

evTell ev0 ev e = do
  v <- ev0 ev e
  tell [(e, v)] 
  return v

eval e = execWriter (fix (evTell ev) e)

main = mapM_ print (eval test)
  where test = (Lit 1 `Plus` Lit 2) `Times` (Lit 3 `Plus` Lit 4)

This is nice, but is there a way to stuff the result alongside the AST, as in
(Times (Plus (Lit 1, 1) (Lit 2, 2), 3) (Plus (Lit 3, 3) (Lit 4, 4), 7),21) ?
IMO it is shorter and context is maintained.