module Minilang.Lambda.Eval where

import Data.Text (Text, unpack)
import Debug.Trace (trace)

data Term
  = Var Text
  | Lam Text Term
  | App Term Term
  deriving (Show, Read, Eq)

type Env = [(Text, Value)]

data Value
  = V Text
  | Abs Text Term Env
  | Ap Value Value
  deriving (Show, Read, Eq)

-- call-by-value evaluator
eval :: Term -> Env -> Value
eval (Var x) env = case lookup x env of
  Just v -> v -- we do not need to eval v again
  Nothing -> V x -- x is not bound in env
eval (Lam x body) env = Abs x body env
eval (App f a) env =
  -- we need to force evaluation of the argument
  -- because haskell's default semantics is non-strict
  -- so if a' is never used, it will not be evaluated!
  let a' = eval a env
   in seq a' $ case eval f env of
        Abs x body env' -> eval body ((x, a') : env')
        f' -> Ap f' a'

evalNeed :: Term -> Env -> Value
evalNeed (Var x) env = case lookup x env of
  Just v -> v
  Nothing -> V x
evalNeed (Lam x body) env = Abs x body env
evalNeed (App f a) env =
  let a' = eval a env
   in case evalNeed f env of
        Abs x body env' -> evalNeed body ((x, a') : env')
        f' -> Ap f' a'