{-# LANGUAGE FlexibleContexts #-}

module Lecture09mtl where

import Control.Applicative
import Control.Monad.Reader
import Data.Map ( Map )
import qualified Data.Map as Map

type Varname = String

data AExpr = Con Integer
           | Var Varname
           | Add AExpr AExpr
           | Mul AExpr AExpr
           deriving ( Eq, Show, Read )

type Assignment = Map Varname Integer

value :: MonadReader Assignment m => Varname -> m Integer
value v = reader (Map.findWithDefault 0 v)

evala :: MonadReader Assignment m => AExpr -> m Integer
evala (Con x)   = pure x
evala (Var x)   = value x
evala (Add x y) = liftA2 (+) (evala x) (evala y)
evala (Mul x y) = liftA2 (*) (evala x) (evala y)