Challenge in parsing math expressions from some kinda AST.

MathExpr.hs

import Control.Monad.Reader
import Data.Map as Map

data Expr = Lit Int | Var String | Add Expr Expr | Let (String, Expr) Expr

eval :: Expr -> Reader (Map String Int) Int
eval (Lit i) = pure i
eval (Var s) = do
  varMap <- ask
  case Map.lookup s varMap of
    Just i -> pure i
    Nothing -> error "No such variable!"
eval (Add expr1 expr2) = do
  x <- eval expr1
  y <- eval expr2
  pure $ x + y
eval (Let (s, expr1) expr2) = do
  val <- eval expr1
  withReader (modifyReader val) (eval expr2)
 where
  modifyReader :: Int -> Map String Int -> Map String Int
  modifyReader val' r = do
    Map.insert s val' r

evalTest :: Bool
evalTest =
  let letExpr = Let ("z", Add (Lit 5) (Lit 5)) (Add (Var "z") (Lit 10))
      x = runReader (eval (Add (Var "x") letExpr)) $ Map.fromList [("x", 3)]
  in x == 23