lec05/Calc.hs


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module Calc where

import ExprT
import Parser

-- Exercise 1

eval :: ExprT -> Integer
eval (Lit n) = n
eval (Add l r) = eval l + eval r
eval (Mul l r) = eval l * eval r

-- Exercise 2

evalStr :: String -> Maybe Integer
evalStr s = case parseExp Lit Add Mul s of
                 Nothing -> Nothing
                 Just t  -> Just (eval t)

-- Exercise 3

class Expr a where
      lit :: Integer -> a
      add :: a -> a -> a
      mul :: a -> a -> a

instance Expr ExprT where
         lit = Lit
         add = Add
         mul = Mul

reify :: ExprT -> ExprT
reify = id