hw5: exercise 3

diff --git a/hw5/Calc.hs b/hw5/Calc.hs
index ef4fd2e2542a1d2ccf8cbb22ed01e335d2f42b74..46f782018c617ae69a0bb440c77af5a753d1cbed 100644 (file)
--- a/hw5/Calc.hs
+++ b/hw5/Calc.hs
@@ -18,3 +18,13 @@ evalStr :: String -> Maybe Integer
 --               Just e -> Just (eval e)
 --              Nothing -> Nothing
 evalStr s = fmap eval (parseExp Lit Add Mul s)
+
+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