From: Ramakrishnan Muthukrishnan <ram@rkrishnan.org>
Date: Sun, 21 Dec 2014 12:07:24 +0000 (+0530)
Subject: hw5: exercise 3
X-Git-Url: https://git.rkrishnan.org/webform_css?a=commitdiff_plain;h=04036d0605e95c06f5d6dea11c2618267c059550;p=yorgey.git

hw5: exercise 3
---

diff --git a/hw5/Calc.hs b/hw5/Calc.hs
index ef4fd2e..46f7820 100644
--- 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