add third exercise of lecture 5

1 files changed, 16 insertions, 1 deletions

diff --git a/lec05/Calc.hs b/lec05/Calc.hs
index cd6f408..f97d784 100644
--- a/lec05/Calc.hs
+++ b/lec05/Calc.hs
@@ -15,4 +15,19 @@ eval (Mul l r) = eval l * eval r
 evalStr :: String -> Maybe Integer
 evalStr s = case parseExp Lit Add Mul s of
                  Nothing -> Nothing
-                 Just t -> Just (eval t)
+                 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