From 2b6de7cb0402e67dd8792d45a9466d7b085666af Mon Sep 17 00:00:00 2001
From: Edoardo La Greca
Date: Thu, 10 Jul 2025 00:37:43 +0200
Subject: add third exercise of lecture 5

---
 lec05/Calc.hs | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

(limited to 'lec05/Calc.hs')

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
-- 
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