add second exercise of lecture 5

1 files changed, 8 insertions, 0 deletions

diff --git a/lec05/Calc.hs b/lec05/Calc.hs
index 8051d76..cd6f408 100644
--- a/lec05/Calc.hs
+++ b/lec05/Calc.hs
@@ -1,6 +1,7 @@
 module Calc where
 
 import ExprT
+import Parser
 
 -- Exercise 1
 
@@ -8,3 +9,10 @@ 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)