cleaned up error reporting

day2.hs

@@ -32,13 +32,13 @@ validB (Password a' b' t p) =
     b = (pred b')
 
 parse :: String -> Maybe Password
-parse [] = error "empty password"
+parse [] = Nothing
 parse s =
   do (l,s') <- parseInt s
      (u,s'') <- parseInt $ dropChar s'
      (t,s''') <- parseChar $ dropChar s''
      let p = (dropChar . dropChar) s'''
-      in (Just $ Password l u t p)
+      in Just $ Password l u t p
 
 parseInt :: String -> Maybe (Int,String)
 parseInt s = case (reads s) of