Seems that quickCheck is telling me that False is not equal to False?

Question

Been trying to write my own Prelude, and to write QuickCheck properties along with it. Writing Prelude is going well, but in doing the QuickCheck I have managed to get blocked early. Here are snippets of the 2 files I have:

-- MyLude.hs

module MyLude where

import Prelude (Bool(..))

(&&) :: Bool -> Bool -> Bool
(&&) True a = a
(&&) _ _ = False

-- MyLudeTest.hs

module MyLudeTest where

import qualified MyLude as P

prop_test_and a b = a && b == a P.&& b

Then in ghci I run:

:load MyLudeTest.hs
:m +Test.QuickCheck
quickCheck prop_test_and

and get the following error:

*** Failed! Falsifiable (after 1 test):
False
False

What confuses me is that I have implemented (||) and implemented a quickcheck property for it that does pretty much the same thing the prop_test_and does, and that seems to have no problems. What am I missing?

Answer 1

This is just operator precedence at work. Your property is parsed as:

prop_test_and a b = a && (b == a) P.&& b

Answer 2

Precedence!

Prelude Test.QuickCheck> (False && False) == myand False False
True
Prelude Test.QuickCheck> False && False == myand False False
False

You're missing parens around your operators. Remember (&&) binds weaker (3) than (==) (4)