Re: [Haskell-cafe] I think I discovered my first Monoid instance

Am 29.09.20 um 10:27 schrieb Mario Lang:

instance Monoid QuadBitboard where mempty = QBB 0 0 0 0

-- | bitwise XOR instance Semigroup QuadBitboard where QBB b0 b1 b2 b3 <> QBB b0' b1' b2' b3' = QBB (b0 `xor` b0') (b1 `xor` b1') (b2 `xor` b2') (b3 `xor` b3')

But maybe I am violating some laws

The Semigroup is okay, since 'xor' is indeed associative, and your instance basically lifts it to 4-tuples. The Monoid instance is wrong, though. There is no unit for 'xor'! Cheers Ben