Must mplus always be associative? Haskell wiki vs. Oleg Kiselyov

Question

The Haskell wikibook asserts that

Instances of MonadPlus are required to fulfill several rules, just as instances of Monad are required to fulfill the

Answer 1

The two laws that everybody agrees that MonadPlus should obey are the identity and associativity laws (a.k.a. the monoid laws):

mplus mempty a = a

mplus a mempty = a

mplus (mplus a b) c = mplus a (mplus b c)

I always assume they hold in all MonadPlus instances that I use and consider instances that violate those laws to be "broken", whether or not they were written by Oleg.

Oleg is right that associativity does not play nicely with breadth-first search, but that just means that MonadPlus is not the abstraction he is looking for.

To answer the point you made in a comment, I would always consider that rewrite rule of yours sound.