Incidentally, it is quite common to define a Monad instance but no Eq instance. (e.g., the IO monad, most parser monads, most state transformer monads, etc.) So you should not interpret the '==' in the monad law as requiring you to define an Eq instance. If you do define an Eq instance, it ought to be reflexive, symmetric and transitive (i.e., an equivalence) if you want functions with Eq constraints to behave in a meaningful way. Also, although there's probably no necessity for your Eq instance to match your notion of equality between computations (i.e., the == used in the monad laws), I think you'll end up very confused if you define an Eq instance which doesn't match in the same way that having Eq on pairs ignore the 2nd field would confuse you. -- Alastair Reid www.haskell-consulting.com
So, the question remains: when the monad laws say that
(return x) >>= f == f x
what is intended in that "=="? Eq."==" for
instance Eq MyCustomMonad where x == y = whatever...
or a more profound kind of equality?