Daniel Fischer wrote:
I think, 1. should be acceptable to everybody, and 2. as a principle too, only the question of which effects are relevant needs to be answered. It's plain that not all measurable effects are relevant. My inclination to ignore the side-effects stemmed from the (irrational) desire to have IO's MonadPlus instance justified, now I'm prepared to say yes, side-effects such as output do count, so the instance MonadPlus IO is erroneous, but may be maintained for practical reasons.
I am sure monads in Haskell (and other functional languages like ML) are defined on types not values. Therefore it only matters that the types are correct and that the operator obeys the associative laws. I am reasonably sure the values whether returned or side-effects are irrelevent. Keean.