On Tue, May 14, 2002 at 04:57:12PM +0200, George Russell wrote: > According to the report > > Instances of Monad should satisfy the following laws: > > > > return a >>= k = k > > m >>= return = m > > m >>= (\x -> k x >>= h) = (m >>= k) >>= h > so neither IO nor my events satisfy this. Up to now I haven't had > any problems with this. > Does GHC or any other Haskell compiler actually rely on instances of > Monad satisfying left identity? If not, I would suggest dropping > the requirement, if it can be done without upsetting category > theorists. (What the hell, they stole the term "Monad" from > philosophy and changed its meaning, so why shouldn't we?)
I don't think this is necessarily wise to drop this from the report altogether. To me, it seems comparable to associativity of addition for instances of Num; many instances don't satisfy it (e.g., Float), but it's a useful guideline to keep in mind. I've often been bothered by the inconsistent treatment of laws in the report; why are there laws for functors, monads, and quot/rem and div/mod, and not much else? I'm pleased to see that the laws that are given actually do have exceptions. --Dylan
msg10876/pgp00000.pgp
Description: PGP signature
