On Thu, 02 Oct 2003 11:22:13 +0200
"Marcin 'Qrczak' Kowalczyk"
As you discovered, there is no meaningful count of operations. If an operation doesn't do anything, do you count it?
It's not about counting the operations (that's just an example), but accumulating any kind of state. For example: data Accum a = Ac [a] a instance Monad Accum where return x = Ac [x] x Ac _ x >>= f = let Ac l x' = f x in Ac (l ++ [x']) x' dupAcc x = Ac [x,x] x m1 = (return 'a') >>= dupAcc -- Ac "aaa" 'a' m2 = dupAcc 'a' -- Ac "aa" 'a'
but monad laws say that "return" *really* doesn't do anything
I don't see it. As I understand, the problem (if any) would be in my implementations of either >>= or f (dupAcc, in this case). I get the same values for m1 and m2 even if I define return x = Ac [] x. I'm not trying to create useful monads (I'm pretty sure they aren't :), but understanding the concepts. 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? Juanma