16 Aug
2011
16 Aug
'11
3:23 a.m.
You can't define (>>=) if your state type changes: (>>=) :: m a -> (a -> m b) -> m c Whereas, your bind is effectively three different parametric types: _bind :: m1 a -> (a -> m2 b) -> m3 b You can use parametric monads to represent state changing within a monad. Oleg Kiselyov has tutorials on their use on his website (search for the Monadish class). The SHE experimental extension of Haskell also has parametric monads built in. bindish :: m s1 a -> (a -> m s2 b) -> m s2 b I don't think parametric monads will solve your problem though, as you want a product of the states as the result of bind. Are you really sure you want this behavior?, I'd imagine it breaks the monad laws anyway. http://okmij.org/ftp/Computation/monads.html