Is there some abstraction in current ghc library that implements something like Reader, but where the value of the environment is updated at every "step"?
It doesn't quite make sense, because one "step" isn't well defined. How many "steps" is "return (f x)" ? how about "return x >>= \y -> return (f y)" ? (...)
I understand. But do you think something like the (obviously not working) code below could respect monad laws, if I could consider (environment->a) a monad over a? update = snd . next ; -- this updates a random number generator instance RandomGen environment => Monad ( environment -> a ) where { -- below, f :: g1 -> ( environment -> g2 ) p >>= f = p2 where { p2 e = ( f . p $ e ) . update } ; return = const ; } Then I would do something like: getStdGen >>= ( return . do { a >>= b >>= c } )
So I think you'd have to make the steps explicit. (...)
advance :: m () -- your primitive which changes the environment
a >>* b = a >> advance >> b a >>*= f = do { r <- a; advance; f r }
The problem is that I need 'a' or 'b' above to sometimes also change the environment. I think with this method I could not get that. Thanks, Maurício