Mauricio wrote:
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"?
do-it-yourself? you can start from reader definition and add what you need. you just need to make "initial state" consisting from state itself and update function so `run` will have just one initialization argument
Sure. I've done a few versions, trying to change the way (>>=) is defined, and learned a lot with that. But I wanted to know if there's already the "right way to do it" instead of my "newbie way to do it" :)
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)" ? Because the monad laws guarantee those two things should be the same, and yet the first is zero steps and the second is one step, going by the crude "counting >>=s" method I'm guess you were thinking of. So I think you'd have to make the steps explicit. You could do this with a custom version of (>>) and (>>=) which automatically do a step, for example. So advance :: m () -- your primitive which changes the environment a >>* b = a >> advance >> b a >>*= f = do { r <- a; advance; f r } Does that help? Jules