Re: [Haskell-cafe] Using a monad to decompose a function into functions

On Friday 13 March 2009, Cristiano Paris wrote:

2009/3/13 Marcin Kosiba :

...

... Threading the state is not the problem. Maybe this will help: what I have now:

fsm world state = case state of  first ->    do_stuff_one    (move_up, succ state)  second ->    do_stuff_two    (move_left, succ state)  third ->     do_stuff_three     (move_right, first)

what I'd want to have is to say: fsm world state = do  do_stuff_one  yield move_up  do_stuff_two  yield move_left  do_stuff_three  yield move_right  fsm world state

and have it "translated" to:

fsm world state =  do_stuff_one  (move_up, \world' state' ->    do_stuff_two      (move_left, \world'' state'' ->         do_stuff_three           (move_right, fsm world'' state'')

Hi,

I've not fully understood your exact problem but I think you might have a look to Continuations and Delimited Continuations.

Both can help you solve the problem with implementing a yield statement. You can have a look at one of my (rather) old blog's posts about how to implement yield/send statements a-la-python:

http://monadicheadaches.blogspot.com/2008/01/python-25s-iterators-in-haskel l-sort-of.html

Notice that blogspot messed up with code blocks so indentation looks bad and some character is even missing.

Hi, I've already checked those out. I tried using your yield implementation and while it works, I couldn't get it to work with the state monad. So while: data RecPair a b = Nil | RP (b, a -> RecPair a b) yield x = Cont $ \k -> RP (x, k) got me half-way to my goal, I couldn't figure out how to make something like: yield' = do state <- get state' <- yield state put state' Thanks! Marcin Kosiba