How to share state between two monads?

Question

I am stuck with following monad problem:

Let's say I have a standard monad State with state S = (LS, RS). I also have another monad:

newtype StateP a = StateP {runP :: S -> (a, RS)}

I want to perform some computation using StateP then merge state with state in State monad:

merge m :: StateP() -> State()
merge m = do
 s@(l,r) <- get
 put (l, snd (runP m s))

It is not working, but I don't get why? Is there another way to achieve such functionality?

In what way is it "not working"? Do you get compile errors (what are they?) or does it fail at runtime? The `merge` code you've posted looks like it shouldn't compile --- is it your actual code? — dave4420, Nov 04 '11 at 18:51

score 2 · Answer 1 · answered Nov 04 '11 at 18:53

2

On the off-chance that your code is nearly right, try

merge :: StateP() -> State()
merge m = do
    s@(l,r) <- get
    put (l, snd (runP m s))

But you need to give us more details really.

answered Nov 04 '11 at 18:53

dave4420

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Hey. It is actually working. I can't really say why I got some type errors before. Thanks for replying. – user1030210 Nov 04 '11 at 19:07
Oh looks like you didn't have `do`. – nponeccop Nov 04 '11 at 19:14
it was not missing. i think i might wrote fst instead of snd. anyway it should be marked for deletion. – user1030210 Nov 05 '11 at 21:51

score 1 · Accepted Answer · answered Nov 04 '11 at 19:55

You can use monad transformers to model these requirements more explicitly using two monad stacks: one that can only read LS and one that can both read and write LS.

type ReadOnlyLS a  = ReaderT LS (State RS) a
type ReadWriteLS a = StateT LS (State RS) a

To run a ReadOnlyLS within ReadWriteLS, we just need to extract LS from the outermost state layer, give it to the reader layer of the inner computation and lift the resulting computation back into the outer monad:

merge :: ReadOnlyLS a -> ReadWriteLS a
merge m = get >>= lift . runReaderT m

score 0 · Answer 3 · answered Nov 04 '11 at 18:41

0

How do you achieve the functionality of RunP? Do you redefine a monad instance for it and have getP/putP? Your code seems fine, can you provide the m you use? What kind of misbehaviour do you get?

answered Nov 04 '11 at 18:41

nponeccop

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This should probably be a comment, not an answer. – Daniel Wagner Nov 04 '11 at 22:50
Well, it was a stub for answer - I planned to edit it as soon as the author adds more details. – nponeccop Nov 05 '11 at 08:12

How to share state between two monads?

3 Answers3