How to share state between pipes consumers without using ioref?

[Ray Qiu]

How to share state between pipes Consumers/Pipes without using ioref?  Thanks!

[Gabriel Gonzalez]

Add a `StateT` layer to each component's base monad.  They will then share that state.  Here is an example:

    import Control.Monad (forever)
    import Control.Monad.Trans.State
    import Pipes
   
    producer :: Producer Int (StateT Int IO) r
    producer = forever (do
        n <- lift get
        lift (put $! n + 1)
        yield n )
   
    consumer :: Consumer Int (StateT Int IO) r
    consumer = forever (do
        n <- await
        lift (lift (print n))
        lift (modify (+ 1)) )
   
    main :: IO ()
    main = evalStateT (runEffect (producer >-> consumer)) 0

That will print:

    0
    2
    4
    6
    ...

On 04/28/2015 09:36 PM, Ray Qiu wrote:

How to share state between pipes Consumers/Pipes without using ioref?  Thanks!

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[Ray Qiu]

Nice!  Thanks!

[Gabriel Gonzalez]

You're welcome!