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Error when combining different types of monads
f#monads

Hi, I have an issue with the following code. I have two types of monads : Attempt, and the FSharp Async. However, for a reason I can't get, it seems that there is an error which arises when returning one from the other as in the following example :

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  module Foo =

    // ... 

    let request  = 
      async
        {  //...

//==================================================
//debugging check :
//the expected value is indeed printed ...
//==================================================
            let x = Attempt.run <| X.decode data_stream
            if AttemptResult.is_success x then
              let x = AttemptResult.get x
              do print_endline ("async task running... works fine")
              do print_endline (X.to_string x) //result is printed as expected !
//==================================================

            //returns a Attempt<X> value
            //Although it raises no error here (see above), it fails to work below...
            return (X.decode data_stream) 
        }  


  let rec announce =     
    let while_locked () =
      attempt 
        { //...
//==================================================
//debugging check :
//the expected value is NOT printed ... an error is instead
//==================================================
             let x=  Attempt.run <| Async.Run(Foo.request)
              if AttemptResult.is_success x then
                let x = AttemptResult.get x
                do print_endline (X.to_string x) //doesn't work
              else
                do print_endline (any_to_string x) //Error: the stream can't be read <-- where could that come from ?
//==================================================
              //the following should work, but it doesn't, see above 
              let! x = Async.Run(X.request)
                
        }
//...

Any idea of what could cause the error ?

Thanks a lot

.

Does bValue have the type you expect?

On the final "let! bvalue", when you say it doesn't work, what happened? Are you saying you tried it instead of the six lines above?

And I concur with the suggestion to simplify the example as much as possible -- even to the point of doing nothing useful except test whether you can get two simple monads to work together. Either there is something fundamentally wrong here, or the problem can be isolated by gradually adding functionality until it breaks. Then you can say "it works until I add this..."

By on 4/6/2008 1:27 AM ()

Any chance you could send a smaller, more isolated example of the problem?

thanks!

don

By on 4/5/2008 4:42 AM ()

And your implementation of the Attempt monad would also probably be good.

By on 4/5/2008 6:21 AM ()

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