f# - Using custom computation expression operator inside match statement

Question

Right now I'm experimenting with F# computation expressions. General idea is to return control mechanism to drive actions executed after each step of recursive function call build from computation expression. Whole example can be seen here.

Using following example:

let rec loop () =
    actor {
        let! msg = m.Receive ()
        match msg with
        | "stop" -> return 0        // expected result: Return (0)
        | "unhandled" -> unhandled  // expected result: Unhandled 
        | x -> 
            mailbox.Sender() <! x
            return! loop ()         // expected result: (Become(fun m -> loop ()))
    }
loop ()

Unfortunately this ends with compile time error on unhandled: A custom operation may not be used in conjunction with 'use', 'try/with', 'try/finally', 'if/then/else' or 'match' operators within this computation expression.

Is it possible in any way to use custom operators inside match statements?

Answer 1

I'm not sure what the details of the actor computation are, but if Unhandled is a value of the underlying computation type, you can certainly produce it using return!

Without knowing the details, I think something like this should work:

match msg with
| "stop" -> return 0
| "unhandled" -> return! Unhandled
| x -> 
    mailbox.Sender() <! x
    return! loop ()