Why is my MailboxProcessor hanging?
f#, mailboxprocessor
Solution
The problem is that you did not start the agent. You can either call `Start` after you create the agent:
let myAgent = (...)
do myAgent.Start()
Alternatively, you can create the agent using `MailboxProcessor<'T>.Start` instead of calling the constructor (I usually prefer this option, because it looks more functional):
let myAgent = MailboxProcessor<Message<'T, 'TState>>.Start(fun inbox -> (...) )
I suppose that you couldn't debug the agent, because the code inside agent wasn't actually running. I tried adding `printfn "Msg: %A" message` right after the call to `Receive` inside the agent (to print incoming messages for debugging) and I noticed that, after calling `Aggregate`, no messages were actually received by the agent... (It only blocked after calling `GetTotal`, which avaits reply)
As a side-note, I would probably turn `GetTotal` into a method, so you'd call `GetTotal()`. Properties are re-evaluated each time you access them, so your code does the same thing, but best practices don't recommend using properties that do complex work.
Problem
I can't work out why the following code is hanging at the call to `GetTotal`. I don't seem to be able to debug inside the MailboxProcessor, so it's hard to see what's going on. ``` module Aggregator open System type Message<'T, 'TState> = | Aggregate of 'T | GetTotal of AsyncReplyChannel<'TState> type Aggregator<'T, 'TState>(initialState, f) = let myAgent = new MailboxProcessor<Message<'T, 'TState>>(fun inbox -> let rec loop agg = async { let! message = inbox.Receive() match message with | Aggregate x -> return! loop (f agg x) | GetTotal replyChannel -> replyChannel.Reply(agg) return! loop agg } loop initialState ) member m.Aggregate x = myAgent.Post(Aggregate(x)) member m.GetTotal = myAgent.PostAndReply(fun replyChannel -> GetTotal(replyChannel)) let myAggregator = new Aggregator<int, int>(0, (+)) myAggregator.Aggregate(3) myAggregator.Aggregate(4) myAggregator.Aggregate(5) let totalSoFar = myAggregator.GetTotal printfn "%d" totalSoFar Console.ReadLine() |> ignore ``` It seems to work fine when using an identical MailboxProcessor directly, rather than wrapping in the `Aggregator` class.