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F# Agents and Retlang: quick comparison

Erlang style message passing is a great way of simplifying concurrency. The open source Retlang library aims to bring some of the benefits to .Net languages like C# and VB.Net. Whereas F# has built-in message passing support with the MailboxProcessor also referred to as Agents. To compare Retlang against F# Agents I have coded up the Retlang Summation example (171 lines) as an equivalent F# implementation (41 lines):

type Agent<'a> = MailboxProcessor<'a>

type Summation =
    | Add of int
    | Total of AsyncReplyChannel<int>    
and SummationAgent () =
    let agent = Agent.Start ( fun inbox ->    
        let rec loop total =
            async {
            let! message = inbox.Receive()
            match message with 
            | Add n -> do! loop (n + total)
            | Total reply -> reply.Reply(total)
        loop 0
    /// Adds value to total
    member this.Add n = Add(n) |> agent.Post
    /// Returns total and ends computation
    member this.Total () = (fun reply -> Total(reply)) |> agent.PostAndReply

/// Invokes specified function with numbers from 1 to limit
let numberSource f limit =
    async {
        for i = 1 to limit do
            f i            
            if i % 10 = 0 then System.Console.WriteLine("{0}\t({1})",i,limit)        

do  /// Summation agent instance
    let agent = SummationAgent ()    
    // Post series of numbers to summation agent in parallel
    |> (numberSource agent.Add)
    |> Async.Parallel
    |> Async.RunSynchronously
    |> ignore    
    // Get total
    let value = agent.Total ()
    System.Diagnostics.Debug.Assert(26425 = value);
    value |> System.Console.WriteLine


If you compare this to the Retlang example implementation, it should be clear that C# is missing the easy message definition (discriminated unions) and pattern matching part of message passing. You may also notice that in the F# version, the functionality is accomplished, not only without locks, but also without any low-level threading primitives (like AutoResetEvent). However, if you find yourself constrained to C# development then Retlang is definitely worth a look.

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