Monadic Retry logic w/ F# and async?

Question

I've found this snippet:

http://fssnip.net/8o

But I'm working not only with retriable functions, but also with asynchronous such, and I was wondering how I make this type properly. I have a tiny piece of retryAsync monad that I'd like to use as a replacement for async computations, but that contains retry logic, and I'm wondering how I combine them?

type AsyncRetryBuilder(retries) =
  member x.Return a = a               // Enable 'return'
  member x.ReturnFrom a = x.Run a
  member x.Delay f = f                // Gets wrapped body and returns it (as it is)
                                       // so that the body is passed to 'Run'
  member x.Bind expr f = async {
    let! tmp = expr
    return tmp
    }
  member x.Zero = failwith "Zero"
  member x.Run (f : unit -> Async<_>) : _ =
    let rec loop = function
      | 0, Some(ex) -> raise ex
      | n, _        -> 
        try 
          async { let! v = f()
                  return v }
        with ex -> loop (n-1, Some(ex))
    loop(retries, None)

let asyncRetry = AsyncRetryBuilder(4)

Consuming code is like this:

module Queue =
  let desc (nm : NamespaceManager) name = asyncRetry {
    let! exists = Async.FromBeginEnd(name, nm.BeginQueueExists, nm.EndQueueExists)
    let beginCreate = nm.BeginCreateQueue : string * AsyncCallback * obj -> IAsyncResult
    return! if exists then Async.FromBeginEnd(name, nm.BeginGetQueue, nm.EndGetQueue)
            else Async.FromBeginEnd(name, beginCreate, nm.EndCreateQueue)
    }

  let recv (client : MessageReceiver) timeout =
    let bRecv = client.BeginReceive : TimeSpan * AsyncCallback * obj -> IAsyncResult
    asyncRetry { 
      let! res = Async.FromBeginEnd(timeout, bRecv, client.EndReceive)
      return res }

Error is:

This expression was expected to have type Async<'a> but here has type 'b -> Async<'c>

Answer 1

Your Bind operation behaves like a normal Bind operation of async, so your code is mostly a re-implementation (or wrapper) over async. However, your Return does not have the right type (it should be 'T -> Async<'T>) and your Delay is also different than normal Delay of async. In general, you should start with Bind and Return - using Run is a bit tricky, because Run is used to wrap the entire foo { .. } block and so it does not give you the usual nice composability.

The F# specification and a free chapter 12 from Real-World Functional Programming both show the usual types that you should follow when implementing these operations, so I won't repeat that here.

The main issue with your approach is that you're trying to retry the computation only in Run, but the retry builder that you're referring to attempts to retry each individual operation called using let!. Your approach may be sufficient, but if that's the case, you can just implement a function that tries to run normal Async<'T> and retries:

 let RetryRun count (work:Async<'T>) = async { 
   try 
     // Try to run the work
     return! work
   with e ->
     // Retry if the count is larger than 0, otherwise fail
     if count > 0 then return! RetryRun (count - 1) work
     else return raise e }

If you actually want to implement a computation builder that will implicitly try to retry every single asynchronous operation, then you can write something like the following (it is just a sketch, but it should point you in the right direction):

// We're working with normal Async<'T> and 
// attempt to retry it until it succeeds, so 
// the computation has type Async<'T>
type RetryAsyncBuilder() =
  member x.ReturnFrom(comp) = comp // Just return the computation
  member x.Return(v) = async { return v } // Return value inside async
  member x.Delay(f) = async { return! f() } // Wrap function inside async
  member x.Bind(work, f) =
    async { 
      try 
        // Try to call the input workflow
        let! v = work
        // If it succeeds, try to do the rest of the work
        return! f v
      with e ->
        // In case of exception, call Bind to try again
        return! x.Bind(work, f) }