Mutually recursive F# functions, typing issue

Question

Having trouble with this code:

  let subscribe (nm : NamespaceManager) (subName : string) (desc : TopicDescription)  : Async<SubscriptionDescription> =
    let rec first_create () =
      async {
        let! exists = desc |> exists nm
        if exists then return! (then_create_subscription () : Async<SubscriptionDescription>)
        try
          let beginCreate = nm.BeginCreateTopic : string * AsyncCallback * obj -> IAsyncResult
          logger.DebugFormat("creating topic '{0}'", desc)
          let! tdesc = Async.FromBeginEnd(desc.Path, beginCreate, nm.EndCreateTopic)
          return! first_create ()
        with | :? MessagingEntityAlreadyExistsException -> return! then_create_subscription () }
    and then_create_subscription ()  : Async<SubscriptionDescription> =
      async {
        let beginCreate = nm.BeginCreateSubscription : string * string * AsyncCallback * obj -> IAsyncResult
        return! Async.FromBeginEnd(desc.Path, subName, beginCreate, nm.EndCreateSubscription) }
    first_create ()

On line 5, it underlines then_create_subscription () : Async<SubscriptionDescription> stating:

Type mismatch. Expecting a Async<unit> but given a Async<SubscriptionDescription> The type 'unit' does not match the type ' SubscriptionDescription '

Exists looks like this:

  let exists (nm : NamespaceManager ) (desc : PathBasedEntity) = 
    async { return! Async.FromBeginEnd(desc.Path, nm.BeginTopicExists, nm.EndTopicExists) }

I want it to create the topic and then go on to create the subscription for it.

Any ideas?

Answer 1

There needs to be an else branch after if, otherwise the if is a statement, not an expression. The else is not implicit.