How to call a handler from a task expression

[s5no5t]

I'm trying to build a handler that opens and closes a document database (RavenDB) session, and calls other handlers in between.

let handler session: HttpHandler =
    choose
        [ other handlers.. ]

let sessionHandler documentStore: HttpHandler =
    fun next ctx ->
        task {
            let session = RavenDb.createSession documentStore
            let result = (handler session) next ctx // <--- what's the correct way to call handler here?
            do! RavenDb.saveChangesAsync session
            return! result
        }

let webApp documentStore = HttpHandlers.sessionHandler documentStore

However, when I try the above code, the database complains about saveChangedAsync being called concurrently:

System.InvalidOperationException: Concurrent usage of async tasks in async session is forbidden, please make sure that async session doesn't execute async methods concurrently.

Any idea how to call other handlers and properly "await" the result before continuing in the task expression?

[64J0]

It could be the case that saveChangeAsync is returning an async operation, which is different from a task operation in .NET @s5no5t. In this case, you need to convert the async to task to use it properly in this CE.

I guess you'll need to do this:

do! (RavenDb.saveChangesAsync session) |> Async.StartAsTask

More information here -> https://learn.microsoft.com/en-us/dotnet/fsharp/tutorials/async#core-concepts.

[s5no5t]

In this case, I'm sure it's returning a Task, as it's a method from a C# library

[64J0]

Got it, thanks for the reference!

[TheJayMann]

I'm not entirely sure as I have not thoroughly evaluated the provided code, nor have I tested anything, but I believe it could be fixed by moving the bang operation from the return to the let, as follows.

 let handler session: HttpHandler =
    choose
        [ other handlers.. ]

let sessionHandler documentStore: HttpHandler =
    fun next ctx ->
        task {
            let session = RavenDb.createSession documentStore
            let! result = (handler session) next ctx // use `let!` to await the result
            do! RavenDb.saveChangesAsync session
            return result  // use a regular return, as the result has been previously awaited
        }

let webApp documentStore = HttpHandlers.sessionHandler documentStore