This document discusses continuation passing style and its relationship to .NET Tasks and F# Async workflows. It explains that both Tasks and Async workflows follow a continuation passing style under the hood. It then describes how the two could be linked together to allow using Tasks in F# Async workflows and vice versa through conversion functions. This would allow awaiting Tasks in F# code and using Async workflows in C# code. Finally, it mentions an implementation of these conversions on GitHub called FusionTasks.

1. How to meets
Async and Task
2016.11.08 MVP SUMMIT SPECIAL: A NIGHT OF LIGHTNING TALKS
KOUJI MATSUI (@KEKYO2)

2. Kouji Matsui - kekyo
• NAGOYA city, AICHI pref., JP
• Twitter – @kekyo2 / Facebook
• ux-spiral corporation
• Microsoft Most Valuable Professional VS
and DevTech 2015-
• Certified Scrum master / Scrum product
owner
• Center CLR organizer.
• .NET/C#/F#/IL/metaprogramming or like…
• Bike rider

3. Agenda
Continuation Passing Style
.NET Task / async-await / F# Async workflow
Continuation Linking

4. Continuation Passing Style
Continuation-passing style (CPS) is a style of programming in which
control is passed explicitly in the form of a continuation.
“continuation-passing style” - Wikipedia

5. Continuation Passing Style
Continuation function
passing to argument
Continuation implement with
nested lambda expressions…
Standard direct style

6. Continuation Passing Style (Details)
Continuation-Passing style
Continuation implementation
nearly equals “Callback”

7. Continuation Passing Style (Details)
The callback technics likely async continuation handler in JavaScript.
Continuation-Passing style
Don’t worry!!
This technics naming “CPS!!”

8. Continuation Passing Style
Understand CPS?

9. Agenda
Continuation Passing Style
.NET Task / async-await / F# Async workflow
Continuation Linking

10. The keyword: .NET Task
The “Task” control interface in .NET 4.0.
In .NET 4.5 with C# 5.0, Task can handle in “async-await” style.
What relationship
between Task and CPS?

11. .NET Task (In .NET 4.0 / C# 4.0)
.NET 4.0 Task is using with ContinueWith function.
Continuation passing into Task.ContinueWith()
--> likely CPS

12. .NET Task (In .NET 4.5/C# 5.0)
And in .NET 4.5/C# 5.0 with “async-await” style:
That means
“Continuation” OK?
Using Task with async-await

13. .NET Task comparison
So, the continuation in async-await:
Means CPS for “async-await” syntax sugar.
In .NET 4.0 (C# 4.0 Task only) And .NET 4.5 (C# 5.0 async-await)

14. F# Async workflow
Async workflow supported from F# 2.0.
Integrated seamless async handling into F# syntax (In .NET 2.0)
What relationship
between Async and CPS?

15. F# Async workflow
Structure nearly equals “async-await.”
That’s CPS?
Returned Async<‘T> type
※ let! (Let-Bang) is awaiting for Async<‘T>, and bound ‘T value.
Means C#’s await keyword.
※ If continuation implement in F# Async workflow, use Async.FromContinuations<‘T>.

16. Overall Async implementations
.NET Task : using ContinueWith
.NET Task : using async-await
F# Async workflow : using let!
Both all patterns nearly with Continuation-Passing style?

17. Agenda
Continuation Passing Style
.NET Task / async-await / F# Async workflow
Continuation Linking

18. Connection both continuations
Can I connect by seamless between .NET Tasks and F# Asyncs?
◦ Before thinking, very nearly structure and interfaces for Task<T> and Async<‘T>.
Then?
◦ Can use await in C# for F#’s Async<‘T>?
◦ And/or can use let! in F# async workflow for Task<T>?
◦ If can, use let!, use!, do!, return! handling directly with HttpClient.GetStreamAsync().

19. Senario for .NET Task / C# side:
Declare augumented function “AsTask” on Async<‘T> class.
Can conversion from Async<‘T> to Task<T>.
◦ Use Async.StartWithContinuations<T> function:
Manipulate “TaskCompletionSource<T> class” (Handled SetResult,
SetException and SetCanceled functions) and returns Task<T>.
Implements Async<‘T>.GetAwaiter function (Augumented).
◦ Can directly awaitable for Async<‘T> in C#.

20. Senario for .NET Task / C# side:
Can directly awaitable for Async<int>.
C# compiler invoking GetAwaiter function.

21. Senario for F# Async workflow side:
Declare augumented function “AsAsync” on Task<T> class.
Can conversion from Task<T> to Async<‘T>.
◦ Use Async.FromContinuations<T> function:
Manipulate Async<‘T> functions from Task.ContinueWith function’s callbacks.
And declare augumented function “Source” on AsyncBuilder<‘T> class.
◦ Source function argument is “Task<T>” type and convet to Async<‘T>,
then can use directly let!, use!, do! and return! for Task<T> in F# Async workflow.

22. Senario for F# Async workflow side:
Can directly awaiting for Task<int>.
F# compiler translate calling “Source” function.

23. Other tips:
Conversion both System.Void and FSharp.Unit in type-safer:
◦ If awaiting for Async<unit> in C#, cannot receive result value (void).
◦ If using Task (Non-generic) in F# async workflow, can only do! expression.
Support the “CancellationToken”:
◦ .NET Task is explicitly handling CancellationToken (ex: argument received).
F# Async workflow is implicitly handling CancellationToken (Async.DefaultCancellationToken).
◦ --> Add CancellationToken argument for AsTask/AsAsync functions.
Don’t use Async.RunSynchronous function:
◦ If use RunSynchronous function, will be hard-blocked current thread nearly invoking Task.Wait.
Implements using for Async.FromContinuations<‘T> or Async.StartWithContinuations<‘T>.
Implement new type “AsyncCompletionSource<‘T>”:
◦ Async.FromContinuations<T> function handle diffecults (by callback structure) for internal impls.
I want to delegation-based interface likely “TaskCompletionSource<T>” class.

24. Thanks join!
The implementations --> GitHub: FusionTasks
◦ https://github.com/kekyo/FSharp.Control.FusionTasks
◦ https://www.nuget.org/packages/FSharp.Control.FusionTasks.FS40/
Progressing:
◦ FusionTasks (for .NET Core waiting for F# RTM :) Adjusting PCL profiles :(
◦ fscx (F# expandable compiler project) https://github.com/fscx-projects/
◦ RelaxVersioner (Git based very easy versioner)
https://github.com/kekyo/RelaxVersioner/
My blog (Sorry default language in Japanese)
◦ http://www.kekyo.net/