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This is a single-file project that implements a computation expression for writing Tasks in F#. It is free and unencumbered software released into the public domain.

F# comes with its own Async type and functions to convert back and forth between Async and Task, but this is a bit of a hassle -- especially since now that Task has language-level support in C# and VB.NET, it's the de facto standard for asynchrony on .NET. Additionally, F#'s Async behaves a little differently from Task, which can be confusing if you're used to the latter.

The goal of this computation expression builder is to let you write asynchronous blocks that behave just like async methods in C# do.

For example, this F# method:

open System
open System.IO
open System.Linq
open FSharp.Control.Tasks.V2

type X() =
  static member WriteFile() =
    task {
      do! Console.Out.WriteLineAsync("Enter a filename:")
      let! name = Console.In.ReadLineAsync()
      use file = File.CreateText(name)
      for i in Enumerable.Range(0, 100) do
        do! file.WriteLineAsync(String.Format("hello {0}", i))
      do! Console.Out.WriteLineAsync("Done")
      return name

Should work exactly the same as this C# method:

using System;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

class X
  public static async Task<string> WriteFile()
    await Console.Out.WriteLineAsync("Enter a filename:");
    var name = await Console.In.ReadLineAsync();
    using (var file = File.CreateText(name))
      foreach (var i in Enumerable.Range(0, 100))
        await file.WriteLineAsync(String.Format("hello {0}", i));
      await Console.Out.WriteLineAsync("Done");
      return name;

In practice there is a small performance hit compared to the C# version, because the C# compiler compiles each async method to a specialized state machine class, while TaskBuilder uses a general-purpose state machine and must chain together continuation functions to represent the computation. However, TaskBuilder should still be faster than using Task.ContinueWith or Async.StartAsTask.


This is public domain code. I encourage you to simply copy TaskBuilder.fs into your own project and use it as you see fit. It is not necessary to credit me or include any legal notice with your copy of the code.

The other files are tests which you do not need to copy (but again, you are free to do so).

Note that by default, if you open FSharp.Control.Tasks.V2, you'll get a task { ... } builder that behaves as closely to C#'s async methods as possible.

However, I have also included a version of the task { ... } builder under FSharp.Control.Tasks.V2.ContextInsensitive which makes one minor change: it will automatically call task.ConfigureAwait(false) on every task you await.

This can improve performance if you're writing library code or server-side code and don't need to interact with thread-unsafe things like Windows forms controls. If you're not sure whether you want to use this version of the builder, reading this MSDN article may help.

What you can bind with let!

As of 7a04419, you should be able to bind anything "awaitable" with let!.

This basically means any type that has:

  • task.GetAwaiter()
  • task.GetAwaiter().GetResult()
  • task.GetAwaiter().IsCompleted

When using FSharp.Control.Tasks.ContextInsensitive, you can also bind any type that has a task.ConfigureAwait(false) returning an "awaitable" type.

Tail calls are not optimized

In F# it is idiomatic to use tail recursion to implement loops more complex than a simple for or while.

This works with some computation expressions (like the built-in F# async builder), but not with TaskBuilder.fs. As far as I know it is not possible to make this work with TPL tasks. C# async/await function are not tail-call optimized either, so at least this is consistent.

To implement a loop that may iterate many times (or indefinitely), use a while loop instead of tail recursion.

For example:

DO ✓

let runPendingJobs() =
    task {
        let mutable anyPending = true
        while anyPending do
            let! jobToRun = checkForJob()
            match jobToRun with
            | None ->
                anyPending <- false
            | Some pendingJob ->
                do! pendingJob()


let rec runPendingJobs() =
    task {
        let! jobToRun = checkForJob()
        match jobToRun with
        | None ->
            return ()
        | Some pendingJob ->
            do! pendingJob()
            return! runPendingJobs()

What's the deal with the V2 module?

For a while, TaskBuilder.fs depended on a compiler behavior that was introduced in F# 4.1.

It wouldn't work with older compiler versions -- more accurately, it would work, but would be unpleasant to use because types would have to be explicitly annotated everywhere.

Thankfully, @gusty rewrote the builder classes and extension methods to work with all F# compiler versions.

But DLLs compiled using the old builder couldn't use the new builder code, since beneath the inline methods, there is a completely different set of classes and methods involved.

Therefore, the old code is still included for binary-compatibility, while the new code lives under the V2 namespace.


F# computation expression builder for System.Threading.Tasks








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