TaskBuilder.fs 2.1.0

F# computation expression builder for System.Threading.Tasks

There is a newer prerelease version of this package available.
See the version list below for details.
Install-Package TaskBuilder.fs -Version 2.1.0
dotnet add package TaskBuilder.fs --version 2.1.0
<PackageReference Include="TaskBuilder.fs" Version="2.1.0" />
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add TaskBuilder.fs --version 2.1.0
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NuGet

About

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.

Usage

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 &#10003;

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

DON'T &#10006;

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.

NuGet

About

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.

Usage

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 &#10003;

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

DON'T &#10006;

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.

NuGet packages (45)

Showing the top 5 NuGet packages that depend on TaskBuilder.fs:

Package Downloads
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A native functional ASP.NET Core web framework for F# developers.
Chauffeur
Chauffeur is a tool for helping with delivering changes to an Umbraco instance.
SBTech.Consul.LeaderElection
Simple library(100% written in F#) to use leader election functionality based on Consul.
FSharp.Azure.Storage
A library that provides an idiomatic F# API for Microsoft Azure services.
Chia
This library Chia contains utils for internal Danpower Reporting and is used by serveral reports.

GitHub repositories (6)

Showing the top 5 popular GitHub repositories that depend on TaskBuilder.fs:

Repository Stars
dotnet/orleans
Orleans is a cross-platform framework for building distributed applications with .NET
Azure/azure-webjobs-sdk
Azure WebJobs SDK
Azure/azure-functions-durable-extension
Durable Task Framework extension for Azure Functions
torhovland/blazor-redux
Connecting a Redux state store with Blazor.
protobuf-net/protobuf-net.Grpc
GRPC bindings for protobuf-net and grpc-dotnet

Version History

Version Downloads Last updated
2.2.0-alpha 1,126 12/5/2019
2.1.0 540,502 9/16/2018
2.0.0 17,363 5/27/2018
1.2.0-rc 1,546 2/26/2018
1.1.1 16,687 2/19/2018
1.1.0 401 2/17/2018
1.0.1 53,417 2/9/2018
1.0.0 3,402 10/7/2017