This is a single-file project that implements a
computation expression
for writing Task
s 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
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
, 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.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.
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.
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:
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()
}