WebSharper supports [F# asynchronous workflows][https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/async-expressions] on the client, implementing them with JavaScript callbacks.

As in F#, a workflow of type Async<'T> represents a program that can be invoked to either succeed with a result 'T asynchronously or fail with an exception. The limitations on the client are:

• All parallelism is cooperative as the JavaScript runtime is single-threaded. You have to yield control inside a workflow to let other workflows execute. Yielding of control happens implicitly every time whenever F# inserts a call to an AsyncBuilder call. This includes where you use a let or let!, every iteration of a for or while loop, or between consecutive statements on the top level of an async block.

• Async.RunSynchronously is not supported, as it would block the JavaScript event loop.

• Cancellation is supported, in standard .NET/F# ways. If a cancellation occurs when waiting on an aynchronous remote call, the response from the server gets discarded without converting the JSON back to an object graph.

Task-based asynchronous workflows

Also supported are the Task and Task<T> types from .NET, which represent a waiting, ongoing or finished operation.

Similar limitations apply as with Async<'T>. Use the task builder to create task workflows.

Task-based asynchronicity in C#

WebSharper supports [C# Task-based Asynchronous Pattern][asyncs] on the client, implementing them with JavaScript callbacks.

• Parallelization is possible with Task.WhenAny and Task.WaitAll methods.

• .RunSynchronously() and .Wait() is not supported, as it would block the JavaScript event loop.

• Use the Async.StartAsTask and Async.AwaitTask to convert between Async<'T> and Task<'T>, same as in .NET.