Nemerle Async Await (2012-07)

Introduction

In the first month of learning Nemerle language I wrote AsyncAwait macro which is an implementation of async/await keywords known from C#. There is a similar approach for asynchronous programming in Nemerle - Computation Expression (which was a starting point for me), but it is not compatible with C#’s Task-based Asynchronous Pattern. You can use my solution to integrate with code written in C#.

You can find excellent introduction to the “Task-based Asynchronous Pattern” here:

http://www.microsoft.com/en-us/download/details.aspx?id=19957

Reading this document before using my macro is highly recommended.

Features

• compatible with C# 5.0 Task-based Asynchronous Pattern
• syntax resembles that of C#
• .NET 4.5 and VS2012 are not required (works fine with .NET 4.0 and VS2010 with AsyncTargetingPack.NET4)
• support for the following language constructs: do / while / for / foreach / try / catch / finally / using
• support for GetAwaiter
• support for ConfigureAwait
• support for IProgress
• support for CancellationToken

Usage

You need to:

• add Macro Reference to Nemerle.Async.Macros assembly
• add normal reference to Nemerle.Async assembly
• add Nemerle.Async using to your source code file

Syntax is very similar to that known from C#:

C# and Nemerle example

C#:

public async Task<int> AsyncMethod(int a)
{
    int res = await AnotherMethod(a);
    return res + 10;
}

Nemerle:

public async AsyncMethod(a : int) : Task[int]
{
    def res = await AnotherMethod(a);
    res + 10;
}

You can also use constructions not possible with C#:

Async function

def proc(cur, max)

{
    async
    {
        def res = await fib(10);
        textBox1.Text = $"fib(10) = $res\n";
    }
}

Async block defined inside method body

await async
{
    def res = await fib(10);
    textBox1.Text = $"fib(10) = $res\n";
}

Exception Handling

What I don’t like in TAP pattern is that’s very easy to miss uncaught exception. The same applies to Nemerle’s and C#’s implementation, so there is no difference here but it deserves a mention. Unhandled exceptions are rethrown outside Task encapsulation (and can cause application crash for example) only in the case of async handlers (async methods that return void):

Unhandled exception

public async OnButtonClick() : void
{
    throw Exception("Unhandled user exception!");
}

In every other situation exceptions are encapsulated inside Task’s Exception property and won’t rethrow it if not checked explicitly. For example, calling fallowing method won’t crash your application:

It won’t crash

public Test() : void
{
    _ = async
    {
        throw Exception("It won't crash!");
    }
}

Because of that I recommend always to use try / catch statements when calling asynchronous code:

Always use try/catch when calling asynchronous code

public async Test() : Task
{
    try
    {
        await async
        {
            throw Exception("This time it will be caught!");
        }
    }
    catch
    {
        | _ => .... exception caught ....
    }
}

Deficiencies

In the current version there are few gaps. I don’t know if I will ever fill them. The reason of this is they are minor for me:

• current solution is not optimized at all and uses closures, so it is slower than C#’s implementation and allocates more memory (rewriting to state machine is a tremendous work and I’m not considering it now), but if you will use it wisely, it should be no problem for you
• block statements are not supported
• you cannot use await as a parameter for a function/method call
• the output program has a dependency of “Nemerle.Async” assembly

Downloads

You can download the latest version from download page. It is also part of snippets for Nemerle. You can download it with Nemerle source code.