Support for iterator state machines (IEnumerable<>)

[rickomax]

Can this code generate an Enumerator which can be called normally instead of a "waitable" method (eg: calling yield ExpressionMethod())?

[avonwyss]

While there are similarities in the way async/await and IEnumerable/yield work, they are not exactly the same. This code currently does only implement the async/await case, as I have not had any use-case for yielding Expressions that return an IEnumerable.

That being said, I might consider implementing this in the future, and maybe even IAsyncEnumerable/yield in order to support async streams.

In order to implement the IEnumerable/yield, one needs to use a different helper class than the currently used TaskCompletionSource<>. That helper class needs to implement IEnumerable<>, and instead of registering the continuation with the awaiter (e.g. OnCompleted) the code would need to wait for Next() to be called. Other than that, the state machine itself and the transformations applied would probably remain the same as they are for async/await.

[rickomax]

Oh, incredible!
The use in my case would be using an expression as a Unity Coroutine (which uses IEnumerators to play/pause a given method).
That is the only thing preventing me from implementing Expression Trees in the project I'm working on.
Keep the good work!

[avonwyss]

@rickomax I put together a first version which allows you to convert a plain LINQ expression tree to a Func<IEnumerable<?>>. Have a look at the readme to see how this can be used, and let me know if that works for you.

Note, since StartCoroutine() takes an IEnumerator an not a IEnumerable<>, you'll have to add a call to GetEnumerator() like so:

Func<string, IEnumerable<object>> compiledIteratorExpression; // let's assume we pass a string parameter to the coroutine expression
StartCoroutine(compiledIteratorExpression("test").GetEnumerator());

[rickomax]

Wow, you're awesome!