Making Lambdas and Iterators Play Nicely Together

The major problem with this idea is in my favourite feature of C#. In an iterator (a function containing the yield keyword), the code is transformed into a state machine represented by a class. It has to be chopped up into sections, so the function can be “paused” after each yield return. Those pieces must all be in the same function, so sadly lambdas don’t play well with iterators.

There are a couple of possible future language features which would completely solve this, however.

Firstly, how about yield foreach. This would take an IEnumerable<T> and yield all the items of it. It would remove the need to write an explicit foreach loop to return a nested sequence. This amazing paper shows how this feature would be implemented (they even picked the same keyword), although the use-case there is to support recursion with good performance.

Secondly, I’d like to be able to write an anonymous iterator. This would be a matter of using yield return in the middle of a lambda, and hey presto, the compiler would build me an iterator instead of a normal anonymous method.

How would these two features help here? Suppose I wanted to use my TraceWriter (see the source download) class in an iterator. Here’s how it would look:

IEnumerable<int> MyIterator(TraceWriter w)
{
    yield return 1;
    w.WriteLine("Not indented yet");

    yield foreach w.Indented(() =>
    {
        yield return 2;
        w.WriteLine("Indented now");
        yield return 3;
    });

    w.WriteLine("Un-indented again");
    yield return 4;
}

So we have the same readability as before. This would require a special implementation of the Indented method, specifically for iterators:

public IEnumerable<T> Indented<T>(IEnumerable<T> sequence)
{
    WriteLine("{");
    _indent++;

    yield foreach sequence;

    _indent--;
    WriteLine("}");
}

It’s handy, that yield foreach thing, isn’t it?