Composition of C# iterator methods

Iterator methods in C# or one of my favorite features of that language. I do not use it all that often, but it is nice to know it is there. If you are not sure what they are, here’s a little example:

public IEnumerable<int> Iota(int from, int count)
{
  for (int offset = 0; offset < count; ++offset)
    yield return from + offset;
}

They allow you to lazily generate any sequence directly in code. For example, I like to use them when generating a list of errors on a complex input (think compiler errors). The presence of the yield contextual keyword transforms the function body into a state machine, allowing you to pause it until you need the next value.

However, this makes it a little more difficult to compose such iterator methods, and in reverse, refactor a complex iterator method into several smaller ones. It was not obvious to me right away how to do it at all in a ‘this always works’ manner, so I am sharing how I do it here. Consider this slightly more complex iterator method:

public IEnumerable<int> IotaAndBack(int from, int count)
{
  for (int offset = 0; offset < count; ++offset)
    yield return from + offset;

  for (int offset = 0; offset < count; ++offset)
    yield return from + count - offset - 1;
}

Now we want to extract both loops into their own functions. My one-size-fits-all solution is this:

public IEnumerable<int> AndBack(int from, int count)
{
  for (int offset = 0; offset < count; ++offset)
    yield return from + count - offset - 1;
}

public IEnumerable<int> IotaAndBack(int from, int count)
{
  foreach (var x in Iota(from, count))
     yield return x;

  foreach (var x in AndBack(from, count))
     yield return x;
}

As you can see, a little ‘foreach harness’ is needed to compose the parts into the outer function. Of course, in a simple case like this, the LINQ version Iota(from, count).Concat(AndBack(from, count)) also works. But that only works when the outer function is sufficiently simple.

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