Give me a yield break

[isaacschemm]

Working in .NET, I've been in a situation where I need to implement a function in an interface that's supposed to return an IEnumerable<T>. (In some cases, I'd argue that IReadOnlyList<T> might make more sense - this way, the caller knows it's not going to be a lazily-evaluated sequence, and you can still return a .NET list, F# list, or array - but I digress).

Maybe the most obvious way to do this is by calling a function that explicitly returns an empty enumerable for you:

IEnumerable<string> SampleInterface.getAll() => Enumerable.Empty<string>();

In F#, it would be even shorter, because of the aggressive type resolution:

interface SampleInterface with
    member _.getAll() = Seq.empty

There are other clever ways to do the same thing, though, and it might just depend on what you think is the clearest or most readable - which might just mean keeping it consistent with the code around it. First, you can always expand out the function (because clearer isn't always shorter - I think it really does depend on the context of what's around it):

IEnumerable<string> SampleInterface.getAll() {
    return Enumerable.Empty<string>();
}

But there's also something clever you can do here, if you want to think of your code in a different way - where instead of resulting in "an empty list", it results in "no elements". C# lets you build iterator functions, where your code defines an IEnumerable<T> (and runs every time the resulting object is enumerated). Any function with a yield return or a yield break is treated in this way by the compiler. This means you can implement a function that returns "no elements" just by doing this:

IEnumerable<string> SampleInterface.getAll() {
    yield break;
}

It's a bit different in VB.NET, where iterator functions are denoted explicitly - so the yield break isn't needed:

Public Iterator Function getAll() As IEnumerable(Of String) Implements SampleInterface.getAll

End Function

That is, in a very literal sense, a function that returns no elements!

Funny thing is that there's no real equivalent to an empty iterator function in F# (not that you'd need it); the compiler won't allow a seq { } workflow without any elements in it, and suggests you use Seq.empty or the empty list [] instead.