C# developers should be familiar with the yield keyword that was introduced in C# 2.0, and you’ll be pleased to know that F# also has the yield keyword which works in conjunction with F#’s equivalent of IEnumerable – sequences.
You can create sequences in F# using sequence expressions:
In addition, you can also use the yield! (pronounced yield bang) keyword to return a subsequence which is merged into the final sequence. Using a rather contrived example (sorry…) let’s compare the output of yield and yield!:
Look at how the signatures differ for these two sequences:
and how the values in the two sequences differ too:
The yield! keyword is similar to IEnumerable.SelectMany in its ability to flatten a collection of collections but it’s far more powerful as you can mix and match it with other yield/yield! statements and give you a level of expressiveness that IEnumerable.SelectMany does not.