F# – yield vs yield!

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 IEnumerablesequences.

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.

Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

You can contact me via Email, Twitter and LinkedIn.

Hire me.