F# – yield vs yield!

Yan Cui

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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:

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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!:

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Look at how the signatures differ for these two sequences:

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and how the values in the two sequences differ too:

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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.

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2 thoughts on “F# – yield vs yield!”

  1. This detailed distinction seems to only apply to sequences? For F#’s computation expressions in general, the meaning of !-variants might differ case by case if I’m not mistaken.

  2. for computations (see Scott Wlaschin’s excellent series if you want a detailed walkthrough of how it works – https://fsharpforfunandprofit.com/series/computation-expressions.html), you can implement both `yield` and `yield!`.

    the difference is that `yield!` accepts an instance of the “wrapper type”, which in the case of the seq computation expression it means a Seq, but for other computation expressions (including your own) can be something entirely different.

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