Recursive Sequence Expressions in F#

One nice feature of F# is sequence expressions which let you write code that emits a sequence (think IEnumerable<T> if you are a C# developer). Within a sequence expression you can use the yield operator to emit values in the sequence much like you can in C#:

let x = seq { 
    yield "Hello"
    yield "World"
}

You can also use the yield! operator to emit all the values from another sequence one by one. So the following code would create a four element sequence:

let x = seq { 
    yield "Hello"
    yield "World"
    yield! ["foo"; "bar"]
}

The yield! operator also opens up the possibility of creating a recursive sequence. So you can do something like this and create a never-ending sequence:

let rec x = seq { 
    yield "Hello"
    yield "World"
    yield! ["foo"; "bar"]
    yield! x
}

But obviously you are likely to want to terminate your sequence at some point, and so you need an equivalent of C#’s yield break. It took me a while to work out how to do this in F#, but eventually I realised it was as simple as returning “unit” which is expressed in F# as (). To illustrate how to use it, I’ve created a simple “countdown” function using a recursive sequence:

let rec countdown from = 
    seq {
        match from with 
            | 0 -> ()
            | n -> 
                yield n
                yield! countdown (from - 1)
    }

Obviously that’s not the best way to implement a countdown sequence in F#. That can be done much more elegantly using another form of sequence expressions:

seq { 10 .. -1 .. 1 }

But hopefully this post helps any C# developers wondering how they can do a yield break in F#.