Programming F# 3.0, 2nd Edition

Chapter 13. Computation Expressions

In Chapters 2, 3, and 4, we covered list, sequence, and array comprehensions, which are ways to write integrated F# code that ultimately produces a collection of ordered elements in list, sequence, or array form. These comprehensions are not just syntactic sugar built into the language, but rather an advanced feature at work called computation expressions (less formally referred to as workflows).

In fact, this same technique that simplifies declaring sequences can also be applied to asynchronous programming or creating domain specific languages.

In short, computation expressions allow you to take F# code and determine how it gets executed. Having the workflow do some of the heavy lifting behind the scenes can dramatically reduce redundant code and in some ways extend the F# language itself.

Toward Computation Expressions

Let’s review briefly what’s possible with sequence expressions—they allow you to define a sequence by embedding some F# code into a seq computation expression. Example 13-1 shows a sequence expression for generating the days of the year.

Notice that you can write pretty much any F# code inside the seq { } block, and elements are produced from the sequence as items are returned with the yield keyword.

Example 13-1. Sequence expression for enumerating days of the year

> // Sequence for producing month, day tuples
let daysOfTheYear =
    seq { let months = [ "Jan"; "Feb"; "Mar"; "Apr"; "May"; "Jun"; "Jul"; "Aug"; "Sep"; "Oct"; "Nov"; "Dec" ...