Patterns are a special syntax in Rust for matching against the structure of
types, both complex and simple. Using patterns in conjunction with match
expressions and other constructs gives you more control over a program’s
control flow. A pattern consists of some combination of the following:
- Literals
- Destructured arrays, enums, structs, or tuples
- Variables
- Wildcards
- Placeholders
Some example patterns include x, (a, 3), and Some(Color::Red). In the
contexts in which patterns are valid, these components describe the shape of
data. Our program then matches values against the patterns to determine whether
it has the correct shape of data to continue running a particular piece of code.
To use a pattern, we compare it to some value. If the pattern matches the
value, we use the value parts in our code. Recall the match expressions in
Chapter 6 that used patterns, such as the coin-sorting machine example. If the
value fits the shape of the pattern, we can use the named pieces. If it
doesn’t, the code associated with the pattern won’t run.
This chapter is a reference on all things related to patterns. We’ll cover the valid places to use patterns, the difference between refutable and irrefutable patterns, and the different kinds of pattern syntax that you might see. By the end of the chapter, you’ll know how to use patterns to express many concepts in a clear way.