All the Places Patterns Can Be Used

2020-01-31

All the Places Patterns Can Be Used

Patterns pop up in a number of places in Rust, and you’ve been using them a lot without realizing it! This section discusses all the places where patterns are valid.

match Arms

As discussed in Chapter 6, we use patterns in the arms of match expressions. Formally, match expressions are defined as the keyword match, a value to match on, and one or more match arms that consist of a pattern and an expression to run if the value matches that arm’s pattern, like this:

match VALUE {
    PATTERN => EXPRESSION,
    PATTERN => EXPRESSION,
    PATTERN => EXPRESSION,
}

One requirement for match expressions is that they need to be exhaustive in the sense that all possibilities for the value in the match expression must be accounted for. One way to ensure you’ve covered every possibility is to have a catchall pattern for the last arm: for example, a variable name matching any value can never fail and thus covers every remaining case.

A particular pattern _ will match anything, but it never binds to a variable, so it’s often used in the last match arm. The _ pattern can be useful when you want to ignore any value not specified, for example. We’ll cover the _ pattern in more detail in the “Ignoring Values in a Pattern” section later in this chapter.

Conditional if let Expressions

In Chapter 6 we discussed how to use if let expressions mainly as a shorter way to write the equivalent of a match that only matches one case. Optionally, if let can have a corresponding else containing code to run if the pattern in the if let doesn’t match.

Listing 18-1 shows that it’s also possible to mix and match if let, else if, and else if let expressions. Doing so gives us more flexibility than a match expression in which we can express only one value to compare with the patterns. Also, the conditions in a series of if let, else if, else if let arms aren’t required to relate to each other.

The code in Listing 18-1 shows a series of checks for several conditions that decide what the background color should be. For this example, we’ve created variables with hardcoded values that a real program might receive from user input.

Filename: src/main.rs

fn main() {
    let favorite_color: Option<&str> = None;
    let is_tuesday = false;
    let age: Result<u8, _> = "34".parse();

    if let Some(color) = favorite_color {
        println!("Using your favorite color, {}, as the background", color);
    } else if is_tuesday {
        println!("Tuesday is green day!");
    } else if let Ok(age) = age {
        if age > 30 {
            println!("Using purple as the background color");
        } else {
            println!("Using orange as the background color");
        }
    } else {
        println!("Using blue as the background color");
    }
}

Listing 18-1: Mixing if let, else if, else if let, and else

If the user specifies a favorite color, that color is the background color. If today is Tuesday, the background color is green. If the user specifies their age as a string and we can parse it as a number successfully, the color is either purple or orange depending on the value of the number. If none of these conditions apply, the background color is blue.

This conditional structure lets us support complex requirements. With the hardcoded values we have here, this example will print Using purple as the background color.

You can see that if let can also introduce shadowed variables in the same way that match arms can: the line if let Ok(age) = age introduces a new shadowed age variable that contains the value inside the Ok variant. This means we need to place the if age > 30 condition within that block: we can’t combine these two conditions into if let Ok(age) = age && age > 30. The shadowed age we want to compare to 30 isn’t valid until the new scope starts with the curly bracket.

The downside of using if let expressions is that the compiler doesn’t check exhaustiveness, whereas with match expressions it does. If we omitted the last else block and therefore missed handling some cases, the compiler would not alert us to the possible logic bug.

while let Conditional Loops

Similar in construction to if let, the while let conditional loop allows a while loop to run for as long as a pattern continues to match. The example in Listing 18-2 shows a while let loop that uses a vector as a stack and prints the values in the vector in the opposite order in which they were pushed.


#![allow(unused_variables)]
fn main() {
let mut stack = Vec::new();

stack.push(1);
stack.push(2);
stack.push(3);

while let Some(top) = stack.pop() {
    println!("{}", top);
}
}

Listing 18-2: Using a while let loop to print values for as long as stack.pop() returns Some

This example prints 3, 2, and then 1. The pop method takes the last element out of the vector and returns Some(value). If the vector is empty, pop returns None. The while loop continues running the code in its block as long as pop returns Some. When pop returns None, the loop stops. We can use while let to pop every element off our stack.

for Loops

In Chapter 3, we mentioned that the for loop is the most common loop construction in Rust code, but we haven’t yet discussed the pattern that for takes. In a for loop, the pattern is the value that directly follows the keyword for, so in for x in y the x is the pattern.

Listing 18-3 demonstrates how to use a pattern in a for loop to destructure, or break apart, a tuple as part of the for loop.


#![allow(unused_variables)]
fn main() {
let v = vec!['a', 'b', 'c'];

for (index, value) in v.iter().enumerate() {
    println!("{} is at index {}", value, index);
}
}

Listing 18-3: Using a pattern in a for loop to destructure a tuple

The code in Listing 18-3 will print the following:

a is at index 0
b is at index 1
c is at index 2

We use the enumerate method to adapt an iterator to produce a value and that value’s index in the iterator, placed into a tuple. The first call to enumerate produces the tuple (0, 'a'). When this value is matched to the pattern (index, value), index will be 0 and value will be 'a', printing the first line of the output.

let Statements

Prior to this chapter, we had only explicitly discussed using patterns with match and if let, but in fact, we’ve used patterns in other places as well, including in let statements. For example, consider this straightforward variable assignment with let:


#![allow(unused_variables)]
fn main() {
let x = 5;
}

Throughout this book, we’ve used let like this hundreds of times, and although you might not have realized it, you were using patterns! More formally, a let statement looks like this:

let PATTERN = EXPRESSION;

In statements like let x = 5; with a variable name in the PATTERN slot, the variable name is just a particularly simple form of a pattern. Rust compares the expression against the pattern and assigns any names it finds. So in the let x = 5; example, x is a pattern that means “bind what matches here to the variable x.” Because the name x is the whole pattern, this pattern effectively means “bind everything to the variable x, whatever the value is.”

To see the pattern matching aspect of let more clearly, consider Listing 18-4, which uses a pattern with let to destructure a tuple.


#![allow(unused_variables)]
fn main() {
let (x, y, z) = (1, 2, 3);
}

Listing 18-4: Using a pattern to destructure a tuple and create three variables at once

Here, we match a tuple against a pattern. Rust compares the value (1, 2, 3) to the pattern (x, y, z) and sees that the value matches the pattern, so Rust binds 1 to x, 2 to y, and 3 to z. You can think of this tuple pattern as nesting three individual variable patterns inside it.

If the number of elements in the pattern doesn’t match the number of elements in the tuple, the overall type won’t match and we’ll get a compiler error. For example, Listing 18-5 shows an attempt to destructure a tuple with three elements into two variables, which won’t work.

let (x, y) = (1, 2, 3);

Listing 18-5: Incorrectly constructing a pattern whose variables don’t match the number of elements in the tuple

Attempting to compile this code results in this type error:

error[E0308]: mismatched types
 --> src/main.rs:2:9
  |
2 |     let (x, y) = (1, 2, 3);
  |         ^^^^^^ expected a tuple with 3 elements, found one with 2 elements
  |
  = note: expected type `({integer}, {integer}, {integer})`
             found type `(_, _)`

If we wanted to ignore one or more of the values in the tuple, we could use _ or .., as you’ll see in the “Ignoring Values in a Pattern” section. If the problem is that we have too many variables in the pattern, the solution is to make the types match by removing variables so the number of variables equals the number of elements in the tuple.

Function Parameters

Function parameters can also be patterns. The code in Listing 18-6, which declares a function named foo that takes one parameter named x of type i32, should by now look familiar.


#![allow(unused_variables)]
fn main() {
fn foo(x: i32) {
    // code goes here
}
}

Listing 18-6: A function signature uses patterns in the parameters

The x part is a pattern! As we did with let, we could match a tuple in a function’s arguments to the pattern. Listing 18-7 splits the values in a tuple as we pass it to a function.

Filename: src/main.rs

fn print_coordinates(&(x, y): &(i32, i32)) {
    println!("Current location: ({}, {})", x, y);
}

fn main() {
    let point = (3, 5);
    print_coordinates(&point);
}

Listing 18-7: A function with parameters that destructure a tuple

This code prints Current location: (3, 5). The values &(3, 5) match the pattern &(x, y), so x is the value 3 and y is the value 5.

We can also use patterns in closure parameter lists in the same way as in function parameter lists, because closures are similar to functions, as discussed in Chapter 13.

At this point, you’ve seen several ways of using patterns, but patterns don’t work the same in every place we can use them. In some places, the patterns must be irrefutable; in other circumstances, they can be refutable. We’ll discuss these two concepts next.