RustBrock/Refutability.md

Refutability: Whether a Pattern Might Fail to Match

Patterns come in two forms:

• Refutable
• Irrefutable Patterns that will match for any possible value passed are irrefutable.

An example would be x in the statement let x = 5; because x matches anything and therefore cannot fail to match.

Patterns that can fail to match for some possible value are refutable.

An example would be Some(x) in the expression if let Some(x) = a_value because if the value in the a_value variable is None rather than Some, the Some(x) pattern will not match.

Function parameters, let statements and for loops can only accept irrefutable patterns, because the program cannot do anything meaningful when values don't match.

The if let and while let expressions and the let-else statement accept refutable and irrefutable patterns, but the compiler warns against irrefutable patterns.

This is because by definition they are intended to handle possible failure.

The functionality of a conditional is in its ability to perform differently depending on success or failure.

Generally you shouldn't have to worry about the distinction between refutable and irrefutable patterns.

However you do need to be familiar with the concept of refutability so you can respond when you see it in an error message.

In these cases, you will need to change either the pattern or the construct you are using the pattern with, depending on the intended behavior of the code.

Now lets take a look at an example of what happens when you try to use a refutable pattern where Rust requires an irrefutable pattern and vice versa.

Here shows a let statement, but for the pattern we have specified Some(x), a refutable pattern.

As expected, this code will not compile.

	let Some(x) = some_option_value;

If some_option_value was a None value, it would fail to match the pattern Some(x), meaning the pattern is refutable.

However, the let statement can only accept an irrefutable pattern because there is nothing valid the code can do with a None value.

At compile time, Rust will complain that we have tried to use a refutable pattern where an irrefutable pattern is required

$ cargo run
   Compiling patterns v0.1.0 (file:///projects/patterns)
error[E0005]: refutable pattern in local binding
 --> src/main.rs:3:9
  |
3 |     let Some(x) = some_option_value;
  |         ^^^^^^^ pattern `None` not covered
  |
  = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
  = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
  = note: the matched value is of type `Option<i32>`
help: you might want to use `let else` to handle the variant that isn't matched
  |
3 |     let Some(x) = some_option_value else { todo!() };
  |                                     ++++++++++++++++

For more information about this error, try `rustc --explain E0005`.
error: could not compile `patterns` (bin "patterns") due to 1 previous error

Because we didn't cover (and couldn't cover) every valid value with the pattern Some(x), Rust rightfully produces a compiler error.

If we have a refutable pattern where an irrefutable pattern is needed, we can fix it by changing the code that uses the pattern.

Instead of using let, we can use if let.

Then if the code doesn't match, the code will just skip the code in the curly brackets, giving a way to continue validly.

Here shows a fix to the code

    if let Some(x) = some_option_value {
        println!("{x}");
    }

We given the code an out, this code is perfectly valid now.

However, if we give if let an irrefutable pattern, such as x, the compiler will give a warning.

    if let x = 5 {
        println!("{x}");
    };

Rust complains that it doesn't make sense to use if let with an irrefutable pattern

$ cargo run
   Compiling patterns v0.1.0 (file:///projects/patterns)
warning: irrefutable `if let` pattern
 --> src/main.rs:2:8
  |
2 |     if let x = 5 {
  |        ^^^^^^^^^
  |
  = note: this pattern will always match, so the `if let` is useless
  = help: consider replacing the `if let` with a `let`
  = note: `#[warn(irrefutable_let_patterns)]` on by default

warning: `patterns` (bin "patterns") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s
     Running `target/debug/patterns`
5

For this reason, match arms must use refutable patterns, except for the last arm, which should match any remaining values with an irrefutable pattern.

Rust allows us to use an irrefutable pattern in a match with only one arm.

This syntax isn't particularly useful and could be replaced with a simpler let statement.

Next we will cover all the syntax we can use to create patterns here