In Rust, a statement is a piece of code that does not return a value and always ends with a semicolon
let x = 12;
println!("x is {}", x);
An expression is code that evaluates to something. Most Rust code contains expressions.
let y = 2 + 2; // 2 + 2 is an expression within a statement
do_something() // do_something() is an expression the returns a value
This distinction is important, especially when reading functions. It might not be obvious what this function is returning at first glance. A function's return value can be the last expression evaluated by body of the function.
fn sum_and_double(x: i32, y: i32) -> i32 {
let summed = x + y;
summed * 2 // since this is the last expression, it is implicitly returned
}
summed * 2
is the last expression so it evaluates and returns that value from the function. This would break if we added a semicolon because that would turn the line into a statement.
This code would result in a compiler error for mismatched types because we are not returning an i32
, we are returning nothing!
fn sum_and_double(x: i32, y: i32) -> i32 {
let summed = x + y;
summed * 2; // Compiler error! This is now a statement and nothing gets returned from the function!
}
You can explicitly use the return
keyword in functions, which is required for returning early, but the implicit return is a de facto standard and you will see it everywhere.
Pay attention to the semicolons!
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