Blocks

A block in Rust has a value and a type: the value is the last expression of the block:

  1. fn main() {
  2.     let x = {
  3.         let y = 10;
  4.         println!("y: {y}");
  5.         let z = {
  6.             let w = {
  7.                 3 + 4
  8.             };
  9.             println!("w: {w}");
  10.             y * w
  11.         };
  12.         println!("z: {z}");
  13.         z - y
  14.     };
  15.     println!("x: {x}");
  16. }

The same rule is used for functions: the value of the function body is the return value:

  1. fn double(x: i32) -> i32 {
  2.     x + x
  3. }
  5. fn main() {
  6.     println!("doubled: {}", double(7));
  7. }

However if the last expression ends with ;, then the resulting value and type is ().

Key Points:

  • The point of this slide is to show that blocks have a type and value in Rust.
  • You can show how the value of the block changes by changing the last line in the block. For instance, adding/removing a semicolon or using a return.