Why does the println! function use an exclamation mark in Rust?

Question

In Swift, ! means to unwrap an optional (possible value).

Answer 1

println! is not a function, it is a macro. Macros use ! to distinguish them from normal method calls. The documentation contains more information.

See also:

• What is the difference between macros and functions in Rust?

---

Rust uses the Option type to denote optional data. It has an unwrap method.

Rust 1.13 added the question mark operator ? as an analog of the try! macro (originally proposed via RFC 243).

An excellent explanation of the question mark operator is in The Rust Programming Language.

fn foo() -> Result<i32, Error> {     Ok(4) }  fn bar() -> Result<i32, Error> {     let a = foo()?;     Ok(a + 4) } 

The question mark operator also extends to Option, so you may see it used to unwrap a value or return None from the function. This is different from just unwrapping as the program will not panic:

fn foo() -> Option<i32> {     None }  fn bar() -> Option<i32> {     let a = foo()?;     Some(a + 4) }