The unit type, written as (), has exactly one possible value and carries no information. It represents nothing while remaining a real, distinct type. section 2.5.3 section 2.6
Use () when an operation succeeds without producing a meaningful success value:
- Functions without an explicit return type return the unit type; writing -> () makes that return explicit. section 2.5.3
- Result<(), E> reports whether an operation succeeded while carrying no additional information in Ok(()). section 2.5.2 section 2.5.3
- Unlike null, () is not a substitute for values of other types; assigning it to String produces a type mismatch. section 2.5.3
Printing with fallible control flow
A FizzBuzz printer returns Result<(), &'static str>. Ok(()) means printing completed; Err(e) carries the failure. The caller matches Ok(()) and handles the error. section 2.5.2
Validation with unit results
Validators can return Result<(), ()> when failure has no detail. A surrounding validate_username function uses map_err to convert those unit errors into NotLowercase or NotUnique, then returns Ok(()). section 2.5.5
Listing 2.31 Three functions that all return the unit type
fn foo() { #1
println!("Hello!");
}
fn bar() -> () { #2
println!("Hello!");
}
fn baz() -> () {
println!("Hello!");
() #3
}
Compare implicit and explicit unit returns, including an explicit final (). section 2.5.3
