The unit type, written `()`, means there is no meaningful value to return. It is a zero-length tuple with exactly one value, also written `()`. A function with no stated return type returns unit implicitly; it can also declare `-> ()`. section 3.1 section 1.6.2
A function that returns unit
`fn report<T: Debug>(item: T) { println!("{:?}", item); }` prints an item but returns no meaningful value. `fn clear(text: &mut String) -> () { *text = String::from(""); }` states its unit return type explicitly. section 3.1
A semicolon matters when a function is meant to return a value. An `add` function whose final expression is `i + j` returns an `i32`; putting a semicolon after that expression makes the function return `()` instead. section 2.2.1 section 3.1
Unit also helps explain a compiler error. An `if` condition needs a Boolean value. Writing `if a = 10` uses assignment, whose result is `()`, rather than the comparison `a == 10`. The compiler therefore reports “expected `bool`, found `()`.” section 1.6.2 section 2.4.6
error[E0308]: mismatched types
--> src/main.rs:4:8
|
4 | if a = 10 {
| ^^^^^^
| |
| expected `bool`, found `()`
| help: try comparing for equality: `a == 10`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`.
To learn more, run the command again with --verbose.
The compiler message pinpoints the mistaken `=` and suggests `==`, showing why unit is not a valid `if` condition. section 1.6.2
