Are functions statements?

1

Rust is expression based language. Its clearly mentioned in the rust book that function definitions are statements and if its a statement , should it not end with ";" ?

fn main() {
println!("hello, world");

} <----- Shouldn't i end with ";" if its a statement?

I am new to rust, could some one explain this ?

2

According to reference, there are five kinds of statements:

  • A lone semicolon. That's what makes code like x.do_something();; (note the double semicolon) syntactically valid Rust, since the second semicolon is parsed as another statement.
  • An item. There are many different kinds of items, then, each with its own syntactic rules, and functions is just one of them.
  • A let statement. It is, essentially, an expression statement (discussed below) with let pattern = added before.
  • An expression statement. That's where the semicolon comes into play: when we have an expression, it can be turned into statement by adding a semicolon at the end.
  • A macro invocation, with, again, its own set of syntactic rules.

You can see that semicolon is not the property of all statements, but rather of a subset - most notably the expression statements, which are the most common ones.

3 Likes
3

Certain expression statements that end with {}-braces don't need a semicolon either. This includes if, loop, match, while, if let, for and blocks (including unsafe blocks). Only if their type is () though.

3 Likes
4

i can see either statements or expressions. are there any kind like " expression statements" ? are these different from expressions themselves?
Function Definitions are also Statements

5

As I said before, this is one of five kinds of statements. This one is defined as one of two things:

  • ExpressionWithoutBlock, with mandatory semicolon;
  • ExpressionWithBlock, with optional semicolon.

These two kinds of expressions are further composed of more different cases, but the gist is simple, as @steffahn already said: expression which ends with a } is also a statement; other expressions are converted to statements with semicolon. In both cases this would be expression statements.

2 Likes
6

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.