v6/site/posts/2021-05-03-statements.md

title = "Part 9: Statements"
tags = ["build a programming language", "rust"]
date = "2021-05-03 17:46:42 -0400"
slug = "statements"
preamble = '<p style="font-style: italic;">This post is part of a <a href="/build-a-programming-language/" data-link="/build-a-programming-language/">series</a> about learning Rust and building a small programming language.</p><hr>'

So the parser can handle a single expression, but since we're not building a Lisp, that's not enough. It needs to handle multiple statements. For context, an expression is a piece of code that represents a value whereas a statement is a piece of code that can be executed but does not result in a value.

In the AST, there's a new top-level type: Statement. For now, the only type of statement is one that contains an expression and nothing else.

enum Statement {
	Expr(Node),
}

The top level parse function has also changed to reflect this. It now returns a vector of statements, instead of a single expression node. The do_parse function continues to work exactly as it has, but is renamed parse_expression to since that's what it's actually doing.

fn parse(tokens: &[Token]) -> Vec<Statement> {
	let mut it = tokens.iter().peekable();
	let mut statements: Vec<Statement> = vec![];
	while let Some(_) = it.peek() {
		match parse_statement(&mut it) {
			Some(statement) => statements.push(statement),
			None => (),
		}
	}
	statements
}

The parse_statement function does exactly what the name suggests.

fn parse_statement<'a, I: Iterator<Item = &'a Token>>(it: &mut Peekable<'a, I>) -> Option<Statement> {
	if it.peek().is_none() {
		return None;
	}

	let node = parse_expression(it).map(|node| Statement::Expr(node));
	node
}

With that in place, parsing multiple statements is easy. The only change is that, after successfully parsing a statement, we need to consume a semicolon if there is one. Then, the parse loop will continue and the next statement can be parsed.

fn parse_statement<'a, I: Iterator<Item = &'a Token>>(it: &mut Peekable<'a, I>) -> Option<Statement> {
	// ...
	match it.peek() {
		Some(Token::Semicolon) => {
			it.next();
		}
		Some(tok) => {
			panic!("unexpected token {:?} after statement", tok);
		}
		None => (),
	}
	
	node
}

I intend to make semicolons optional and allow newline-delimited statements, but that is more complicated and will have to wait for another time. For now, this is good enough:

fn main() {
	let tokens = tokenize("1 + 2; foo();");
	print("statements: {:?}", parse(&tokens));
}

$ cargo run
statements: [
	Expr(
		BinaryOp {
			left: Integer(1),
			op: Add,
			right: Integer(2),
		},
	),
	Expr(
		Call {
			name: "foo",
			params: [],
		},
	),
]