Add Part 9: Statements

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

@@ -0,0 +1,97 @@
+```
+metadata.title = "Part 9: Statements"
+metadata.tags = ["build a programming language", "rust"]
+metadata.date = "2021-05-03 17:46:42 -0400"
+metadata.shortDesc = ""
+metadata.slug = "statements"
+metadata.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.
+
+<!-- excerpt-end -->
+
+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.
+
+```rust
+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.
+
+```rust
+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.
+
+```rust
+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.
+
+```rust
+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:
+
+```rust
+fn main() {
+	let tokens = tokenize("1 + 2; foo();");
+	print("statements: {:?}", parse(&tokens));
+}
+```
+
+```sh
+$ cargo run
+statements: [
+	Expr(
+		BinaryOp {
+			left: Integer(1),
+			op: Add,
+			right: Integer(2),
+		},
+	),
+	Expr(
+		Call {
+			name: "foo",
+			params: [],
+		},
+	),
+]
+```
+