rust-crafting-interpreters-.../src/parser.rs

737 lines
24 KiB
Rust

use crate::{
ast,
errors::{ErrorHandler, ParserError},
tokens::{Token, TokenType},
};
/// The parser contains the input tokens and the current input position.
#[derive(Debug)]
pub struct Parser {
tokens: Vec<Token>,
current: usize,
loop_state: Vec<LoopParsingState>,
}
/// The state of the parser regarding loops. We may be parsing an unnamed or
/// named loop, or we might not be parsing a loop at all.
#[derive(Debug, Clone, PartialEq)]
enum LoopParsingState {
None,
Unnamed,
Named(String),
}
impl From<&Option<Token>> for LoopParsingState {
fn from(value: &Option<Token>) -> Self {
match &value {
None => LoopParsingState::Unnamed,
Some(name) => LoopParsingState::Named(name.lexeme.clone()),
}
}
}
/// The result of one of the parser's functions.
type ParserResult<T> = Result<T, ParserError>;
impl Parser {
/// Initialize the parser.
pub fn new(tokens: Vec<Token>) -> Self {
Self {
tokens,
current: 0,
loop_state: Vec::default(),
}
}
/// Parse the tokens into an AST and return it, or return nothing if a
/// parser error occurs.
pub fn parse(mut self, err_hdl: &mut ErrorHandler) -> Option<ast::ProgramNode> {
self.loop_state.push(LoopParsingState::None);
let result = self.parse_program(err_hdl);
self.loop_state.pop();
result
}
/// Synchronize the parser after an error.
fn synchronize(&mut self) {
self.advance();
while !self.is_at_end() {
if self.previous().token_type == TokenType::Semicolon
|| matches!(
self.peek().token_type,
TokenType::Class
| TokenType::Fun
| TokenType::If
| TokenType::Print
| TokenType::Return
| TokenType::Var
| TokenType::While
)
{
return;
}
self.current += 1;
}
}
/* ------------------------ *
* RECURSIVE DESCENT PARSER *
* ------------------------ */
/// Parse the following rule:
/// ```
/// program := statement*
/// ```
fn parse_program(&mut self, err_hdl: &mut ErrorHandler) -> Option<ast::ProgramNode> {
let mut stmts: Vec<ast::StmtNode> = Vec::new();
while !self.is_at_end() {
match self.parse_statement() {
Ok(node) => stmts.push(node),
Err(err) => {
err.report(err_hdl);
self.synchronize()
}
}
}
if err_hdl.had_error().is_none() {
Some(ast::ProgramNode(stmts))
} else {
None
}
}
/// Parse the following rule:
/// ```
/// statement := expression ";"
/// statement := "print" expression ";"
/// statement := declaration ";"
/// statement := block
/// statement := labelled_loop
/// statement := if_statement
/// statement := while_statement
/// statement := for_statement
/// statement := loop_control_statement
/// ```
fn parse_statement(&mut self) -> ParserResult<ast::StmtNode> {
if self.expect(&[TokenType::Var]).is_some() {
self.parse_var_declaration()
} else if self.expect(&[TokenType::Fun]).is_some() {
self.parse_fun_declaration("function")
} else if self.expect(&[TokenType::LeftBrace]).is_some() {
self.parse_block()
} else if self.expect(&[TokenType::Address]).is_some() {
self.parse_labelled_loop()
} else if self.expect(&[TokenType::If]).is_some() {
self.parse_if_statement()
} else if self.expect(&[TokenType::While]).is_some() {
self.parse_while_statement(None)
} else if self.expect(&[TokenType::For]).is_some() {
self.parse_for_statement(None)
} else if let Some(lcs) = self.expect(&[TokenType::Break, TokenType::Continue]) {
self.parse_loop_control_statement(&lcs)
} else if self.expect(&[TokenType::Print]).is_some() {
let expression = self.parse_expression()?;
self.consume(&TokenType::Semicolon, "expected ';' after value")?;
Ok(ast::StmtNode::Print(expression))
} else {
self.parse_expression_stmt()
}
}
/// Parse the following rule:
/// ```
/// expression_stmt := expression ";"
/// ```
fn parse_expression_stmt(&mut self) -> ParserResult<ast::StmtNode> {
let expression = self.parse_expression()?;
self.consume(&TokenType::Semicolon, "expected ';' after expression")?;
Ok(ast::StmtNode::Expression(expression))
}
/// Parse the following rule:
/// ```
/// var_declaration := "var" IDENTIFIER ";"
/// var_declaration := "var" IDENTIFIER "=" expression ";"
/// ```
fn parse_var_declaration(&mut self) -> ParserResult<ast::StmtNode> {
let name = match self.peek().token_type {
TokenType::Identifier(_) => self.advance().clone(),
_ => return Err(ParserError::new(self.peek(), "expected variable name")),
};
let initializer: Option<ast::ExprNode> = match self.expect(&[TokenType::Equal]) {
Some(_) => Some(self.parse_expression()?),
None => None,
};
self.consume(
&TokenType::Semicolon,
"expected ';' after variable declaration",
)?;
Ok(ast::StmtNode::VarDecl(name, initializer))
}
/// Parse the following rule:
/// ```
/// fun_declaration := "fun" function
/// function := IDENTIFIER function_info
/// function_info := "(" parameters? ")" block
/// parameters := IDENTIFIER ( "," IDENTIFIER )*
/// ```
/// The `kind` parameter is used to generate error messages.
fn parse_fun_declaration(&mut self, kind: &'static str) -> ParserResult<ast::StmtNode> {
// Read the name
let name = match self.peek().token_type {
TokenType::Identifier(_) => self.advance().clone(),
_ => {
return Err(ParserError::new(
self.peek(),
&format!("expected {} name", kind),
))
}
};
// Read the list of parameter names
self.consume(
&TokenType::LeftParen,
&format!("expected '(' after {} name", kind),
)?;
let params = {
let mut params = Vec::new();
if !self.check(&TokenType::RightParen) {
loop {
if params.len() >= 255 {
return Err(ParserError::new(
self.peek(),
&format!("{} can't have more than 255 parameters", kind),
));
}
if let TokenType::Identifier(_) = self.peek().token_type {
params.push(self.advance().clone());
} else {
return Err(ParserError::new(self.peek(), "parameter name expected"));
}
if self.expect(&[TokenType::Comma]).is_none() {
break;
}
}
self.consume(&TokenType::RightParen, "')' expected after parameters")?;
}
params
};
// Read the function's body
self.consume(
&TokenType::LeftBrace,
&format!("'{{' expected before {} body", kind),
)?;
let block = self.parse_block()?;
Ok(ast::StmtNode::FunDecl {
name,
params,
body: block.extract_block_statements(),
})
}
/// Parse the following rule:
/// ```
/// block := "{" statement* "}"
/// ```
fn parse_block(&mut self) -> ParserResult<ast::StmtNode> {
let mut stmts: Vec<ast::StmtNode> = Vec::new();
while !(self.check(&TokenType::RightBrace) || self.is_at_end()) {
stmts.push(self.parse_statement()?);
}
self.consume(&TokenType::RightBrace, "expected '}' after block.")?;
Ok(ast::StmtNode::Block(stmts))
}
/// Parse the following rule:
/// ```
/// if_statement := "if" "(" expression ")" statement
/// if_statement := "if" "(" expression ")" statement "else" statement
/// ```
fn parse_if_statement(&mut self) -> ParserResult<ast::StmtNode> {
self.consume(&TokenType::LeftParen, "expected '(' after 'if'")?;
let expression = self.parse_expression()?;
self.consume(
&TokenType::RightParen,
"expected ')' after condition in 'if' statement",
)?;
let then_branch = Box::new(self.parse_statement()?);
let else_branch = match self.expect(&[TokenType::Else]) {
Some(_) => Some(Box::new(self.parse_statement()?)),
None => None,
};
Ok(ast::StmtNode::IfStmt {
condition: expression,
then_branch,
else_branch,
})
}
/// Parse the following rule:
/// ```
/// labelled_loop := "@" IDENTIFIER while_statement
/// labelled_loop := "@" IDENTIFIER for_statement
/// ```
fn parse_labelled_loop(&mut self) -> ParserResult<ast::StmtNode> {
let name_token = match self.peek().token_type {
TokenType::Identifier(_) => self.advance().clone(),
_ => {
return Err(ParserError::new(
self.peek(),
"identifier expected after '@'",
))
}
};
if self.expect(&[TokenType::While]).is_some() {
self.parse_while_statement(Some(name_token))
} else if self.expect(&[TokenType::For]).is_some() {
self.parse_for_statement(Some(name_token))
} else {
Err(ParserError::new(
self.peek(),
"'while' or 'for' expected after loop label",
))
}
}
/// Parse the following rule:
/// ```
/// while_statement := "while" "(" expression ")" statement
/// ```
fn parse_while_statement(&mut self, label: Option<Token>) -> ParserResult<ast::StmtNode> {
self.consume(&TokenType::LeftParen, "expected '(' after 'while'")?;
let condition = self.parse_expression()?;
self.consume(
&TokenType::RightParen,
"expected ')' after condition in 'while' statement",
)?;
let body = Box::new({
self.loop_state.push(LoopParsingState::from(&label));
let result = self.parse_statement();
self.loop_state.pop();
result?
});
Ok(ast::StmtNode::LoopStmt {
label,
condition,
body,
after_body: None,
})
}
/// Parse the following rules:
/// ```
/// for_statement := "for" "(" for_initializer ";" expression ";" expression ")" statement
/// for_initializer := declaration
/// for_initializer := expression
/// for_initializer :=
/// ```
fn parse_for_statement(&mut self, label: Option<Token>) -> ParserResult<ast::StmtNode> {
self.consume(&TokenType::LeftParen, "expected '(' after 'for'")?;
let initializer = if self.expect(&[TokenType::Semicolon]).is_some() {
None
} else if self.expect(&[TokenType::Var]).is_some() {
Some(self.parse_var_declaration()?)
} else {
Some(self.parse_expression_stmt()?)
};
let condition = if self.check(&TokenType::Semicolon) {
ast::ExprNode::Litteral {
value: Token {
token_type: TokenType::True,
lexeme: String::from("true"),
line: self.peek().line,
},
}
} else {
self.parse_expression()?
};
self.consume(
&TokenType::Semicolon,
"expected ';' after condition in 'for' statement",
)?;
let increment = if self.check(&TokenType::RightParen) {
None
} else {
Some(self.parse_expression()?)
};
self.consume(
&TokenType::RightParen,
"expected ')' after increment in 'for' statement",
)?;
// Generate a while loop, with an optional initializer which may be
// inside a specific block if the initializer declares a variable.
let body_stmt = {
self.loop_state.push(LoopParsingState::from(&label));
let result = self.parse_statement();
self.loop_state.pop();
result?
};
let while_stmt = ast::StmtNode::LoopStmt {
label,
condition,
body: Box::new(body_stmt),
after_body: increment.map(|incr| Box::new(ast::StmtNode::Expression(incr))),
};
if let Some(init_stmt) = initializer {
Ok(ast::StmtNode::Block(vec![init_stmt, while_stmt]))
} else {
Ok(while_stmt)
}
}
/// Parse the following rule:
/// ```
/// loop_control_statement := "break" ( IDENTIFIER )? ";"
/// loop_control_statement := "continue" ( IDENTIFIER )? ";"
/// ```
fn parse_loop_control_statement(&mut self, stmt_token: &Token) -> ParserResult<ast::StmtNode> {
if self.loop_state() == &LoopParsingState::None {
return Err(ParserError::new(
stmt_token,
&format!(
"'{}' statement found outside of loop body",
stmt_token.lexeme
),
));
}
let loop_name = if let TokenType::Identifier(_) = self.peek().token_type {
let name_token = self.advance().clone();
if !self.find_named_loop(&name_token.lexeme) {
self.expect(&[TokenType::Semicolon]);
return Err(ParserError::new(
&name_token,
&format!("no reachable loop named '{}'", name_token.lexeme),
));
}
Some(name_token)
} else {
None
};
self.consume(
&TokenType::Semicolon,
"';' expected after loop control statement",
)?;
Ok(ast::StmtNode::LoopControlStmt {
is_break: stmt_token.token_type == TokenType::Break,
loop_name,
})
}
/// Parse the following rule:
/// ```
/// expression := assignment
/// ```
fn parse_expression(&mut self) -> ParserResult<ast::ExprNode> {
self.parse_assignment()
}
/// Parse the following rule:
/// ```
/// assignment := IDENTIFIER "=" equality
/// assignment := equality
/// ```
fn parse_assignment(&mut self) -> ParserResult<ast::ExprNode> {
let expr = self.parse_logic_or()?;
if let Some(equals) = self.expect(&[TokenType::Equal]) {
let value = self.parse_assignment()?;
if let ast::ExprNode::Variable { name } = expr {
Ok(ast::ExprNode::Assignment {
name,
value: Box::new(value),
})
} else {
Err(ParserError::new(&equals, "invalid assignment target"))
}
} else {
Ok(expr)
}
}
/// Parse the following rule:
/// ```
/// logic_or := logic_and ( "or" logic_and )*
/// ```
fn parse_logic_or(&mut self) -> ParserResult<ast::ExprNode> {
let mut expr = self.parse_logic_and()?;
while let Some(operator) = self.expect(&[TokenType::Or]) {
let right = self.parse_logic_and()?;
expr = ast::ExprNode::Logical {
left: Box::new(expr),
operator: operator.clone(),
right: Box::new(right),
};
}
Ok(expr)
}
/// Parse the following rule:
/// ```
/// logic_and := equality ( "and" equality )*
/// ```
fn parse_logic_and(&mut self) -> ParserResult<ast::ExprNode> {
let mut expr = self.parse_equality()?;
while let Some(operator) = self.expect(&[TokenType::And]) {
let right = self.parse_equality()?;
expr = ast::ExprNode::Logical {
left: Box::new(expr),
operator: operator.clone(),
right: Box::new(right),
};
}
Ok(expr)
}
/// Parse the following rule:
/// ```
/// equality := comparison "==" comparison
/// equality := comparison "!=" comparison
/// ```
fn parse_equality(&mut self) -> ParserResult<ast::ExprNode> {
let mut expr = self.parse_comparison()?;
while let Some(operator) = self.expect(&[TokenType::BangEqual, TokenType::EqualEqual]) {
let right = self.parse_comparison()?;
expr = ast::ExprNode::Binary {
left: Box::new(expr),
operator: operator.clone(),
right: Box::new(right),
};
}
Ok(expr)
}
/// Parse the following rule:
/// ```
/// comparison := term comparison_operator term
/// comparison_operator := "<" | "<=" | ">" | ">="
/// ```
fn parse_comparison(&mut self) -> ParserResult<ast::ExprNode> {
let mut expr = self.parse_term()?;
while let Some(operator) = self.expect(&[
TokenType::Greater,
TokenType::GreaterEqual,
TokenType::Less,
TokenType::LessEqual,
]) {
let right = self.parse_term()?;
expr = ast::ExprNode::Binary {
left: Box::new(expr),
operator: operator.clone(),
right: Box::new(right),
};
}
Ok(expr)
}
/// Parse the following rule:
/// ```
/// term := factor ( "+" factor )*
/// term := factor ( "-" factor )*
/// ```
fn parse_term(&mut self) -> ParserResult<ast::ExprNode> {
let mut expr = self.parse_factor()?;
while let Some(operator) = self.expect(&[TokenType::Minus, TokenType::Plus]) {
let right = self.parse_factor()?;
expr = ast::ExprNode::Binary {
left: Box::new(expr),
operator: operator.clone(),
right: Box::new(right),
};
}
Ok(expr)
}
/// Parse the following rule:
/// ```
/// factor := unary ( "*" unary )*
/// factor := unary ( "/" unary )*
/// ```
fn parse_factor(&mut self) -> ParserResult<ast::ExprNode> {
let mut expr = self.parse_unary()?;
while let Some(operator) = self.expect(&[TokenType::Slash, TokenType::Star]) {
let right = self.parse_unary()?;
expr = ast::ExprNode::Binary {
left: Box::new(expr),
operator: operator.clone(),
right: Box::new(right),
};
}
Ok(expr)
}
/// Parse the following rule:
/// ```
/// unary := "-" unary
/// unary := "!" unary
/// unary := primary call_arguments*
/// ```
fn parse_unary(&mut self) -> ParserResult<ast::ExprNode> {
if let Some(operator) = self.expect(&[TokenType::Bang, TokenType::Minus]) {
Ok(ast::ExprNode::Unary {
operator,
right: Box::new(self.parse_unary()?),
})
} else {
let mut expr = self.parse_primary()?;
while self.expect(&[TokenType::LeftParen]).is_some() {
expr = self.parse_call_arguments(expr)?;
}
Ok(expr)
}
}
/// Parse the following rule:
/// ```
/// primary := "(" expression ")"
/// primary := FALSE | TRUE | NIL | STRING | NUMBER
/// primary := IDENTIFIER
/// primary := call
/// ```
fn parse_primary(&mut self) -> ParserResult<ast::ExprNode> {
if self.expect(&[TokenType::LeftParen]).is_some() {
let expr = self.parse_expression()?;
self.consume(&TokenType::RightParen, "expected ')' after expression")?;
Ok(ast::ExprNode::Grouping {
expression: Box::new(expr),
})
} else if let Some(token) =
self.expect(&[TokenType::False, TokenType::True, TokenType::Nil])
{
Ok(ast::ExprNode::Litteral { value: token })
} else {
match &self.peek().token_type {
TokenType::Number(_) | &TokenType::String(_) => Ok(ast::ExprNode::Litteral {
value: self.advance().clone(),
}),
TokenType::Identifier(_) => Ok(ast::ExprNode::Variable {
name: self.advance().clone(),
}),
_ => Err(ParserError::new(self.peek(), "expected expression")),
}
}
}
/// Help parsing the following rules:
/// ```
/// call := expression "(" arguments? ")"
/// arguments := expression ( "," expression )*
/// ```
fn parse_call_arguments(
&mut self,
callee: ast::ExprNode,
) -> Result<ast::ExprNode, ParserError> {
let mut arguments = Vec::new();
if !self.check(&TokenType::RightParen) {
loop {
if arguments.len() == 255 {
return Err(ParserError::new(
self.peek(),
"functions may not have more than 255 arguments",
));
}
arguments.push(self.parse_expression()?);
if self.expect(&[TokenType::Comma]).is_none() {
break;
}
}
}
let right_paren = self
.consume(&TokenType::RightParen, "')' expected after arguments")?
.clone();
Ok(ast::ExprNode::Call {
callee: Box::new(callee),
right_paren,
arguments,
})
}
/* -------------- *
* HELPER METHODS *
* -------------- */
/// Expect a token of some types. If a matching token is found, the read
/// pointer is moved and a clone of the token is returned.
fn expect(&mut self, accepts: &[TokenType]) -> Option<Token> {
for tt in accepts {
if self.check(tt) {
return Some(self.advance().clone());
}
}
None
}
/// Consume a token of a given type. If no matching token is found, a
/// parse error is returned instead. Otherwise the read pointer is moved.
fn consume(&mut self, token_type: &TokenType, error: &str) -> ParserResult<&Token> {
if self.check(token_type) {
Ok(self.advance())
} else {
Err(ParserError::new(self.peek(), error))
}
}
/// Check for a token of some type. Returns `false` if the end of the input
/// has been reached. The read pointer isn't affected.
fn check(&self, token_type: &TokenType) -> bool {
if self.is_at_end() {
false
} else {
&self.peek().token_type == token_type
}
}
/// Move the read pointer forward if the end hasn't been reached. In all
/// cases, return the previous element (so either the element that was
/// current before the pointer moved, or the last, non-`EOF` token).
fn advance(&mut self) -> &Token {
if !self.is_at_end() {
self.current += 1
}
self.previous()
}
/// Check whether the end of token stream has been reached by checking
/// for the `EOF` token.
fn is_at_end(&self) -> bool {
self.peek().token_type == TokenType::Eof
}
/// Return a reference to the current token in the stream.
fn peek(&self) -> &Token {
&self.tokens[self.current]
}
/// Return a reference to the previous token in the stream.
fn previous(&self) -> &Token {
&self.tokens[self.current - 1]
}
/// Take a peek at the current loop state.
fn loop_state(&self) -> &LoopParsingState {
&self.loop_state[self.loop_state.len() - 1]
}
/// Find a loop with a given name in the loop state. Stops looking when
/// the first non-loop entry is reached.
fn find_named_loop(&self, name: &str) -> bool {
let mut pos = self.loop_state.len() - 1;
loop {
match &self.loop_state[pos] {
LoopParsingState::None => break,
LoopParsingState::Unnamed => (),
LoopParsingState::Named(n) if n == name => return true,
LoopParsingState::Named(_) => (),
}
pos -= 1;
}
false
}
}