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// This implements the grammar of Lox as described starting in the
// Crafting Interpreters chapter "Representing Code". Note that the
// upstream Java implementation works around Java being bad at value
// classes by writing a code generator for Java.
//
// My Rust implementation skips this step because it's unnecessary, we
// have real types.
use crate::errors::{Error, ErrorKind};
use crate::scanner::{Token, TokenKind};
// AST
#[derive(Debug)]
struct Binary<'a> {
left: Box<Expr<'a>>,
operator: Token<'a>,
right: Box<Expr<'a>>,
}
#[derive(Debug)]
struct Grouping<'a>(Box<Expr<'a>>);
#[derive(Debug)]
enum Literal {
Boolean(bool),
Number(f64),
String(String),
Nil,
}
#[derive(Debug)]
struct Unary<'a> {
operator: Token<'a>,
right: Box<Expr<'a>>,
}
#[derive(Debug)]
enum Expr<'a> {
Binary(Binary<'a>),
Grouping(Grouping<'a>),
Literal(Literal),
Unary(Unary<'a>),
}
// Parser
/*
expression → equality ;
equality → comparison ( ( "!=" | "==" ) comparison )* ;
comparison → term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
term → factor ( ( "-" | "+" ) factor )* ;
factor → unary ( ( "/" | "*" ) unary )* ;
unary → ( "!" | "-" ) unary
| primary ;
primary → NUMBER | STRING | "true" | "false" | "nil"
| "(" expression ")" ;
*/
struct Parser<'a> {
tokens: Vec<Token<'a>>,
current: usize,
}
type ExprResult<'a> = Result<Expr<'a>, Error>;
impl<'a> Parser<'a> {
// recursive-descent parser functions
fn expression(&mut self) -> ExprResult<'a> {
self.equality()
}
fn equality(&mut self) -> ExprResult<'a> {
self.binary_operator(
&[TokenKind::BangEqual, TokenKind::EqualEqual],
Self::comparison,
)
}
fn comparison(&mut self) -> ExprResult<'a> {
self.binary_operator(
&[
TokenKind::Greater,
TokenKind::GreaterEqual,
TokenKind::Less,
TokenKind::LessEqual,
],
Self::term,
)
}
fn term(&mut self) -> ExprResult<'a> {
self.binary_operator(&[TokenKind::Minus, TokenKind::Plus], Self::factor)
}
fn factor(&mut self) -> ExprResult<'a> {
self.binary_operator(&[TokenKind::Slash, TokenKind::Star], Self::unary)
}
fn unary(&mut self) -> ExprResult<'a> {
if self.match_token(&[TokenKind::Bang, TokenKind::Minus]) {
return Ok(Expr::Unary(Unary {
operator: self.previous(),
right: Box::new(self.unary()?),
}));
}
return self.primary();
}
fn primary(&mut self) -> ExprResult<'a> {
let next = self.advance();
let literal = match next.kind {
TokenKind::True => Literal::Boolean(true),
TokenKind::False => Literal::Boolean(false),
TokenKind::Nil => Literal::Nil,
TokenKind::Number(num) => Literal::Number(num),
TokenKind::String(string) => Literal::String(string),
TokenKind::LeftParen => {
unimplemented!("need error handling to deal with unbalanced parens");
}
// This branch indicates a parser bug, not invalid input.
unexpected => panic!("Parser encountered unexpected token '{:?}'", unexpected),
};
Ok(Expr::Literal(literal))
}
// internal helpers
/// Check if the next token is in `oneof`, and advance if it is.
fn match_token(&mut self, oneof: &[TokenKind]) -> bool {
for token in oneof {
if self.check_token(token) {
self.advance();
return true;
}
}
return false;
}
/// Return the next token and advance parser state.
fn advance(&mut self) -> Token<'a> {
if !self.is_at_end() {
self.current += 1;
}
return self.previous();
}
fn is_at_end(&self) -> bool {
self.check_token(&TokenKind::Eof)
}
/// Is the next token `token`?
fn check_token(&self, token: &TokenKind) -> bool {
self.peek().kind == *token
}
fn peek(&self) -> &Token<'a> {
&self.tokens[self.current]
}
fn previous(&self) -> Token<'a> {
self.tokens[self.current - 1].clone()
}
fn binary_operator(
&mut self,
oneof: &[TokenKind],
each: fn(&mut Parser<'a>) -> ExprResult<'a>,
) -> ExprResult<'a> {
let mut expr = each(self)?;
while self.match_token(oneof) {
expr = Expr::Binary(Binary {
left: Box::new(expr),
operator: self.previous(),
right: Box::new(each(self)?),
})
}
return Ok(expr);
}
}
|
