feat(tazjin/rlox): Implement tree-walk interpreter of exprs
This is only a subset of the Lox spec so far. It implements the language up to the runtime error chapter on https://craftinginterpreters.com/evaluating-expressions.html Change-Id: I295dbf4b6544420d6fe80b6aaba661fb21acdea6 Reviewed-on: https://cl.tvl.fyi/c/depot/+/2281 Reviewed-by: tazjin <mail@tazj.in> Tested-by: BuildkiteCI
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2 changed files with 74 additions and 10 deletions
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@ -1,6 +1,6 @@
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use crate::errors::{report, Error};
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use crate::parser;
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use crate::scanner::{self, Token};
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use crate::parser::{self, Expr, Literal};
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use crate::scanner::{self, Token, TokenKind};
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// Run some Lox code and print it to stdout
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pub fn run(code: &str) {
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@ -10,7 +10,10 @@ pub fn run(code: &str) {
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Ok(tokens) => {
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print_tokens(&tokens);
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match parser::parse(tokens) {
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Ok(expr) => println!("Expression:\n{:?}", expr),
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Ok(expr) => {
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println!("Expression:\n{:?}", expr);
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println!("Result: {:?}", eval(&expr));
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}
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Err(errors) => report_errors(errors),
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}
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}
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@ -30,3 +33,64 @@ fn report_errors(errors: Vec<Error>) {
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report(&error);
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}
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}
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// Tree-walk interpreter
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fn eval_truthy(lit: &Literal) -> bool {
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match lit {
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Literal::Nil => false,
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Literal::Boolean(b) => *b,
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_ => true,
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}
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}
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fn eval_unary<'a>(expr: &parser::Unary<'a>) -> Literal {
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let right = eval(&*expr.right);
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match (&expr.operator.kind, right) {
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(TokenKind::Minus, Literal::Number(num)) => Literal::Number(-num),
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(TokenKind::Bang, right) => Literal::Boolean(!eval_truthy(&right)),
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_ => unimplemented!("no handling of type errors"),
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}
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}
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fn eval_binary<'a>(expr: &parser::Binary<'a>) -> Literal {
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let left = eval(&*expr.left);
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let right = eval(&*expr.right);
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match (&expr.operator.kind, left, right) {
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// Numeric
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(TokenKind::Minus, Literal::Number(l), Literal::Number(r)) => Literal::Number(l - r),
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(TokenKind::Slash, Literal::Number(l), Literal::Number(r)) => Literal::Number(l / r),
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(TokenKind::Star, Literal::Number(l), Literal::Number(r)) => Literal::Number(l * r),
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(TokenKind::Plus, Literal::Number(l), Literal::Number(r)) => Literal::Number(l + r),
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// Strings
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(TokenKind::Plus, Literal::String(l), Literal::String(r)) => {
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Literal::String(format!("{}{}", l, r))
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}
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// Comparators (on numbers only?)
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(TokenKind::Greater, Literal::Number(l), Literal::Number(r)) => Literal::Boolean(l > r),
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(TokenKind::GreaterEqual, Literal::Number(l), Literal::Number(r)) => {
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Literal::Boolean(l >= r)
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}
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(TokenKind::Less, Literal::Number(l), Literal::Number(r)) => Literal::Boolean(l < r),
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(TokenKind::LessEqual, Literal::Number(l), Literal::Number(r)) => Literal::Boolean(l <= r),
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// Equality
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(TokenKind::Equal, l, r) => Literal::Boolean(l == r),
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(TokenKind::BangEqual, l, r) => Literal::Boolean(l != r),
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_ => unimplemented!("type errors unhandled"),
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}
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}
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fn eval<'a>(expr: &Expr<'a>) -> Literal {
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match expr {
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Expr::Literal(lit) => lit.clone(),
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Expr::Grouping(grouping) => eval(&*grouping.0),
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Expr::Unary(unary) => eval_unary(unary),
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Expr::Binary(binary) => eval_binary(binary),
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}
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}
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@ -12,15 +12,15 @@ use crate::scanner::{Token, TokenKind};
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#[derive(Debug)]
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pub struct Binary<'a> {
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left: Box<Expr<'a>>,
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operator: Token<'a>,
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right: Box<Expr<'a>>,
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pub left: Box<Expr<'a>>,
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pub operator: Token<'a>,
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pub right: Box<Expr<'a>>,
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}
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#[derive(Debug)]
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pub struct Grouping<'a>(Box<Expr<'a>>);
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pub struct Grouping<'a>(pub Box<Expr<'a>>);
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#[derive(Debug)]
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#[derive(Debug, Clone, PartialEq)]
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pub enum Literal {
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Boolean(bool),
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Number(f64),
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@ -30,8 +30,8 @@ pub enum Literal {
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#[derive(Debug)]
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pub struct Unary<'a> {
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operator: Token<'a>,
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right: Box<Expr<'a>>,
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pub operator: Token<'a>,
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pub right: Box<Expr<'a>>,
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}
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#[derive(Debug)]
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