path: root/users/tazjin/rlox/src/parser.rs

// 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)]
pub struct Assign<'a> {
    pub name: Token<'a>,
    pub value: Box<Expr<'a>>,
}

#[derive(Debug)]
pub struct Binary<'a> {
    pub left: Box<Expr<'a>>,
    pub operator: Token<'a>,
    pub right: Box<Expr<'a>>,
}

#[derive(Debug)]
pub struct Grouping<'a>(pub Box<Expr<'a>>);

#[derive(Debug, Clone, PartialEq)]
pub enum Literal {
    Boolean(bool),
    Number(f64),
    String(String),
    Nil,
}

#[derive(Debug)]
pub struct Unary<'a> {
    pub operator: Token<'a>,
    pub right: Box<Expr<'a>>,
}

// Not to be confused with `Var`, which is for assignment.
#[derive(Debug)]
pub struct Variable<'a>(pub Token<'a>);

#[derive(Debug)]
pub enum Expr<'a> {
    Assign(Assign<'a>),
    Binary(Binary<'a>),
    Grouping(Grouping<'a>),
    Literal(Literal),
    Unary(Unary<'a>),
    Variable(Variable<'a>),
}

// Variable assignment. Not to be confused with `Variable`, which is
// for access.
#[derive(Debug)]
pub struct Var<'a> {
    pub name: Token<'a>,
    pub initialiser: Option<Expr<'a>>,
}

pub type Block<'a> = Vec<Statement<'a>>;

#[derive(Debug)]
pub enum Statement<'a> {
    Expr(Expr<'a>),
    Print(Expr<'a>),
    Var(Var<'a>),
    Block(Block<'a>),
}

// Parser

/*
program        → declaration* EOF ;

declaration    → varDecl
               | statement ;

statement      → exprStmt
               | printStmt ;

exprStmt       → expression ";" ;
printStmt      → "print" expression ";" ;

expression     → assignment ;
assignment     → IDENTIFIER "=" assignment
               | 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>;
type StmtResult<'a> = Result<Statement<'a>, Error>;

impl<'a> Parser<'a> {
    // recursive-descent parser functions

    fn declaration(&mut self) -> StmtResult<'a> {
        if self.match_token(&[TokenKind::Var]) {
            return self.var_declaration();
        }

        self.statement()
    }

    fn var_declaration(&mut self) -> StmtResult<'a> {
        // Since `TokenKind::Identifier` carries data, we can't use
        // `consume`.
        if let TokenKind::Identifier(_) = self.peek().kind {
            let mut var = Var {
                name: self.advance(),
                initialiser: None,
            };

            if self.match_token(&[TokenKind::Equal]) {
                var.initialiser = Some(self.expression()?);
            }

            self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;
            return Ok(Statement::Var(var));
        }

        return Err(Error {
            line: self.peek().line,
            kind: ErrorKind::ExpectedVariableName,
        });
    }

    fn statement(&mut self) -> StmtResult<'a> {
        if self.match_token(&[TokenKind::Print]) {
            self.print_statement()
        } else if self.match_token(&[TokenKind::LeftBrace]) {
            self.block_statement()
        } else {
            self.expr_statement()
        }
    }

    fn print_statement(&mut self) -> StmtResult<'a> {
        let expr = self.expression()?;
        self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;
        Ok(Statement::Print(expr))
    }

    fn block_statement(&mut self) -> StmtResult<'a> {
        let mut block: Block<'a> = vec![];

        while !self.check_token(&TokenKind::RightBrace) && !self.is_at_end() {
            block.push(self.declaration()?);
        }

        self.consume(&TokenKind::RightBrace, ErrorKind::ExpectedClosingBrace)?;

        Ok(Statement::Block(block))
    }

    fn expr_statement(&mut self) -> StmtResult<'a> {
        let expr = self.expression()?;
        self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;
        Ok(Statement::Expr(expr))
    }

    fn expression(&mut self) -> ExprResult<'a> {
        self.assignment()
    }

    fn assignment(&mut self) -> ExprResult<'a> {
        let expr = self.equality()?;

        if self.match_token(&[TokenKind::Equal]) {
            let equals = self.previous().clone();
            let value = self.assignment()?;

            if let Expr::Variable(Variable(name)) = expr {
                return Ok(Expr::Assign(Assign {
                    name,
                    value: Box::new(value),
                }));
            }

            return Err(Error {
                line: equals.line,
                kind: ErrorKind::InvalidAssignmentTarget(format!("{:?}", equals)),
            });
        }

        Ok(expr)
    }

    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().clone(),
                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 => {
                let expr = self.expression()?;
                self.consume(&TokenKind::RightParen, ErrorKind::UnmatchedParens)?;
                return Ok(Expr::Grouping(Grouping(Box::new(expr))));
            }

            TokenKind::Identifier(_) => return Ok(Expr::Variable(Variable(next))),

            unexpected => {
                eprintln!("encountered {:?}", unexpected);
                return Err(Error {
                    line: next.line,
                    kind: ErrorKind::ExpectedExpression(next.lexeme.into_iter().collect()),
                });
            }
        };

        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().clone();
    }

    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]
    }

    fn consume(&mut self, kind: &TokenKind, err: ErrorKind) -> Result<Token, Error> {
        if self.check_token(kind) {
            return Ok(self.advance());
        }

        Err(Error {
            line: self.peek().line,
            kind: err,
        })
    }

    fn synchronise(&mut self) {
        self.advance();

        while !self.is_at_end() {
            if self.previous().kind == TokenKind::Semicolon {
                return;
            }

            match self.peek().kind {
                TokenKind::Class
                | TokenKind::Fun
                | TokenKind::Var
                | TokenKind::For
                | TokenKind::If
                | TokenKind::While
                | TokenKind::Print
                | TokenKind::Return => return,

                _ => {
                    self.advance();
                }
            }
        }
    }

    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().clone(),
                right: Box::new(each(self)?),
            })
        }

        return Ok(expr);
    }
}

pub fn parse<'a>(tokens: Vec<Token<'a>>) -> Result<Block<'a>, Vec<Error>> {
    let mut parser = Parser { tokens, current: 0 };
    let mut program: Block<'a> = vec![];
    let mut errors: Vec<Error> = vec![];

    while !parser.is_at_end() {
        match parser.declaration() {
            Err(err) => {
                errors.push(err);
                parser.synchronise();
            }
            Ok(decl) => {
                program.push(decl);
            }
        }
    }

    if errors.is_empty() {
        Ok(program)
    } else {
        Err(errors)
    }
}