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};
use std::rc::Rc;

// AST

#[derive(Debug)]
pub struct Assign {
    pub name: Token,
    pub value: Box<Expr>,
}

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

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

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

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

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

#[derive(Debug)]
pub struct Call {
    pub callee: Box<Expr>,
    pub paren: Token,
    pub args: Vec<Expr>,
}

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

#[derive(Debug)]
pub enum Expr {
    Assign(Assign),
    Binary(Binary),
    Grouping(Grouping),
    Literal(Literal),
    Unary(Unary),
    Call(Call),
    Variable(Variable),
    Logical(Logical),
}

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

#[derive(Debug)]
pub struct Return {
    pub value: Expr,
}

#[derive(Debug)]
pub struct If {
    pub condition: Expr,
    pub then_branch: Box<Statement>,
    pub else_branch: Option<Box<Statement>>,
}

#[derive(Debug)]
pub struct While {
    pub condition: Expr,
    pub body: Box<Statement>,
}

pub type Block = Vec<Statement>;

#[derive(Debug)]
pub struct Function {
    pub name: Token,
    pub params: Vec<Token>,
    pub body: Block,
}

#[derive(Debug)]
pub enum Statement {
    Expr(Expr),
    Print(Expr),
    Var(Var),
    Block(Block),
    If(If),
    While(While),
    Function(Rc<Function>),
    Return(Return),
}

// Parser

/*
program        → declaration* EOF ;

declaration    → funDecl
               | varDecl
               | statement ;

funDecl        → "fun" function ;
function       → IDENTIFIER "(" parameters? ")" block ;
parameters     → IDENTIFIER ( "," IDENTIFIER )* ;


statement      → exprStmt
               | forStmt
               | ifStmt
               | printStmt
               | returnStmt
               | whileStmt
               | block ;

forStmt        → "for" "(" ( varDecl | exprStmt | ";" )
                 expression? ";"
                 expression? ")" statement ;

returnStmt     → "return" expression? ";" ;

whileStmt      → "while" "(" expression ")" statement ;

exprStmt       → expression ";" ;

ifStmt         → "if" "(" expression ")" statement
               ( "else" statement )? ;

printStmt      → "print" expression ";" ;

expression     → assignment ;
assignment     → IDENTIFIER "=" assignment
               | logic_or ;
logic_or       → logic_and ( "or" logic_and )* ;
logic_and      → equality ( "and" equality )* ;
equality       → comparison ( ( "!=" | "==" ) comparison )* ;
comparison     → term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
term           → factor ( ( "-" | "+" ) factor )* ;
factor         → unary ( ( "/" | "*" ) unary )* ;
unary          → ( "!" | "-" ) unary | call ;
call           → primary ( "(" arguments? ")" )* ;
arguments      → expression ( "," expression )* ;
primary        → NUMBER | STRING | "true" | "false" | "nil"
               | "(" expression ")" ;
*/

struct Parser {
    tokens: Vec<Token>,
    current: usize,
}

type ExprResult = Result<Expr, Error>;
type StmtResult = Result<Statement, Error>;

impl Parser {
    // recursive-descent parser functions

    fn declaration(&mut self) -> StmtResult {
        if self.match_token(&TokenKind::Fun) {
            return self.function();
        }

        if self.match_token(&TokenKind::Var) {
            return self.var_declaration();
        }

        self.statement()
    }

    fn function(&mut self) -> StmtResult {
        let name = self.identifier("Expected function name.")?;

        self.consume(
            &TokenKind::LeftParen,
            ErrorKind::ExpectedToken("Expect '(' after function name."),
        )?;

        let mut params = vec![];

        if !self.check_token(&TokenKind::RightParen) {
            loop {
                if params.len() >= 255 {
                    return Err(Error {
                        line: self.peek().line,
                        kind: ErrorKind::InternalError("255 parameter limit exceeded.".into()),
                    });
                }

                params.push(self.identifier("Expected parameter name.")?);

                if !self.match_token(&TokenKind::Comma) {
                    break;
                }
            }
        }

        self.consume(
            &TokenKind::RightParen,
            ErrorKind::ExpectedToken("Expect ')' after parameters."),
        )?;

        self.consume(
            &TokenKind::LeftBrace,
            ErrorKind::ExpectedToken("Expect '{' before function body."),
        )?;

        Ok(Statement::Function(Rc::new(Function {
            name,
            params,
            body: self.block_statement()?,
        })))
    }

    fn var_declaration(&mut self) -> StmtResult {
        // Since `TokenKind::Identifier` carries data, we can't use
        // `consume`.
        let mut var = Var {
            name: self.identifier("Expected variable name.")?,
            initialiser: None,
        };

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

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

    fn statement(&mut self) -> StmtResult {
        if self.match_token(&TokenKind::Print) {
            self.print_statement()
        } else if self.match_token(&TokenKind::LeftBrace) {
            Ok(Statement::Block(self.block_statement()?))
        } else if self.match_token(&TokenKind::If) {
            self.if_statement()
        } else if self.match_token(&TokenKind::While) {
            self.while_statement()
        } else if self.match_token(&TokenKind::For) {
            self.for_statement()
        } else if self.match_token(&TokenKind::Return) {
            self.return_statement()
        } else {
            self.expr_statement()
        }
    }

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

    fn block_statement(&mut self) -> Result<Block, Error> {
        let mut block: Block = vec![];

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

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

        Ok(block)
    }

    fn if_statement(&mut self) -> StmtResult {
        self.consume(
            &TokenKind::LeftParen,
            ErrorKind::ExpectedToken("Expected '(' after 'if'"),
        )?;
        let condition = self.expression()?;
        self.consume(
            &TokenKind::RightParen,
            ErrorKind::ExpectedToken("Expected ')' after condition"),
        )?;

        let then_branch = Box::new(self.statement()?);

        let mut stmt = If {
            condition,
            then_branch,
            else_branch: Option::None,
        };

        if self.match_token(&TokenKind::Else) {
            stmt.else_branch = Some(Box::new(self.statement()?));
        }

        Ok(Statement::If(stmt))
    }

    fn while_statement(&mut self) -> StmtResult {
        self.consume(
            &TokenKind::LeftParen,
            ErrorKind::ExpectedToken("Expected '(' after 'while'"),
        )?;

        let condition = self.expression()?;

        self.consume(
            &TokenKind::RightParen,
            ErrorKind::ExpectedToken("Expected ')' after 'while'"),
        )?;

        Ok(Statement::While(While {
            condition,
            body: Box::new(self.statement()?),
        }))
    }

    fn for_statement(&mut self) -> StmtResult {
        // Parsing of clauses ...
        self.consume(
            &TokenKind::LeftParen,
            ErrorKind::ExpectedToken("Expected '(' after 'for'"),
        )?;

        let initialiser = if self.match_token(&TokenKind::Semicolon) {
            None
        } else if self.match_token(&TokenKind::Var) {
            Some(self.var_declaration()?)
        } else {
            Some(self.expr_statement()?)
        };

        let condition = if self.check_token(&TokenKind::Semicolon) {
            // unspecified condition => infinite loop
            Expr::Literal(Literal::Boolean(true))
        } else {
            self.expression()?
        };

        self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;

        let increment = if self.check_token(&TokenKind::RightParen) {
            None
        } else {
            Some(self.expression()?)
        };

        self.consume(
            &TokenKind::RightParen,
            ErrorKind::ExpectedToken("Expected ')' after for clauses"),
        )?;

        let mut body = self.statement()?;

        // ... desugaring to while

        if let Some(inc) = increment {
            body = Statement::Block(vec![body, Statement::Expr(inc)]);
        }

        body = Statement::While(While {
            condition,
            body: Box::new(body),
        });

        if let Some(init) = initialiser {
            body = Statement::Block(vec![init, body]);
        }

        Ok(body)
    }

    fn return_statement(&mut self) -> StmtResult {
        let value = self.expression()?;
        self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;
        Ok(Statement::Return(Return { value }))
    }

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

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

    fn assignment(&mut self) -> ExprResult {
        let expr = self.logic_or()?;

        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 logic_or(&mut self) -> ExprResult {
        let mut expr = self.logic_and()?;

        while self.match_token(&TokenKind::Or) {
            expr = Expr::Logical(Logical {
                left: Box::new(expr),
                operator: self.previous().clone(),
                right: Box::new(self.logic_and()?),
            })
        }

        Ok(expr)
    }

    fn logic_and(&mut self) -> ExprResult {
        let mut expr = self.equality()?;

        while self.match_token(&TokenKind::And) {
            expr = Expr::Logical(Logical {
                left: Box::new(expr),
                operator: self.previous().clone(),
                right: Box::new(self.equality()?),
            })
        }

        Ok(expr)
    }

    fn equality(&mut self) -> ExprResult {
        self.binary_operator(
            &[TokenKind::BangEqual, TokenKind::EqualEqual],
            Self::comparison,
        )
    }

    fn comparison(&mut self) -> ExprResult {
        self.binary_operator(
            &[
                TokenKind::Greater,
                TokenKind::GreaterEqual,
                TokenKind::Less,
                TokenKind::LessEqual,
            ],
            Self::term,
        )
    }

    fn term(&mut self) -> ExprResult {
        self.binary_operator(&[TokenKind::Minus, TokenKind::Plus], Self::factor)
    }

    fn factor(&mut self) -> ExprResult {
        self.binary_operator(&[TokenKind::Slash, TokenKind::Star], Self::unary)
    }

    fn unary(&mut self) -> ExprResult {
        if self.match_token(&TokenKind::Bang) || self.match_token(&TokenKind::Minus) {
            return Ok(Expr::Unary(Unary {
                operator: self.previous().clone(),
                right: Box::new(self.unary()?),
            }));
        }

        return self.call();
    }

    fn call(&mut self) -> ExprResult {
        let mut expr = self.primary()?;

        loop {
            if self.match_token(&TokenKind::LeftParen) {
                expr = self.finish_call(expr)?;
            } else {
                break;
            }
        }

        Ok(expr)
    }

    fn finish_call(&mut self, callee: Expr) -> ExprResult {
        let mut args = vec![];

        if !self.check_token(&TokenKind::RightParen) {
            loop {
                // TODO(tazjin): Check for max args count
                args.push(self.expression()?);
                if !self.match_token(&TokenKind::Comma) {
                    break;
                }
            }
        }

        let paren = self.consume(
            &TokenKind::RightParen,
            ErrorKind::ExpectedToken("Expect ')' after arguments."),
        )?;

        Ok(Expr::Call(Call {
            args,
            callee: Box::new(callee),
            paren,
        }))
    }

    fn primary(&mut self) -> ExprResult {
        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),
                });
            }
        };

        Ok(Expr::Literal(literal))
    }

    // internal helpers

    fn identifier(&mut self, err: &'static str) -> Result<Token, Error> {
        if let TokenKind::Identifier(_) = self.peek().kind {
            Ok(self.advance())
        } else {
            Err(Error {
                line: self.peek().line,
                kind: ErrorKind::ExpectedToken(err),
            })
        }
    }

    /// Check if the next token is in `oneof`, and advance if it is.
    fn match_token(&mut self, token: &TokenKind) -> bool {
        if self.check_token(token) {
            self.advance();
            return true;
        }

        false
    }

    /// Return the next token and advance parser state.
    fn advance(&mut self) -> Token {
        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 {
        &self.tokens[self.current]
    }

    fn previous(&self) -> &Token {
        &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) -> ExprResult,
    ) -> ExprResult {
        let mut expr = each(self)?;

        while oneof.iter().any(|t| self.match_token(t)) {
            expr = Expr::Binary(Binary {
                left: Box::new(expr),
                operator: self.previous().clone(),
                right: Box::new(each(self)?),
            })
        }

        return Ok(expr);
    }
}

pub fn parse(tokens: Vec<Token>) -> Result<Block, Vec<Error>> {
    let mut parser = Parser { tokens, current: 0 };
    let mut program: Block = 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)
    }
}