lox-go/parser.go

package main

type parseError interface {
	Error() string
	Token() *token
}

type syntaxError struct {
	token   *token
	message string
}

func newSyntaxError(token *token, message string) *syntaxError {
	return &syntaxError{token, message}
}

func (s *syntaxError) Token() *token {
	return s.token
}

func (s *syntaxError) Error() string {
	return s.message
}

type operandFunc func() (expr, error)

type parser struct {
	tokens  []*token
	current int
}

func newParser(tokens []*token) *parser {
	return &parser{
		tokens:  tokens,
		current: 0,
	}
}

func (p *parser) Parse() (expr, error) {
	e, err := p.expression()
	if err != nil {
		return e, err
	}

	return e, err
}

// expression -> equality
func (p *parser) expression() (expr, error) {
	return p.equality()
}

func (p *parser) parseLeftAssocBinOps(operand operandFunc, tokenTypes ...tokenType) (expr, error) {
	e, err := operand()
	if err != nil {
		return e, err
	}

	for p.match(tokenTypes...) {
		op := p.previous()
		r, err := operand()
		if err != nil {
			return e, err
		}
		e = &binaryExpr{e, op, r}
	}

	return e, nil
}

// eqality -> comparison ( ("!=" | "==") comparison )*
func (p *parser) equality() (expr, error) {
	return p.parseLeftAssocBinOps(
		p.comparison,
		tokenTypeBangEq, tokenTypeEqualEqual,
	)
}

// comparison -> term ( ( ">" | ">=" | "<" | "<=" ) term )*
func (p *parser) comparison() (expr, error) {
	return p.parseLeftAssocBinOps(
		p.term,
		tokenTypeGreater, tokenTypeGreaterEq,
		tokenTypeLess, tokenTypeLessEq,
	)
}

// term -> factor ( ( "-" | "+" ) factor )*
func (p *parser) term() (expr, error) {
	return p.parseLeftAssocBinOps(
		p.factor,
		tokenTypeMinus, tokenTypePlus,
	)
}

// factor -> unary ( ( "*" | "/" ) unary )*
func (p *parser) factor() (expr, error) {
	return p.parseLeftAssocBinOps(
		p.unary,
		tokenTypeSlash, tokenTypeStar,
	)
}

// unary -> ( "!" | "-" ) unary | primary;
func (p *parser) unary() (expr, error) {
	if p.match(tokenTypeBang, tokenTypeMinus) {
		op := p.previous()
		r, err := p.unary()
		return &unaryExpr{op, r}, err
	}

	return p.primary()
}

// primary -> STRING | NUMBER | "true" | "false" | "nil" | "(" expression ")"
func (p *parser) primary() (expr, error) {
	if p.match(tokenTypeTrue) {
		return &literalExpr{true}, nil
	}
	if p.match(tokenTypeFalse) {
		return &literalExpr{false}, nil
	}
	if p.match(tokenTypeNil) {
		return &literalExpr{nil}, nil
	}

	if p.match(tokenTypeString, tokenTypeNumber) {
		return &literalExpr{p.previous().Literal}, nil
	}

	if p.match(tokenTypeLeftParen) {
		e, err := p.expression()
		if err != nil {
			return nil, err
		}
		_, err = p.consume(tokenTypeRightParen, "expected ')' after expression")
		return &groupingExpr{e}, err
	}

	return nil, newSyntaxError(p.peek(), "expected expression")
}

func (p *parser) consume(tokenType tokenType, msg string) (*token, error) {
	if p.check(tokenType) {
		return p.advance(), nil
	}

	return nil, newSyntaxError(p.peek(), msg)
}

func (p *parser) synchronize() {
	p.advance()

	for !p.isAtEnd() {
		if p.previous().Type == tokenTypeSemicolon {
			return
		}

		if isKeyword(p.peek()) {
			return
		}
		p.advance()
	}
}

func (p *parser) match(tokenTypes ...tokenType) bool {
	for _, t := range tokenTypes {
		if p.check(t) {
			p.advance()
			return true
		}
	}

	return false
}

func (p *parser) check(t tokenType) bool {
	if p.isAtEnd() {
		return false
	}
	return p.peek().Type == t
}

func (p *parser) advance() *token {
	if !p.isAtEnd() {
		p.current += 1
	}
	return p.previous()
}

func (p *parser) isAtEnd() bool {
	return p.peek().Type == tokenTypeEOF
}

func (p *parser) peek() *token {
	return p.tokens[p.current]
}

func (p *parser) previous() *token {
	return p.tokens[p.current-1]
}