lox-go/parser/parser.go

package parser

import (
	"git.red-panda.pet/pandaware/lox-go/ast"
	"git.red-panda.pet/pandaware/lox-go/lexer"
)

type ParseError interface {
	Error() string
	Token() *lexer.Token
}

type SyntaxError struct {
	token   *lexer.Token
	message string
}

func newSyntaxError(token *lexer.Token, message string) *SyntaxError {
	return &SyntaxError{token, message}
}

func (s *SyntaxError) Token() *lexer.Token {
	return s.token
}

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

type operandFunc func() (ast.Expr, error)

type Parser struct {
	tokens  []*lexer.Token
	current int
}

func New(tokens []*lexer.Token) *Parser {
	return &Parser{
		tokens:  tokens,
		current: 0,
	}
}

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

	return e, err
}

// expression -> equality
func (p *Parser) expression() (ast.Expr, error) {
	return p.equality()
}

func (p *Parser) parseLeftAssocBinOps(operand operandFunc, tokenTypes ...lexer.TokenType) (ast.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 = &ast.BinaryExpr{Left: e, Op: op, Right: r}
	}

	return e, nil
}

// eqality -> comparison ( ("!=" | "==") comparison )*
func (p *Parser) equality() (ast.Expr, error) {
	return p.parseLeftAssocBinOps(
		p.comparison,
		lexer.TokenTypeBangEq, lexer.TokenTypeEqualEqual,
	)
}

// comparison -> term ( ( ">" | ">=" | "<" | "<=" ) term )*
func (p *Parser) comparison() (ast.Expr, error) {
	return p.parseLeftAssocBinOps(
		p.term,
		lexer.TokenTypeGreater, lexer.TokenTypeGreaterEq,
		lexer.TokenTypeLess, lexer.TokenTypeLessEq,
	)
}

// term -> factor ( ( "-" | "+" ) factor )*
func (p *Parser) term() (ast.Expr, error) {
	return p.parseLeftAssocBinOps(
		p.factor,
		lexer.TokenTypeMinus, lexer.TokenTypePlus,
	)
}

// factor -> unary ( ( "*" | "/" ) unary )*
func (p *Parser) factor() (ast.Expr, error) {
	return p.parseLeftAssocBinOps(
		p.unary,
		lexer.TokenTypeSlash, lexer.TokenTypeStar,
	)
}

// unary -> ( "!" | "-" ) unary | primary;
func (p *Parser) unary() (ast.Expr, error) {
	if p.match(lexer.TokenTypeBang, lexer.TokenTypeMinus) {
		op := p.previous()
		r, err := p.unary()
		return &ast.UnaryExpr{Op: op, Right: r}, err
	}

	return p.primary()
}

// primary -> STRING | NUMBER | "true" | "false" | "nil" | "(" expression ")"
func (p *Parser) primary() (ast.Expr, error) {
	if p.match(lexer.TokenTypeTrue) {
		return &ast.LiteralExpr{Value: true}, nil
	}
	if p.match(lexer.TokenTypeFalse) {
		return &ast.LiteralExpr{Value: false}, nil
	}
	if p.match(lexer.TokenTypeNil) {
		return &ast.LiteralExpr{Value: nil}, nil
	}

	if p.match(lexer.TokenTypeString, lexer.TokenTypeNumber) {
		return &ast.LiteralExpr{Value: p.previous().Literal}, nil
	}

	if p.match(lexer.TokenTypeLeftParen) {
		e, err := p.expression()
		if err != nil {
			return nil, err
		}
		_, err = p.consume(lexer.TokenTypeRightParen, "expected ')' after expression")
		return &ast.GroupingExpr{Expr: e}, err
	}

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

func (p *Parser) consume(tokenType lexer.TokenType, msg string) (*lexer.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 == lexer.TokenTypeSemicolon {
			return
		}

		if lexer.IsKeyword(p.peek()) {
			return
		}
		p.advance()
	}
}

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

	return false
}

func (p *Parser) check(t lexer.TokenType) bool {
	if p.isAtEnd() {
		return false
	}
	return p.peek().Type == t
}

func (p *Parser) advance() *lexer.Token {
	if !p.isAtEnd() {
		p.current += 1
	}
	return p.previous()
}

func (p *Parser) isAtEnd() bool {
	return p.peek().Type == lexer.TokenTypeEOF
}

func (p *Parser) peek() *lexer.Token {
	return p.tokens[p.current]
}

func (p *Parser) previous() *lexer.Token {
	return p.tokens[p.current-1]
}