lox-go/ast/gen/parser.go

package main

import (
	"errors"
	"fmt"
)

type visitor interface {
	visitASTDefinitionsNode(a *astDefinitionsNode) (string, error)
	visitName(n *nameNode) (string, error)
	visitIdentifier(i *identifierNode) (string, error)
	visitField(g *fieldNode) (string, error)
	visitDefinition(d *definitionNode) (string, error)
}

type node interface {
	accept(v visitor) (string, error)
}

type astDefinitionsNode struct {
	name        string
	definitions []node
}

// accept implements node.
func (a *astDefinitionsNode) accept(v visitor) (string, error) {
	return v.visitASTDefinitionsNode(a)
}

var _ node = new(astDefinitionsNode)

type nameNode struct {
	value string
}

// accept implements node.
func (n *nameNode) accept(v visitor) (string, error) {
	return v.visitName(n)
}

var _ node = new(nameNode)

type identifierNode struct {
	value string
}

// accept implements node.
func (i *identifierNode) accept(v visitor) (string, error) {
	return v.visitIdentifier(i)
}

var _ node = new(identifierNode)

type fieldNode struct {
	left  node
	right node
}

// accept implements node.
func (f *fieldNode) accept(v visitor) (string, error) {
	return v.visitField(f)
}

var _ node = new(fieldNode)

type definitionNode struct {
	identifier node
	fields     []node
}

// accept implements node.
func (d *definitionNode) accept(v visitor) (string, error) {
	return v.visitDefinition(d)
}

var _ node = new(definitionNode)

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

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

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

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

	return p.peek().Type == t
}

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

	return false
}

func (p *parser) advance() *token {
	t := p.tokens[p.current]
	p.current += 1
	return t
}

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

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

func (p *parser) astDefinitions() (node, error) {
	if p.match(tokenTypeName) {
		name := p.previous()
		defs := []node{}

		for !p.isAtEnd() {
			def, err := p.definition()
			if err != nil {
				return nil, err
			}
			defs = append(defs, def)
		}

		return &astDefinitionsNode{
			name:        name.Lexeme,
			definitions: defs,
		}, nil
	}

	return nil, errors.New("expected name definition at start of file")
}

// definition -> identifier "[" field+ "]"
func (p *parser) definition() (node, error) {
	id, err := p.identifier()
	if err != nil {
		return nil, err
	}

	if p.match(tokenTypeLeftBracket) {
		fields := []node{}

		for !p.check(tokenTypeRightBracket) && !p.isAtEnd() {
			f, err := p.field()

			if err != nil {
				return nil, err
			}

			fields = append(fields, f)
		}

		if p.isAtEnd() {
			return nil, errors.New(fmt.Sprintf("expected ']' after field definitions in '%s', got EOF", debug(id)))
		}

		_, err := p.consume(tokenTypeRightBracket, fmt.Sprintf("expected ']' after field definitions in '%s', got EOF", debug(id)))
		if err != nil {
			return nil, err
		}

		return &definitionNode{
			identifier: id,
			fields:     fields,
		}, nil
	}

	return nil, errors.New(fmt.Sprintf("expected '[' after identifier '%s'", debug(id)))
}

// field -> identifier "=" identifier ";"
func (p *parser) field() (node, error) {
	left, err := p.identifier()
	if err != nil {
		return nil, err
	}

	if p.match(tokenTypeEqual) {
		right, err := p.identifier()
		if err != nil {
			return nil, err
		}

		if p.match(tokenTypeSemicolon) {
			return &fieldNode{
				left:  left,
				right: right,
			}, nil
		}

		return nil, errors.New(fmt.Sprintf("expected ';' at end of field '%s'", debug(left)))
	}

	return nil, errors.New(fmt.Sprintf("expected '=' after identifier '%s'", debug(left)))
}

func (p *parser) identifier() (node, error) {
	if p.match(tokenTypeIdentifier) {
		return &identifierNode{
			value: p.previous().Lexeme,
		}, nil
	}

	return nil, errors.New("expected identifier")
}

func parse(tokens []*token) (node, error) {
	p := new(parser)
	p.tokens = tokens
	p.current = 0

	return p.astDefinitions()
}