waiig15/parser/parser.go

package parser

import (
	"code.osinet.fr/fgm/waiig15/ast"
	"code.osinet.fr/fgm/waiig15/lexer"
	"code.osinet.fr/fgm/waiig15/token"
	"fmt"
)

type Parser struct {
	errors []string
	l      *lexer.Lexer

	curToken  token.Token
	peekToken token.Token
}

func New(l *lexer.Lexer) *Parser {
	p := &Parser{
		l:      l,
		errors: []string{},
	}

	// Read two tokens, so curToken and peeToken are both set.
	p.nextToken()
	p.nextToken()

	return p
}

func (p *Parser) Errors() []string {
	return p.errors
}

func (p *Parser) ParseProgram() *ast.Program {
	program := &ast.Program{
		Statements: []ast.Statement{},
	}
	for !p.curTokenIs(token.EOF) {
		stmt := p.parseStatement()
		if stmt != nil {
			program.Statements = append(program.Statements, stmt)
		}
		p.nextToken()
	}

	return program
}

// Is the current token in the parser of the given type ?
func (p *Parser) curTokenIs(t token.TokenType) bool {
	return p.curToken.Type == t
}

// Is the next token in the parser of the given type ? If it is, consume it,
// else don't.
func (p *Parser) expectPeek(t token.TokenType) bool {
	if p.peekTokenIs(t) {
		p.nextToken()
		return true
	} else {
		p.peekError(t)
		return false
	}
}

func (p *Parser) nextToken() {
	p.curToken = p.peekToken
	p.peekToken = p.l.NextToken()
}

func (p *Parser) parseLetStatement() *ast.LetStatement {
	stmt := &ast.LetStatement{
		Token: p.curToken,
	}

	// Let statement starts with an IDENT token, so if next token is not an
	// IDENT, the next statement cannot be a Let statement.
	if !p.expectPeek(token.IDENT) {
		return nil
	}

	stmt.Name = &ast.Identifier{
		Token: p.curToken,
		Value: p.curToken.Literal,
	}

	// The previous expectPeek() call fetched the next token, so we should now
	// be on the assignment.
	if !p.expectPeek(token.ASSIGN) {
		return nil
	}

	// Skip the expression for now, progress to the semicolon terminating the
	// statement.
	for !p.curTokenIs(token.SEMICOLON) {
		p.nextToken()
	}

	return stmt
}

func (p *Parser) parseStatement() ast.Statement {
	switch p.curToken.Type {
	case token.LET:
		return p.parseLetStatement()
	default:
		return nil
	}
}

// Log a mismatch error on the peek token type in the parser instance.
//
//   - t is the type of token that was expected
func (p *Parser) peekError(t token.TokenType) {
	msg := fmt.Sprintf("expected next token to be %s, got %s instead",
		t, p.peekToken.Type)
	p.errors = append(p.errors, msg)
}

// Is the next token in the parser of the given type ? Don't consume it.
func (p *Parser) peekTokenIs(t token.TokenType) bool {
	return p.peekToken.Type == t
}