monkey/parser/parser.go

package parser

import (
	"fmt"
	"monkey/ast"
	"monkey/lexer"
	"monkey/token"
	"strconv"
)

const (
	_ int = iota
	LOWEST
	EQUALS      // ==
	LESSGREATER // > or <
	SUM         // +
	PRODUCT     // *
	PREFIX      // -X or !X
	CALL        // myFunction(X)
)

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

	curToken  token.Token
	peekToken token.Token

	prefixParseFns map[token.TokenType]prefixParseFn
	infixParseFns  map[token.TokenType]infixParseFn
}

type (
	prefixParseFn func() ast.Expression
	infixParseFn  func(ast.Expression) ast.Expression
)

func (p *Parser) registerPrefix(tokenType token.TokenType, fn prefixParseFn) {
	p.prefixParseFns[tokenType] = fn
}

func (p *Parser) registerInfix(tokenType token.TokenType, fn infixParseFn) {
	p.infixParseFns[tokenType] = fn
}

func New(l *lexer.Lexer) *Parser {
	p := &Parser{
		l:      l,
		errors: []string{},
	}
	p.infixParseFns = make(map[token.TokenType]infixParseFn)
    p.registerInfix(token.PLUS, p.parseInfixExpression)
    p.registerInfix(token.MINUS, p.parseInfixExpression)
    p.registerInfix(token.SLASH, p.parseInfixExpression)
    p.registerInfix(token.ASTERISK, p.parseInfixExpression)
    p.registerInfix(token.EQ, p.parseInfixExpression)
    p.registerInfix(token.NOT_EQ, p.parseInfixExpression)
    p.registerInfix(token.LT, p.parseInfixExpression)
    p.registerInfix(token.GT, p.parseInfixExpression)
	p.prefixParseFns = make(map[token.TokenType]prefixParseFn)
	p.registerPrefix(token.IDENT, p.parseIdentifier)
    p.registerPrefix(token.INT, p.parseIntegerLiteral)
	p.registerPrefix(token.BANG, p.parsePrefixExpression)
	p.registerPrefix(token.MINUS, p.parsePrefixExpression)

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

	return p
}

func (p *Parser) parsePrefixExpression() ast.Expression {
    expression := &ast.PrefixExpression{
        Token:    p.curToken,
        Operator: p.curToken.Literal,
    }

    p.nextToken()

    expression.Right = p.parseExpression(PREFIX)

    return expression
}

var precedences = map[token.TokenType]int{
    token.EQ:       EQUALS,
    token.NOT_EQ:   EQUALS,
    token.LT:       LESSGREATER,
    token.GT:       LESSGREATER,
    token.PLUS:     SUM,
    token.MINUS:    SUM,
    token.SLASH:    PRODUCT,
    token.ASTERISK: PRODUCT,
}

func (p *Parser) peekPrecedence() int {
    if p, ok := precedences[p.peekToken.Type]; ok {
        return p
    }

    return LOWEST
}

func (p *Parser) curPrecedence() int {
    if p, ok := precedences[p.curToken.Type]; ok {
        return p
    }

    return LOWEST
}

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

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

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

func (p *Parser) parseReturnStatement() *ast.ReturnStatement {
	stmt := &ast.ReturnStatement{Token: p.curToken}

	p.nextToken()

	// TODO: We're skipping the expressions until we
	// encounter a semicolon
	for !p.curTokenIs(token.SEMICOLON) {
		p.nextToken()
	}

	return stmt
}

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

	if !p.expectPeek(token.IDENT) {
		return nil
	}

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

	if !p.expectPeek(token.ASSIGN) {
		return nil
	}

	// TODO: We're skipping the expressions until we
	// encounter a semicolon
	for !p.curTokenIs(token.SEMICOLON) {
		p.nextToken()
	}

	return stmt
}

func (p *Parser) parseExpressionStatement() *ast.ExpressionStatement {
	stmt := &ast.ExpressionStatement{Token: p.curToken}

	stmt.Expression = p.parseExpression(LOWEST)

	if p.peekTokenIs(token.SEMICOLON) {
		p.nextToken()
	}

	return stmt
}

func (p *Parser) parseExpression(precedence int) ast.Expression {
    prefix := p.prefixParseFns[p.curToken.Type]
    if prefix == nil {
        p.noPrefixParseFnError(p.curToken.Type)
        return nil
    }
    leftExp := prefix()

    for !p.peekTokenIs(token.SEMICOLON) && precedence < p.peekPrecedence() {
        infix := p.infixParseFns[p.peekToken.Type]
        if infix == nil {
            return leftExp
        }

        p.nextToken()

        leftExp = infix(leftExp)
    }

    return leftExp
}

func (p *Parser) parseIntegerLiteral() ast.Expression {
	lit := &ast.IntegerLiteral{Token: p.curToken}

	value, err := strconv.ParseInt(p.curToken.Literal, 0, 64)
	if err != nil {
		msg := fmt.Sprintf("could not parse %q as integer", p.curToken.Literal)
		p.errors = append(p.errors, msg)
		return nil
	}

	lit.Value = value

	return lit
}

func (p *Parser) curTokenIs(t token.TokenType) bool {
	return p.curToken.Type == t
}

func (p *Parser) peekTokenIs(t token.TokenType) bool {
	return p.peekToken.Type == 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) Errors() []string {
	return p.errors
}

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)
}

func (p *Parser) parseIdentifier() ast.Expression {
	return &ast.Identifier{Token: p.curToken, Value: p.curToken.Literal}
}


func (p *Parser) noPrefixParseFnError(t token.TokenType) {
    msg := fmt.Sprintf("no prefix parse function for %s found", t)
    p.errors = append(p.errors, msg)
}
func (p *Parser) parseInfixExpression(left ast.Expression) ast.Expression {
    expression := &ast.InfixExpression{
        Token:    p.curToken,
        Operator: p.curToken.Literal,
        Left:     left,
    }

    precedence := p.curPrecedence()
    p.nextToken()
    expression.Right = p.parseExpression(precedence)

    return expression
}