Mako/parser/parser.go

package parser

import (
	"fmt"
	"strconv"
)

// Parser implements a recursive descent Pratt parser
type Parser struct {
	lexer *Lexer

	curToken  Token
	peekToken Token

	prefixParseFns map[TokenType]func() Expression
	infixParseFns  map[TokenType]func(Expression) Expression

	errors []string
}

// NewParser creates a new parser instance
func NewParser(lexer *Lexer) *Parser {
	p := &Parser{
		lexer:  lexer,
		errors: []string{},
	}

	p.prefixParseFns = make(map[TokenType]func() Expression)
	p.registerPrefix(IDENT, p.parseIdentifier)
	p.registerPrefix(NUMBER, p.parseNumberLiteral)
	p.registerPrefix(STRING, p.parseStringLiteral)
	p.registerPrefix(LPAREN, p.parseGroupedExpression)

	p.infixParseFns = make(map[TokenType]func(Expression) Expression)
	p.registerInfix(PLUS, p.parseInfixExpression)
	p.registerInfix(MINUS, p.parseInfixExpression)
	p.registerInfix(SLASH, p.parseInfixExpression)
	p.registerInfix(STAR, p.parseInfixExpression)

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

	return p
}

// registerPrefix registers a prefix parse function
func (p *Parser) registerPrefix(tokenType TokenType, fn func() Expression) {
	p.prefixParseFns[tokenType] = fn
}

// registerInfix registers an infix parse function
func (p *Parser) registerInfix(tokenType TokenType, fn func(Expression) Expression) {
	p.infixParseFns[tokenType] = fn
}

// nextToken advances to the next token
func (p *Parser) nextToken() {
	p.curToken = p.peekToken
	p.peekToken = p.lexer.NextToken()
}

// ParseProgram parses the entire program
func (p *Parser) ParseProgram() *Program {
	program := &Program{}
	program.Statements = []Statement{}

	for !p.curTokenIs(EOF) {
		stmt := p.parseStatement()
		if stmt != nil {
			program.Statements = append(program.Statements, stmt)
		}
		p.nextToken()
	}

	return program
}

// parseStatement parses a statement
func (p *Parser) parseStatement() Statement {
	if p.curTokenIs(IDENT) && p.peekTokenIs(ASSIGN) {
		return p.parseAssignStatement()
	}

	// Skip unknown statements for now
	return nil
}

// parseAssignStatement parses variable assignment
func (p *Parser) parseAssignStatement() *AssignStatement {
	stmt := &AssignStatement{}

	if !p.curTokenIs(IDENT) {
		return nil
	}

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

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

	p.nextToken()

	stmt.Value = p.parseExpression(LOWEST)

	return stmt
}

// parseExpression parses expressions using Pratt parsing
func (p *Parser) parseExpression(precedence Precedence) Expression {
	prefix := p.prefixParseFns[p.curToken.Type]
	if prefix == nil {
		p.noPrefixParseFnError(p.curToken.Type)
		return nil
	}

	leftExp := prefix()

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

		p.nextToken()
		leftExp = infix(leftExp)
	}

	return leftExp
}

// Expression parsing functions
func (p *Parser) parseIdentifier() Expression {
	return &Identifier{Value: p.curToken.Literal}
}

func (p *Parser) parseNumberLiteral() Expression {
	lit := &NumberLiteral{}

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

	lit.Value = value
	return lit
}

func (p *Parser) parseStringLiteral() Expression {
	return &StringLiteral{Value: p.curToken.Literal}
}

func (p *Parser) parseGroupedExpression() Expression {
	p.nextToken()

	exp := p.parseExpression(LOWEST)

	if !p.expectPeek(RPAREN) {
		return nil
	}

	return exp
}

func (p *Parser) parseInfixExpression(left Expression) Expression {
	expression := &InfixExpression{
		Left:     left,
		Operator: p.curToken.Literal,
	}

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

	return expression
}

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

func (p *Parser) peekTokenIs(t TokenType) bool {
	return p.peekToken.Type == t
}

func (p *Parser) expectPeek(t TokenType) bool {
	if p.peekTokenIs(t) {
		p.nextToken()
		return true
	} else {
		p.peekError(t)
		return false
	}
}

func (p *Parser) peekError(t TokenType) {
	msg := fmt.Sprintf("expected next token to be %v, got %v instead",
		t, p.peekToken.Type)
	p.errors = append(p.errors, msg)
}

func (p *Parser) noPrefixParseFnError(t TokenType) {
	msg := fmt.Sprintf("no prefix parse function for %v found", t)
	p.errors = append(p.errors, msg)
}

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

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

// Errors returns all parsing errors
func (p *Parser) Errors() []string {
	return p.errors
}