parse let and return

ast/ast.go

@@ -0,0 +1,63 @@
+package ast
+
+import "monkey/token"
+
+type Node interface {
+	TokenLiteral() string
+}
+
+type Statement interface {
+	Node
+	statementNode()
+}
+
+type Expression interface {
+	Node
+	expressionNode()
+}
+
+type Program struct {
+	Statements []Statement
+}
+
+func (p *Program) TokenLiteral() string {
+	if len(p.Statements) > 0 {
+		return p.Statements[0].TokenLiteral()
+	} else {
+		return ""
+	}
+}
+
+type Identifier struct {
+	Token token.Token // the token.Ident token
+	Value string
+}
+
+type LetStatement struct {
+	Token token.Token // the token.Let token
+	Name  *Identifier
+	Value Expression
+}
+
+type ReturnStatement struct {
+	Token       token.Token // the 'return token
+	ReturnValue Expression
+}
+
+func (ls *LetStatement) statementNode() {
+}
+func (ls *LetStatement) TokenLiteral() string {
+	return ls.Token.Literal
+}
+
+func (i *Identifier) expressionNode() {
+}
+func (i *Identifier) TokenLiteral() string {
+	return i.Token.Literal
+}
+
+func (i *ReturnStatement) statementNode() {
+}
+func (i *ReturnStatement) TokenLiteral() string {
+	return i.Token.Literal
+}

parser/parser.go

@@ -0,0 +1,125 @@
+package parser
+
+import (
+	"fmt"
+	"monkey/ast"
+	"monkey/lexer"
+	"monkey/token"
+)
+
+type Parser struct {
+	l      *lexer.Lexer
+	errors []string
+
+	curToken  token.Token
+	peekToken token.Token
+}
+
+func (p *Parser) nextToken() {
+	p.curToken = p.peekToken
+	p.peekToken = p.l.NextToken()
+}
+
+func New(l *lexer.Lexer) *Parser {
+	p := &Parser{
+		l:      l,
+		errors: []string{},
+	}
+
+	// Read two token, so curToken and peekToken are both set
+	p.nextToken()
+	p.nextToken()
+
+	return p
+}
+
+func (p *Parser) ParseProgram() *ast.Program {
+	program := &ast.Program{}
+	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
+}
+
+func (p *Parser) parseStatement() ast.Statement {
+	switch p.curToken.Type {
+	case token.LET:
+		return p.parseLetStatement()
+	case token.RETURN:
+		return p.parseReturnStatement()
+	default:
+		return nil
+	}
+}
+
+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 expressions until we
+	// encounter a semicolon
+	for !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+
+	return stmt
+}
+
+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) 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) 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) parseReturnStatement() *ast.ReturnStatement {
+	stmt := &ast.ReturnStatement{Token: p.curToken}
+
+	p.nextToken()
+
+	// TODO: We're skipping the expression until we encounter a semicolon
+	for !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+
+	return stmt
+}

parser/parser_test.go

@@ -0,0 +1,75 @@
+package parser
+
+import (
+	"monkey/ast"
+	"monkey/lexer"
+	"testing"
+)
+
+func TestLetStatements(t *testing.T) {
+	input := `
+return 5;
+return 10;
+return 993322;
+`
+	l := lexer.New(input)
+	p := New(l)
+
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	if len(program.Statements) != 3 {
+		t.Fatalf("program.Statements does not contain 3 statements. got=%d",
+			len(program.Statements))
+	}
+
+	for _, stmt := range program.Statements {
+		returnStmt, ok := stmt.(*ast.ReturnStatement)
+		if !ok {
+			t.Errorf("stmt not *ast.ReturnStatement. got=%T", stmt)
+			continue
+		}
+		if returnStmt.TokenLiteral() != "return" {
+			t.Errorf("returnStmt.TokenLiteral not 'return', got %q",
+				returnStmt.TokenLiteral())
+		}
+	}
+}
+
+func checkParserErrors(t *testing.T, p *Parser) {
+	errors := p.Errors()
+	if len(errors) == 0 {
+		return
+	}
+
+	t.Errorf("parser has %d errors", len(errors))
+	for _, msg := range errors {
+		t.Errorf("parser error %q", msg)
+	}
+	t.FailNow()
+}
+
+func testLetStatements(t *testing.T, s ast.Statement, name string) bool {
+	if s.TokenLiteral() != "let" {
+		t.Errorf("s.TokenLiteral not 'let'. got=%q", s.TokenLiteral())
+		return false
+	}
+
+	letStmt, ok := s.(*ast.LetStatement)
+	if !ok {
+		t.Errorf("s not *ast.LetStatement. got=%T", s)
+		return false
+	}
+
+	if letStmt.Name.Value != name {
+		t.Errorf("letStmt.Name.Value not '%s'. got=%s", name, letStmt.Name.Value)
+		return false
+	}
+
+	if letStmt.Name.TokenLiteral() != name {
+		t.Errorf("letStmt.Name.TokenLiteral not '%s'. got=%s", name, letStmt.Name.TokenLiteral())
+		return false
+	}
+
+	return true
+}