Parsing statements (skipping expressions).

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/parser/parser.go

@@ -0,0 +1,116 @@
+package parser
+
+import (
+	"fmt"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/ast"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/lexer"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/token"
+)
+
+type Parser struct {
+	l         *lexer.Lexer
+	errors    []string
+	curToken  token.Token
+	peekToken token.Token
+}
+
+func New(l *lexer.Lexer) *Parser {
+	p := &Parser{
+		l:      l,
+		errors: []string{},
+	}
+	// TODO: figure out why this can't be done from `parseProgram`
+	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()
+	}
+	return nil
+}
+
+func (p *Parser) parseLetStatement() ast.Statement {
+	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: Skipping until we find the semicolon to avoid parsing the expression.
+	if !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+	return stmt
+}
+
+func (p *Parser) parseReturnStatement() ast.Statement {
+	stmt := &ast.ReturnStatement{Token: p.curToken}
+	p.nextToken()
+	// TODO: Skipping until we find the semicolon to avoid parsing the expression.
+	if !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+	return stmt
+}
+
+func (p *Parser) curTokenIs(typ token.TokenType) bool {
+	return p.curToken.Type == typ
+}
+
+func (p *Parser) peekTokenIs(typ token.TokenType) bool {
+	return p.peekToken.Type == typ
+}
+
+// NOTE: I'll leave the name as-is to avoid deviating from the book (maybe a
+// rename at the end?), but I think `nextTokenIfPeek` would be a much better
+// name for this.
+func (p *Parser) expectPeek(typ token.TokenType) bool {
+	if p.peekTokenIs(typ) {
+		p.nextToken()
+		return true
+	}
+	p.peekError(typ)
+	return false
+}
+
+func (p *Parser) nextToken() {
+	p.curToken = p.peekToken
+	p.peekToken = p.l.NextToken()
+}
+
+func (p *Parser) Errors() []string {
+	return p.errors
+}
+
+func (p *Parser) peekError(typ token.TokenType) {
+	p.errors = append(
+		p.errors,
+		fmt.Sprintf(
+			"expected next token to be %q, got %q instead",
+			typ,
+			p.peekToken.Type,
+		),
+	)
+}

pkg/parser/parser_test.go

@@ -0,0 +1,112 @@
+package parser
+
+import (
+	"testing"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/ast"
+	"code.jmug.me/jmug/interpreter-in-go/pkg/lexer"
+)
+
+func TestLetStatements(t *testing.T) {
+	input := `
+let x = 5;
+let y = 10;
+let foobar = 838383;
+	`
+	l := lexer.New(input)
+	p := New(l)
+
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+	if program == nil {
+		t.Fatalf("ParseProgram() returned nil")
+	}
+	if len(program.Statements) != 3 {
+		t.Fatalf("program.Statements does not contain 3 statements. got=%d",
+			len(program.Statements))
+	}
+
+	tests := []struct {
+		expectedIdentifier string
+	}{
+		{"x"},
+		{"y"},
+		{"foobar"},
+	}
+
+	for i, tt := range tests {
+		stmt := program.Statements[i]
+		if !testLetStatement(t, stmt, tt.expectedIdentifier) {
+			return
+		}
+	}
+}
+
+func testLetStatement(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
+}
+
+func TestReturnStatements(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()
+}