Implement parser of 'let' statement

.gitignore

@@ -1 +1,2 @@
 .idea/
+monkey_interpreter.exe

ast/ast.go

@@ -0,0 +1,46 @@
+package ast
+
+import "monkey_interpreter/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 LetStatement struct {
+	Token token.Token // the token.LET token
+	Name  *Identifier
+	Value Expression
+}
+
+func (ls *LetStatement) statementNode()       {}
+func (ls *LetStatement) TokenLiteral() string { return ls.Token.Literal }
+
+type Identifier struct {
+	Token token.Token // the token.IDENT token
+	Value string
+}
+
+func (i *Identifier) statementNode()       {}
+func (i *Identifier) TokenLiteral() string { return i.Token.Literal }

parser/parser.go

@@ -0,0 +1,93 @@
+package parser
+
+import (
+	"monkey_interpreter/ast"
+	"monkey_interpreter/lexer"
+	"monkey_interpreter/token"
+)
+
+type Parser struct {
+	l *lexer.Lexer
+
+	curToken  token.Token
+	peekToken token.Token
+}
+
+func New(l *lexer.Lexer) *Parser {
+	p := &Parser{l: l}
+
+	// Read two tokens, curToken and peekToken are both set
+	p.nextToken()
+	p.nextToken()
+
+	return p
+}
+
+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()
+	default:
+		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
+	}
+
+	for !p.curTokenIs(token.SEMICOLON) {
+		p.nextToken()
+	}
+
+	return stmt
+}
+
+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 {
+		return false
+	}
+}

parser/parser_test.go

@@ -0,0 +1,62 @@
+package parser
+
+import (
+	"monkey_interpreter/ast"
+	"monkey_interpreter/lexer"
+	"testing"
+)
+
+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()
+	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
+}