Support Array literal

ast/ast.go

@@ -267,3 +267,45 @@ func (fl *FunctionLiteral) String() string {
 
 	return out.String()
 }
+
+type ArrayLiteral struct {
+	Token    token.Token // the '[' token
+	Elements []Expression
+}
+
+func (al *ArrayLiteral) expressionNode()      {}
+func (al *ArrayLiteral) TokenLiteral() string { return al.Token.Literal }
+func (al *ArrayLiteral) String() string {
+	var out bytes.Buffer
+
+	var elements []string
+	for _, el := range al.Elements {
+		elements = append(elements, el.String())
+	}
+
+	out.WriteString("[")
+	out.WriteString(strings.Join(elements, ", "))
+	out.WriteString("]")
+
+	return out.String()
+}
+
+type IndexExpression struct {
+	Token token.Token // The [ token
+	Left  Expression
+	Index Expression
+}
+
+func (ie *IndexExpression) expressionNode()      {}
+func (ie *IndexExpression) TokenLiteral() string { return ie.Token.Literal }
+func (ie *IndexExpression) String() string {
+	var out bytes.Buffer
+
+	out.WriteString("(")
+	out.WriteString(ie.Left.String())
+	out.WriteString("[")
+	out.WriteString(ie.Index.String())
+	out.WriteString("])")
+
+	return out.String()
+}

evaluator/evaluator.go

@@ -71,6 +71,17 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		}
 
 		return applyFunction(function, args)
+	case *ast.IndexExpression:
+		left := Eval(node.Left, env)
+		if isError(left) {
+			return left
+		}
+
+		index := Eval(node.Index, env)
+		if isError(index) {
+			return index
+		}
+		return evalIndexExpression(left, index)
 	case *ast.Identifier:
 		return evalIdentifier(node, env)
 	case *ast.FunctionLiteral:
@@ -83,6 +94,12 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		return &object.String{Value: node.Value}
 	case *ast.Boolean:
 		return nativeBoolToBooleanObject(node.Value)
+	case *ast.ArrayLiteral:
+		elements := evalExpressions(node.Elements, env)
+		if len(elements) == 1 && isError(elements[0]) {
+			return elements[0]
+		}
+		return &object.Array{Elements: elements}
 	}
 
 	return nil
@@ -238,6 +255,27 @@ func evalStringInfixExpression(operator string, left, right object.Object) objec
 	return &object.String{Value: leftVal + rightVal}
 }
 
+func evalIndexExpression(left, index object.Object) object.Object {
+	switch {
+	case left.Type() == object.ArrayObj && index.Type() == object.IntegerObj:
+		return evalArrayIndexExpression(left, index)
+	default:
+		return newError("index operator not supported: %s", left.Type())
+	}
+}
+
+func evalArrayIndexExpression(array, index object.Object) object.Object {
+	arrayObject := array.(*object.Array)
+	idx := index.(*object.Integer).Value
+	max := int64(len(arrayObject.Elements) - 1)
+
+	if idx < 0 || idx > max {
+		return Null
+	}
+
+	return arrayObject.Elements[idx]
+}
+
 func evalIdentifier(node *ast.Identifier, env *object.Environment) object.Object {
 	if val, ok := env.Get(node.Value); ok {
 		return val

evaluator/evaluator_test.go

@@ -293,6 +293,82 @@ func TestBuiltinFunctions(t *testing.T) {
 	}
 }
 
+func TestArrayLiterals(t *testing.T) {
+	input := "[1, 2 * 2, 3 + 3]"
+
+	evaluated := testEval(input)
+	result, ok := evaluated.(*object.Array)
+	if !ok {
+		t.Fatalf("object is not %T. got=%T (%+v)", object.Array{}, evaluated, evaluated)
+	}
+
+	if len(result.Elements) != 3 {
+		t.Fatalf("array has wrong num of elements. got=%d", len(result.Elements))
+	}
+
+	testIntegerObject(t, result.Elements[0], 1)
+	testIntegerObject(t, result.Elements[1], 4)
+	testIntegerObject(t, result.Elements[2], 6)
+}
+
+func TestArrayIndexExpressions(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{
+			"[1, 2, 3][0]",
+			1,
+		},
+		{
+			"[1, 2, 3][1]",
+			2,
+		},
+		{
+			"[1, 2, 3][2]",
+			3,
+		},
+		{
+			"let i = 0; [1][i];",
+			1,
+		},
+		{
+			"[1, 2, 3][1 + 1];",
+			3,
+		},
+		{
+			"let myArray = [1, 2, 3]; myArray[2];",
+			3,
+		},
+		{
+			"let myArray = [1, 2, 3]; myArray[0] + myArray[1] + myArray[2];",
+			6,
+		},
+		{
+			"let myArray = [1, 2, 3]; let i = myArray[0]; myArray[i]",
+			2,
+		},
+		{
+			"[1, 2, 3][3]",
+			nil,
+		},
+		{
+			"[1, 2, 3][-1]",
+			nil,
+		},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		integer, ok := tt.expected.(int)
+		if ok {
+			testIntegerObject(t, evaluated, int64(integer))
+		} else {
+			testNullObject(t, evaluated)
+		}
+	}
+}
+
 func testEval(input string) object.Object {
 	l := lexer.New(input)
 	p := parser.New(l)

lexer/lexer.go

@@ -65,6 +65,10 @@ func (l *Lexer) NextToken() token.Token {
 		tok = newToken(token.LeftBrace, l.ch)
 	case '}':
 		tok = newToken(token.RightBrace, l.ch)
+	case '[':
+		tok = newToken(token.LeftBracket, l.ch)
+	case ']':
+		tok = newToken(token.RightBracket, l.ch)
 	case '"':
 		tok.Type = token.String
 		tok.Literal = l.readString()

lexer/lexer_test.go

@@ -28,6 +28,7 @@ func TestNextToken(t *testing.T) {
 		10 != 9;
 		"foobar"
 		"foo bar"
+		[1, 2];
 	`
 
 	tests := []struct {
@@ -109,6 +110,12 @@ func TestNextToken(t *testing.T) {
 		{token.Semicolon, ";"},
 		{token.String, "foobar"},
 		{token.String, "foo bar"},
+		{token.LeftBracket, "["},
+		{token.Int, "1"},
+		{token.Comma, ","},
+		{token.Int, "2"},
+		{token.RightBracket, "]"},
+		{token.Semicolon, ";"},
 		{token.EOF, ""},
 	}
 

object/object.go

@@ -19,6 +19,7 @@ const (
 	FunctionObj    = "Function"
 	StringObj      = "String"
 	BuiltinObj     = "Builtin"
+	ArrayObj       = "Array"
 )
 
 type Object interface {
@@ -99,3 +100,23 @@ type Builtin struct {
 
 func (b *Builtin) Type() ObjectType { return BuiltinObj }
 func (b *Builtin) Inspect() string  { return "builtin function" }
+
+type Array struct {
+	Elements []Object
+}
+
+func (ao *Array) Type() ObjectType { return ArrayObj }
+func (ao *Array) Inspect() string {
+	var out bytes.Buffer
+
+	var elements []string
+	for _, e := range ao.Elements {
+		elements = append(elements, e.Inspect())
+	}
+
+	out.WriteString("[")
+	out.WriteString(strings.Join(elements, ", "))
+	out.WriteString("]")
+
+	return out.String()
+}

parser/parser.go

@@ -18,6 +18,7 @@ const (
 	Product       // *
 	Prefix        // -X or !X
 	Call          // myFunction(X)
+	Index         // array[index]
 )
 
 var precedences = map[token.TokenType]int{
@@ -30,6 +31,7 @@ var precedences = map[token.TokenType]int{
 	token.Slash:       Product,
 	token.Asterisk:    Product,
 	token.LeftParen:   Call,
+	token.LeftBracket: Index,
 }
 
 type (
@@ -62,6 +64,7 @@ func New(l *lexer.Lexer) *Parser {
 	p.registerPrefix(token.If, p.parseIfExpression)
 	p.registerPrefix(token.Function, p.parseFunctionLiteral)
 	p.registerPrefix(token.String, p.parseStringLiteral)
+	p.registerPrefix(token.LeftBracket, p.parseArrayLiteral)
 
 	p.infixParseFns = make(map[token.TokenType]infixParseFn)
 	p.registerInfix(token.Plus, p.parseInfixExpression)
@@ -73,6 +76,7 @@ func New(l *lexer.Lexer) *Parser {
 	p.registerInfix(token.LessThan, p.parseInfixExpression)
 	p.registerInfix(token.GreaterThan, p.parseInfixExpression)
 	p.registerInfix(token.LeftParen, p.parseCallExpression)
+	p.registerInfix(token.LeftBracket, p.parseIndexExpression)
 
 	// Read two tokens, so currentToken and peekToken are both set
 	p.nextToken()
@@ -337,49 +341,70 @@ func (p *Parser) parseStringLiteral() ast.Expression {
 	return &ast.StringLiteral{Token: p.currentToken, Value: p.currentToken.Literal}
 }
 
-// Infix expressions
-
-func (p *Parser) parseInfixExpression(left ast.Expression) ast.Expression {
-	expression := &ast.InfixExpression{
-		Token:    p.currentToken,
-		Operator: p.currentToken.Literal,
-		Left:     left,
-	}
+func (p *Parser) parseArrayLiteral() ast.Expression {
+	array := &ast.ArrayLiteral{Token: p.currentToken}
 
-	precedence := p.currentPrecedence()
-	p.nextToken()
-	expression.Right = p.parseExpression(precedence)
-	return expression
-}
+	array.Elements = p.parseExpressionList(token.RightBracket)
 
-func (p *Parser) parseCallExpression(function ast.Expression) ast.Expression {
-	exp := &ast.CallExpression{Token: p.currentToken, Function: function}
-	exp.Arguments = p.parseCallArguments()
-	return exp
+	return array
 }
 
-func (p *Parser) parseCallArguments() []ast.Expression {
-	var args []ast.Expression
+func (p *Parser) parseExpressionList(end token.TokenType) []ast.Expression {
+	var list []ast.Expression
 
-	if p.peekTokenIs(token.RightParen) {
+	if p.peekTokenIs(end) {
 		p.nextToken()
-		return args
+		return list
 	}
 
 	p.nextToken()
-	args = append(args, p.parseExpression(Lowest))
+	list = append(list, p.parseExpression(Lowest))
 
 	for p.peekTokenIs(token.Comma) {
 		p.nextToken()
 		p.nextToken()
-		args = append(args, p.parseExpression(Lowest))
+		list = append(list, p.parseExpression(Lowest))
 	}
 
-	if !p.setNextTokenIf(token.RightParen) {
+	if !p.setNextTokenIf(end) {
 		return nil
 	}
 
-	return args
+	return list
+}
+
+func (p *Parser) parseIndexExpression(left ast.Expression) ast.Expression {
+	exp := &ast.IndexExpression{Token: p.currentToken, Left: left}
+
+	p.nextToken()
+	exp.Index = p.parseExpression(Lowest)
+
+	if !p.setNextTokenIf(token.RightBracket) {
+		return nil
+	}
+
+	return exp
+}
+
+// Infix expressions
+
+func (p *Parser) parseInfixExpression(left ast.Expression) ast.Expression {
+	expression := &ast.InfixExpression{
+		Token:    p.currentToken,
+		Operator: p.currentToken.Literal,
+		Left:     left,
+	}
+
+	precedence := p.currentPrecedence()
+	p.nextToken()
+	expression.Right = p.parseExpression(precedence)
+	return expression
+}
+
+func (p *Parser) parseCallExpression(function ast.Expression) ast.Expression {
+	exp := &ast.CallExpression{Token: p.currentToken, Function: function}
+	exp.Arguments = p.parseExpressionList(token.RightParen)
+	return exp
 }
 
 // Token