Null my old friend

code/code.go

@@ -26,6 +26,7 @@ const (
 	OpBang
 	OpJumpNotTruthy
 	OpJump
+	OpNull
 )
 
 type Definition struct {
@@ -49,6 +50,7 @@ var definitions = map[Opcode]*Definition{
 	OpBang:          {"OpBang", []int{}},
 	OpJumpNotTruthy: {"OpJumpNotTruthy", []int{2}},
 	OpJump:          {"OpJump", []int{2}},
+	OpNull:          {"OpNull", []int{}},
 }
 
 func Lookup(op byte) (*Definition, error) {

compiler/compiler.go

@@ -134,16 +134,15 @@ func (c *Compiler) Compile(node ast.Node) error {
 			c.removeLastPop()
 		}
 
+		// Emit an `OnJump` with a bogus value
+		jumpPos := c.emit(code.OpJump, 9999)
+
+		afterConsequencePos := len(c.instructions)
+		c.changeOperand(jumpNotTruthyPos, afterConsequencePos)
+
 		if node.Alternative == nil {
-			afterConsequencePos := len(c.instructions)
-			c.changeOperand(jumpNotTruthyPos, afterConsequencePos)
+			c.emit(code.OpNull)
 		} else {
-			// Emit an `OnJump` with a bogus value
-			jumpPos := c.emit(code.OpJump, 9999)
-
-			afterConsequencePos := len(c.instructions)
-			c.changeOperand(jumpNotTruthyPos, afterConsequencePos)
-
 			err = c.Compile(node.Alternative)
 			if err != nil {
 				return err
@@ -152,12 +151,11 @@ func (c *Compiler) Compile(node ast.Node) error {
 			if c.lastInstructionIsPop() {
 				c.removeLastPop()
 			}
-
-			afterAlternativePos := len(c.instructions)
-			c.changeOperand(jumpPos, afterAlternativePos)
-
 		}
 
+		afterAlternativePos := len(c.instructions)
+		c.changeOperand(jumpPos, afterAlternativePos)
+
 	case *ast.BlockStatement:
 		for _, s := range node.Statements {
 			err := c.Compile(s)

compiler/compiler_test.go

@@ -182,17 +182,22 @@ func TestConditionals(t *testing.T) {
 			`,
 			expectedConstants: []interface{}{10, 3333},
 			expectedInstructions: []code.Instructions{
+				// 0000
 				code.Make(code.OpTrue),
 				// 0001
-				code.Make(code.OpJumpNotTruthy, 7),
+				code.Make(code.OpJumpNotTruthy, 10),
 				// 0004
 				code.Make(code.OpConstant, 0),
 				// 0007
-				code.Make(code.OpPop),
-				// 0008
-				code.Make(code.OpConstant, 1),
+				code.Make(code.OpJump, 11),
+				// 0010
+				code.Make(code.OpNull),
 				// 0011
 				code.Make(code.OpPop),
+				// 0012
+				code.Make(code.OpConstant, 1),
+				// 0015
+				code.Make(code.OpPop),
 			},
 		}, {
 			input: `

vm/vm.go

@@ -9,6 +9,7 @@ import (
 
 const StackSize = 2048
 
+var Null = &object.Null{}
 var True = &object.Boolean{Value: true}
 var False = &object.Boolean{Value: false}
 
@@ -99,6 +100,12 @@ func (vm *VM) Run() error {
 			if !isTruthy(condition) {
 				ip = pos - 1
 			}
+
+		case code.OpNull:
+			err := vm.push(Null)
+			if err != nil {
+				return err
+			}
 		}
 	}
 
@@ -111,6 +118,9 @@ func isTruthy(obj object.Object) bool {
 	case *object.Boolean:
 		return obj.Value
 
+	case *object.Null:
+		return false
+
 	default:
 		return true
 	}
@@ -211,6 +221,8 @@ func (vm *VM) executeBangOperator() error {
 		return vm.push(False)
 	case False:
 		return vm.push(True)
+	case Null:
+		return vm.push(True)
 	default:
 		return vm.push(False)
 	}

vm/vm_test.go

@@ -54,6 +54,11 @@ func testExpectedObject(t *testing.T, expected interface{}, actual object.Object
 		if err != nil {
 			t.Errorf("testBooleanObject failed: %s", err)
 		}
+
+	case *object.Null:
+		if actual != Null {
+			t.Errorf("object is not Null: %T (%+v)", actual, actual)
+		}
 	}
 }
 
@@ -138,6 +143,7 @@ func TestBooleanExpressions(t *testing.T) {
 		{"!!true", true},
 		{"!!false", false},
 		{"!!5", true},
+		{"!(if (false) { 5; })", true},
 	}
 
 	runVmTests(t, tests)
@@ -152,6 +158,9 @@ func TestConditionals(t *testing.T) {
 		{"if (1 < 2) { 10 }", 10},
 		{"if (1 < 2) { 10 } else { 20 }", 10},
 		{"if (1 > 2) { 10 } else { 20 }", 20},
+		{"if (1 > 2) { 10 }", Null},
+		{"if (false) { 10 }", Null},
+		{"if ((if (false) { 10 })) { 10 } else { 20 }", 20},
 	}
 
 	runVmTests(t, tests)