3.5: Evaluate boolean and prefix expressions

evaluator/evaluator.go

@@ -5,6 +5,12 @@ import (
 	"monkey_interpreter/object"
 )
 
+var (
+	NULL  = &object.Null{}
+	TRUE  = &object.Boolean{Value: true}
+	FALSE = &object.Boolean{Value: false}
+)
+
 func Eval(node ast.Node) object.Object {
 	switch node := node.(type) {
 
@@ -17,6 +23,12 @@ func Eval(node ast.Node) object.Object {
 	// expressions
 	case *ast.IntegerLiteral:
 		return &object.Integer{Value: node.Value}
+	case *ast.Boolean:
+		return nativeBoolToBooleanObject(node.Value)
+	case *ast.PrefixExpression:
+		right := Eval(node.Right)
+		return evalPrefixExpression(node.Operator, right)
+
 	}
 
 	return nil
@@ -31,3 +43,43 @@ func evalStatements(stmts []ast.Statement) object.Object {
 
 	return result
 }
+
+func evalPrefixExpression(operator string, right object.Object) object.Object {
+	switch operator {
+	case "!":
+		return evalBangOperatorExpression(right)
+	case "-":
+		return evalMinusPrefixOperatorExpression(right)
+	default:
+		return NULL
+	}
+}
+
+func evalBangOperatorExpression(right object.Object) object.Object {
+	switch right {
+	case TRUE:
+		return FALSE
+	case FALSE:
+		return TRUE
+	case NULL:
+		return TRUE
+	default:
+		return FALSE
+	}
+}
+
+func evalMinusPrefixOperatorExpression(right object.Object) object.Object {
+	if right.Type() != object.INTEGER_OBJ {
+		return nil
+	}
+
+	value := right.(*object.Integer).Value
+	return &object.Integer{Value: -value}
+}
+
+func nativeBoolToBooleanObject(input bool) *object.Boolean {
+	if input {
+		return TRUE
+	}
+	return FALSE
+}

evaluator/evaluator_test.go

@@ -14,6 +14,8 @@ func TestEvalIntegerExpression(t *testing.T) {
 	}{
 		{"5", 5},
 		{"10", 10},
+		{"-5", -5},
+		{"-10", -10},
 	}
 
 	for _, tt := range tests {
@@ -44,3 +46,51 @@ func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
 
 	return true
 }
+
+func TestEvalBooleanExpression(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected bool
+	}{
+		{"true", true},
+		{"false", false},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		testBooleanObject(t, evaluated, tt.expected)
+	}
+}
+
+func testBooleanObject(t *testing.T, obj object.Object, expected bool) bool {
+	result, ok := obj.(*object.Boolean)
+	if !ok {
+		t.Errorf("object is not Boolean. got=%T (%+v)", obj, obj)
+		return false
+	}
+	if result.Value != expected {
+		t.Errorf("object has wrong value. got=%t, want=%t",
+			result.Value, expected)
+		return false
+	}
+	return true
+}
+
+func TestBangOperator(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected bool
+	}{
+		{"!true", false},
+		{"!false", true},
+		{"!5", false},
+		{"!!true", true},
+		{"!!false", false},
+		{"!!5", true},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		testBooleanObject(t, evaluated, tt.expected)
+	}
+}