Eval Minus Prefix Operator

2 files changed, 13 insertions, 0 deletions

diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index 0cc003f..b8940a9 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -57,6 +57,8 @@ func evalPrefixExpression(operator string, right object.Object) object.Object {
 	switch operator {
 	case "!":
 		return evalBangOperatorExpression(right)
+	case "-":
+		return evalMinusPrefixOperatorExpression(right)
 	default:
 		return NULL
 	}
@@ -74,3 +76,12 @@ func evalBangOperatorExpression(right object.Object) object.Object {
 		return FALSE
 	}
 }
+
+func evalMinusPrefixOperatorExpression(right object.Object) object.Object {
+	if right.Type() != object.INTEGER_OBJ {
+		return NULL
+	}
+
+	value := right.(*object.Integer).Value
+	return &object.Integer{Value: -value}
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index aa1833a..1221de6 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -15,6 +15,8 @@ func TestEvalIntegerExpression(t *testing.T) {
 	}{
 		{"5", 5},
 		{"10", 10},
+		{"-5", -5},
+		{"-10", -10},
 	}
 
 	for _, tt := range tests {