string concatenation

evaluator/evaluator.go

@@ -163,6 +163,8 @@ func evalInfixExpression(
 	case left.Type() != right.Type():
 		return newError("type mismatch: %s %s %s",
 			left.Type(), operator, right.Type())
+	case left.Type() == object.STRING_OBJ && right.Type() == object.STRING_OBJ:
+		return evalStringInfixExpression(operator, left, right)
 	default:
 		return newError("unknown operator: %s %s %s",
 			left.Type(), operator, right.Type())
@@ -224,6 +226,19 @@ func evalIntegerInfixExpression(
 	}
 }
 
+func evalStringInfixExpression(
+	operator string,
+	left, right object.Object,
+) object.Object {
+	if operator != "+" {
+		return newError("unknown operator: %s %s %s",
+			left.Type(), operator, right.Type())
+	}
+	leftVal := left.(*object.String).Value
+	rightVal := right.(*object.String).Value
+	return &object.String{Value: leftVal + rightVal}
+}
+
 func evalIfExpression(ie *ast.IfExpression, env *object.Environment) object.Object {
 	condition := Eval(ie.Condition, env)
 	if isError(condition) {