From 7c9ed7ca27ca07653fc240530b3441d748fea6d4 Mon Sep 17 00:00:00 2001
From: Pekka Laiho <pekka.i.laiho@gmail.com>
Date: Thu, 4 Jun 2020 16:06:56 +0700
Subject: [PATCH] overwrite protection for root env

---
 src/Env.php | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/src/Env.php b/src/Env.php
index c8b9416..63aff5f 100644
--- a/src/Env.php
+++ b/src/Env.php
@@ -20,4 +20,14 @@ class Env extends Hash
 
         throw new MadLispException("symbol $key not defined in env");
     }
+
+    public function set(string $key, $value)
+    {
+        // Do not allow overwriting values in root env
+        if ($this->has($key) && $this->parent == null) {
+            throw new MadLispException("attempt to overwrite $key in root env");
+        }
+
+        return parent::set($key, $value);
+    }
 }