diff --git a/language/interpreter.cc b/language/interpreter.cc
old mode 100644
new mode 100755
index 58f52fe..22de0cf
--- a/language/interpreter.cc
+++ b/language/interpreter.cc
@@ -7,7 +7,7 @@ using namespace std;
 
 struct Eval
 {
-    static Value eval(const Ast& ast, Env& env) {
+    static Value eval(const Ast& ast, shared_ptr<Environment>& env) {
         switch (ast.type) {
             case Statements:   return eval_statements(ast, env);
             case While:        return eval_while(ast, env);
@@ -32,7 +32,7 @@ struct Eval
     }
 
 private:
-    static Value eval_statements(const Ast& ast, Env& env) {
+    static Value eval_statements(const Ast& ast, shared_ptr<Environment>& env) {
         if (ast.is_token) {
             return eval(ast, env);
         } else if (ast.nodes.empty()) {
@@ -46,7 +46,7 @@ private:
         return eval(**it, env);
     }
 
-    static Value eval_while(const Ast& ast, Env& env) {
+    static Value eval_while(const Ast& ast, shared_ptr<Environment>& env) {
         for (;;) {
             auto cond = eval(*ast.nodes[0], env);
             if (!cond.to_bool()) {
@@ -57,7 +57,7 @@ private:
         return Value();
     }
 
-    static Value eval_if(const Ast& ast, Env& env) {
+    static Value eval_if(const Ast& ast, shared_ptr<Environment>& env) {
         const auto& nodes = ast.nodes;
 
         for (auto i = 0u; i < nodes.size(); i += 2) {
@@ -74,7 +74,7 @@ private:
         return Value();
     }
 
-    static Value eval_function(const Ast& ast, Env& env) {
+    static Value eval_function(const Ast& ast, shared_ptr<Environment>& env) {
         vector<string> params;
         for (auto node: ast.nodes[0]->nodes) {
             params.push_back(node->token);
@@ -84,12 +84,14 @@ private:
 
         auto f = Value::FunctionValue {
             params,
-            [=](Env& env) { return eval(*body, env); }
+            [=](shared_ptr<Environment>& callEnv) {
+	            return eval(*body, callEnv);
+	        }
         };
         return Value(f);
     };
 
-    static Value eval_function_call(const Ast& ast, Env& env) {
+    static Value eval_function_call(const Ast& ast, shared_ptr<Environment>& env) {
         const auto& var = ast.nodes[0]->token;
         const auto& args = ast.nodes[1]->nodes;
 
@@ -97,15 +99,14 @@ private:
         const auto& fv = f.to_function();
 
         if (fv.params.size() <= args.size()) {
-            Env callEnv(env);
-
-            callEnv.set("self", f);
+            auto callEnv = make_shared<Environment>(env);
+            callEnv->set("self", f);
 
             for (auto i = 0u; i < fv.params.size(); i++) {
                 auto var = fv.params[i];
                 auto arg = args[i];
                 auto val = eval(*arg, env);
-                callEnv.set(var, val);
+                callEnv->set(var, val);
             }
             return fv.eval(callEnv);
         }
@@ -114,7 +115,7 @@ private:
         throw runtime_error(msg);
     }
 
-    static Value eval_condition(const Ast& ast, Env& env) {
+    static Value eval_condition(const Ast& ast, shared_ptr<Environment>& env) {
         auto lhs = eval(*ast.nodes[0], env);
         if (ast.nodes.size() > 1) {
             auto ope = eval(*ast.nodes[1], env).to_string();
@@ -139,7 +140,7 @@ private:
         return lhs; // Any
     }
 
-    static Value eval_bin_expression(const Ast& ast, Env& env) {
+    static Value eval_bin_expression(const Ast& ast, shared_ptr<Environment>& env) {
         auto ret = eval(*ast.nodes[0], env).to_long();
         for (auto i = 1u; i < ast.nodes.size(); i += 2) {
             auto val = eval(*ast.nodes[i + 1], env).to_long();
@@ -155,27 +156,27 @@ private:
         return Value(ret);
     }
 
-    static Value eval_assignment(const Ast& ast, Env& env) {
+    static Value eval_assignment(const Ast& ast, shared_ptr<Environment>& env) {
         const auto& var = ast.nodes[0]->token;
         auto val = eval(*ast.nodes[1], env);
-        env.set(var, val);
+        env->set(var, val);
         return val;
     };
 
-    static Value eval_identifier(const Ast& ast, Env& env) {
+    static Value eval_identifier(const Ast& ast, shared_ptr<Environment>& env) {
         const auto& var = ast.token;
         return dereference_identirier(env, var);
     };
 
-    static Value eval_number(const Ast& ast, Env& env) {
+    static Value eval_number(const Ast& ast, shared_ptr<Environment>& env) {
         return Value(stol(ast.token));
     };
 
-    static Value eval_bool(const Ast& ast, Env& env) {
+    static Value eval_bool(const Ast& ast, shared_ptr<Environment>& env) {
         return Value(ast.token == "true");
     };
 
-    static Value eval_interpolated_string(const Ast& ast, Env& env) {
+    static Value eval_interpolated_string(const Ast& ast, shared_ptr<Environment>& env) {
         string s;
         for (auto node: ast.nodes) {
             const auto& val = eval(*node, env);
@@ -184,12 +185,12 @@ private:
         return Value(s);
     };
 
-    static Value dereference_identirier(Env& env, const string& var) {
-        if (!env.has(var)) {
+    static Value dereference_identirier(shared_ptr<Environment>& env, const string& var) {
+        if (!env->has(var)) {
             string msg = "undefined variable '" + var + "'...";
             throw runtime_error(msg);
         }
-        return env.get(var);
+        return env->get(var);
     };
 };
 
@@ -198,10 +199,10 @@ std::ostream& operator<<(std::ostream& os, const Value& val)
     return val.out(os);
 }
 
-bool run(const string& path, Env& env, const char* expr, size_t len, Value& val, std::string& msg, bool print_ast)
+bool run(const string& path, shared_ptr<Environment>& env, const char* expr, size_t len, Value& val, std::string& msg, bool print_ast)
 {
     try {
-        shared_ptr<Ast> ast;        
+        shared_ptr<Ast> ast;
 
         auto& parser = get_parser();
 
diff --git a/language/interpreter.hpp b/language/interpreter.hpp
old mode 100644
new mode 100755
index e96fe64..ddc670a
--- a/language/interpreter.hpp
+++ b/language/interpreter.hpp
@@ -2,7 +2,7 @@
 #include <string>
 #include <peglib.h>
 
-struct Env;
+struct Environment;
 
 struct Value
 {
@@ -14,7 +14,7 @@ struct Value
 
     struct FunctionValue {
         std::vector<std::string>        params;
-        std::function<Value (Env& env)> eval;
+        std::function<Value (std::shared_ptr<Environment>& env)> eval;
     };
 
     explicit Value() : type(Undefined) {}
@@ -163,10 +163,10 @@ private:
 
 std::ostream& operator<<(std::ostream& os, const Value& val);
 
-struct Env
+struct Environment
 {
-    Env() : outer_(nullptr) {}
-    Env(Env& outer) : outer_(&outer) {}
+    Environment() = default;
+    Environment(std::shared_ptr<Environment>& outer) : outer_(outer) {}
 
     bool has(const std::string& s) const {
         if (dic_.find(s) != dic_.end()) {
@@ -187,14 +187,20 @@ struct Env
     }
 
     void set(const std::string& s, const Value& val) {
+        if (dic_.find(s) != dic_.end()) {
+            dic_[s] = val;
+        }
+        if (outer_ && outer_->has(s)) {
+            return outer_->set(s, val);
+        }
         dic_[s] = val;
     }
 
     void setup_built_in_functions() {
         auto func_pretty_print = Value::FunctionValue {
             { "arg" },
-            [](Env& env) {
-                std::cout << env.get("arg").str() << std::endl;
+            [](std::shared_ptr<Environment>& env) {
+                std::cout << env->get("arg").str() << std::endl;
                 return Value();
             }
         };
@@ -202,8 +208,8 @@ struct Env
     }
 
 private:
-    Env*                         outer_;
+    std::shared_ptr<Environment> outer_;
     std::map<std::string, Value> dic_;
 };
 
-bool run(const std::string& path, Env& env, const char* expr, size_t len, Value& val, std::string& msg, bool print_ast);
+bool run(const std::string& path, std::shared_ptr<Environment>& env, const char* expr, size_t len, Value& val, std::string& msg, bool print_ast);
diff --git a/language/main.cc b/language/main.cc
old mode 100644
new mode 100755
index 376dd81..0bc6818
--- a/language/main.cc
+++ b/language/main.cc
@@ -45,8 +45,8 @@ int main(int argc, const char** argv)
     }
 
     try {
-        Env env;
-        env.setup_built_in_functions();
+        auto env = make_shared<Environment>();
+        env->setup_built_in_functions();
 
         for (auto path: path_list) {
             vector<char> buff;
diff --git a/language/repl.cc b/language/repl.cc
old mode 100644
new mode 100755
index 688b18a..9baf442
--- a/language/repl.cc
+++ b/language/repl.cc
@@ -4,7 +4,7 @@
 
 using namespace std;
 
-int repl(Env& env, bool print_ast)
+int repl(shared_ptr<Environment>& env, bool print_ast)
 {
     for (;;) {
         auto line = linenoise::Readline("cul> ");
diff --git a/language/repl.hpp b/language/repl.hpp
old mode 100644
new mode 100755
index 21dfebd..1ae8005
--- a/language/repl.hpp
+++ b/language/repl.hpp
@@ -1,5 +1,5 @@
 #include "interpreter.hpp"
 
-int repl(Env& env, bool print_ast);
+int repl(std::shared_ptr<Environment>& env, bool print_ast);
 
 // vim: et ts=4 sw=4 cin cino={1s ff=unix