diff --git a/language/interpreter.cc b/language/interpreter.cc
index 68900ae..cc8f522 100755
--- a/language/interpreter.cc
+++ b/language/interpreter.cc
@@ -75,9 +75,11 @@ private:
     }
 
     static Value eval_function(const Ast& ast, shared_ptr<Environment> env) {
-        vector<string> params;
+        vector<Value::FunctionValue::Parameter> params;
         for (auto node: ast.nodes[0]->nodes) {
-            params.push_back(node->token);
+            auto mut = node->nodes[0]->token == "mut";
+            const auto& name = node->nodes[1]->token;
+            params.push_back({ name, mut });
         }
 
         auto body = ast.nodes[1];
@@ -95,19 +97,19 @@ private:
         const auto& var = ast.nodes[0]->token;
         const auto& args = ast.nodes[1]->nodes;
 
-        const auto& f = dereference_identirier(env, var);
+        const auto& f = env->get(var);
         const auto& fv = f.to_function();
 
         if (fv.params.size() <= args.size()) {
             auto callEnv = make_shared<Environment>();
 
-            callEnv->set("self", f);
+            callEnv->declare("self", f, false);
 
             for (auto i = 0u; i < fv.params.size(); i++) {
-                auto var = fv.params[i];
+                auto param = fv.params[i];
                 auto arg = args[i];
                 auto val = eval(*arg, env);
-                callEnv->set(var, val);
+                callEnv->declare(param.name, val, param.mut);
             }
 
             return fv.eval(callEnv);
@@ -159,15 +161,20 @@ private:
     }
 
     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);
+        const auto& mut = ast.nodes[0]->token;
+        const auto& var = ast.nodes[1]->token;
+        auto val = eval(*ast.nodes[2], env);
+        if (env->has(var)) {
+            env->assign(var, val);
+        } else {
+            env->declare(var, val, mut == "mut");
+        }
         return val;
     };
 
     static Value eval_identifier(const Ast& ast, shared_ptr<Environment> env) {
         const auto& var = ast.token;
-        return dereference_identirier(env, var);
+        return env->get(var);
     };
 
     static Value eval_number(const Ast& ast, shared_ptr<Environment> env) {
@@ -186,14 +193,6 @@ private:
         }
         return Value(std::move(s));
     };
-
-    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);
-    };
 };
 
 bool run(const string& path, shared_ptr<Environment> env, const char* expr, size_t len, Value& val, std::string& msg, bool print_ast)
diff --git a/language/interpreter.hpp b/language/interpreter.hpp
index 71fde03..cb2b3de 100755
--- a/language/interpreter.hpp
+++ b/language/interpreter.hpp
@@ -13,7 +13,11 @@ struct Value
     };
 
     struct FunctionValue {
-        std::vector<std::string>                                params;
+        struct Parameter {
+            std::string name;
+            bool        mut;
+        };
+        std::vector<Parameter>                            params;
         std::function<Value (std::shared_ptr<Environment> env)> eval;
     };
 
@@ -189,40 +193,58 @@ struct Environment
 
     Value get(const std::string& s) const {
         if (dic_.find(s) != dic_.end()) {
-            return dic_.at(s);
+            return dic_.at(s).val;
         }
         if (outer_) {
             return outer_->get(s);
         }
-        // NOTREACHED
-        throw std::logic_error("invalid internal condition.");
+        std::string msg = "undefined variable '" + s + "'...";
+        throw std::runtime_error(msg);
     }
 
-    void set(const std::string& s, const Value& val) {
+    void assign(const std::string& s, const Value& val) {
+        assert(has(s));
         if (dic_.find(s) != dic_.end()) {
-            dic_[s] = val;
+            auto& sym = dic_[s];
+            if (!sym.mut) {
+                std::string msg = "immutable variable '" + s + "'...";
+                throw std::runtime_error(msg);
+            }
+            sym.val = val;
             return;
         }
         if (outer_ && outer_->has(s)) {
-            outer_->set(s, val);
+            outer_->assign(s, val);
             return;
         }
-        dic_[s] = val;
+    }
+
+    void declare(const std::string& s, const Value& val, bool mut) {
+        assert(!has(s));
+        dic_[s] = Symbol{val, mut};
     }
 
     void setup_built_in_functions() {
-        set("pp", Value(Value::FunctionValue {
-            { "arg" },
-            [](std::shared_ptr<Environment> env) {
-                std::cout << env->get("arg").str() << std::endl;
-                return Value();
-            }
-        }));
+        declare(
+            "pp",
+            Value(Value::FunctionValue {
+                { {"arg", true} },
+                [](std::shared_ptr<Environment> env) {
+                    std::cout << env->get("arg").str() << std::endl;
+                    return Value();
+                }
+            }),
+            false);
     }
 
 private:
+    struct Symbol {
+        Value val;
+        bool  mut;
+    };
+
     std::shared_ptr<Environment> outer_;
-    std::map<std::string, Value> dic_;
+    std::map<std::string, Symbol> dic_;
 };
 
 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/parser.cc b/language/parser.cc
index 80f9871..bfd21a3 100755
--- a/language/parser.cc
+++ b/language/parser.cc
@@ -9,11 +9,12 @@ static auto g_grammar = R"(
     STATEMENTS            <-  EXPRESSION*
 
     EXPRESSION            <-  ASSIGNMENT / PRIMARY
-    ASSIGNMENT            <-  IDENTIFIER '=' _ EXPRESSION
+    ASSIGNMENT            <-  MUTABLE IDENTIFIER '=' _ EXPRESSION
     WHILE                 <-  'while' _ EXPRESSION BLOCK
     IF                    <-  'if' _ EXPRESSION BLOCK ('else' _ 'if' _ EXPRESSION BLOCK)* ('else' _ BLOCK)?
-    FUNCTION              <-  'fun' _ PARAMETERS BLOCK
-    PARAMETERS            <-  '(' _ (IDENTIFIER (',' _ IDENTIFIER)*)? ')' _
+    FUNCTION              <-  'fn' _ PARAMETERS BLOCK
+    PARAMETERS            <-  '(' _ (PARAMETER (',' _ PARAMETER)*)? ')' _
+    PARAMETER             <-  MUTABLE IDENTIFIER
     FUNCTION_CALL         <-  IDENTIFIER ARGUMENTS
     ARGUMENTS             <-  '(' _ (EXPRESSION (', ' _ EXPRESSION)*)? ')' _
 
@@ -36,6 +37,8 @@ static auto g_grammar = R"(
     INTERPOLATED_STRING   <-  '"' ('{' _ EXPRESSION '}' / INTERPOLATED_CONTENT)* '"' _
     INTERPOLATED_CONTENT  <-  (!["{] .) (!["{] .)*
 
+    MUTABLE               <-  < 'mut'? > _
+
     ~_                    <-  (Space / EndOfLine / Comment)*
     Space                 <-  ' ' / '\t'
     EndOfLine             <-  '\r\n' / '\n' / '\r'
@@ -81,6 +84,7 @@ peg& get_parser()
             { peg::AstNodeType::Token,       "IDENTIFIER",           Identifier         },
             { peg::AstNodeType::Regular,     "INTERPOLATED_STRING",  InterpolatedString },
             { peg::AstNodeType::Token,       "INTERPOLATED_CONTENT", Undefined          },
+            { peg::AstNodeType::Token,       "MUTABLE",              Undefined          },
         },
         Undefined);
     }