diff --git a/language/interpreter.cc b/language/interpreter.cc
index cc8f522..4e7f60d 100755
--- a/language/interpreter.cc
+++ b/language/interpreter.cc
@@ -103,13 +103,13 @@ private:
         if (fv.params.size() <= args.size()) {
             auto callEnv = make_shared<Environment>();
 
-            callEnv->declare("self", f, false);
+            callEnv->initialize("self", f, false);
 
             for (auto i = 0u; i < fv.params.size(); i++) {
                 auto param = fv.params[i];
                 auto arg = args[i];
                 auto val = eval(*arg, env);
-                callEnv->declare(param.name, val, param.mut);
+                callEnv->initialize(param.name, val, param.mut);
             }
 
             return fv.eval(callEnv);
@@ -167,7 +167,7 @@ private:
         if (env->has(var)) {
             env->assign(var, val);
         } else {
-            env->declare(var, val, mut == "mut");
+            env->initialize(var, val, mut == "mut");
         }
         return val;
     };
diff --git a/language/interpreter.hpp b/language/interpreter.hpp
index cb2b3de..defceb8 100755
--- a/language/interpreter.hpp
+++ b/language/interpreter.hpp
@@ -217,15 +217,17 @@ struct Environment
             outer_->assign(s, val);
             return;
         }
+        // NOTREACHED
+        throw std::logic_error("invalid internal condition.");
     }
 
-    void declare(const std::string& s, const Value& val, bool mut) {
+    void initialize(const std::string& s, const Value& val, bool mut) {
         assert(!has(s));
         dic_[s] = Symbol{val, mut};
     }
 
     void setup_built_in_functions() {
-        declare(
+        initialize(
             "pp",
             Value(Value::FunctionValue {
                 { {"arg", true} },