Change implementation from prepend to UnboundMethod

README.md

@@ -82,7 +82,7 @@ B.call # => 42
 ```
 
 ## Difference with other gems
-Memery is very similar to [Memoist](https://github.com/matthewrudy/memoist). The difference is that it doesn't override methods, instead it uses Ruby 2 `Module.prepend` feature. This approach is cleaner and it allows subclasses' methods to work properly: if you redefine a memoized method in a subclass, it's not memoized by default, but you can memoize it normally (without using awkward `identifier: ` argument) and it will just work:
+Memery is very similar to [Memoist](https://github.com/matthewrudy/memoist). The difference is that it doesn't create additional prefixed methods, instead it uses `UnboundMethod` as local variable. This approach is cleaner and it allows subclasses' methods to work properly: if you redefine a memoized method in a subclass, it's not memoized by default, but you can memoize it normally (without using awkward `identifier: ` argument) and it will just work:
 
 ```ruby
 class A

lib/memery.rb

@@ -21,41 +21,25 @@ def self.method_visibility(klass, method_name)
 
   module ClassMethods
     def memoize(method_name)
-      prepend_memery_module!
-      define_memoized_method!(method_name)
-    end
-
-    private
-
-    def prepend_memery_module!
-      return if defined?(@_memery_module)
-      @_memery_module = Module.new
-      prepend @_memery_module
-    end
-
-    def define_memoized_method!(method_name)
-      mod_id = @_memery_module.object_id
       visibility = Memery.method_visibility(self, method_name)
-      raise ArgumentError, "Method #{method_name} is not defined on #{self}" unless visibility
+      old_method = instance_method(method_name)
 
-      @_memery_module.module_eval do
-        define_method(method_name) do |*args, &block|
-          return super(*args, &block) if block
+      define_method(method_name) do |*args, &block|
+        return old_method.bind(self).call(*args, &block) if block
 
-          @_memery_memoized_values ||= {}
+        @_memery_memoized_values ||= {}
 
-          key = [method_name, mod_id].join("_").to_sym
-          store = @_memery_memoized_values[key] ||= {}
+        key = :"#{method_name}_#{old_method.object_id}"
+        store = @_memery_memoized_values[key] ||= {}
 
-          if store.key?(args)
-            store[args]
-          else
-            store[args] = super(*args)
-          end
+        if store.key?(args)
+          store[args]
+        else
+          store[args] = old_method.bind(self).call(*args)
         end
-
-        send(visibility, method_name)
       end
+
+      send(visibility, method_name)
     end
   end
 

spec/memery_spec.rb

@@ -163,7 +163,9 @@ class << self
     end
 
     specify do
-      expect { klass }.to raise_error(ArgumentError, /Method foo is not defined/)
+      expect { klass }.to raise_error(
+        NameError, /undefined method `foo' for class `#<Class:0x\w+>'/
+      )
     end
   end
 end