From: ynnor@...
Date: 2015-10-12T10:47:51+00:00
Subject: [ruby-core:71055] [Ruby trunk - Feature #11550] Current behaviour of super(...) is dangerous in the presence of more than one included modules.

Issue #11550 has been updated by Ronald Fischer.


Benoit Daloze wrote:
> Why not simply having `super` in M1 and M2 #initialize?
> All constructors should call `super`, unless they just inherit from Object/BasicObject.

First, M1 and M2 don't know where they are going to be mixed in, so they can not invoke `suuper` - they don't know what parameters are being passed.

Second, the class which mixes in M1 and M2 needs to initialize two modules, which is not possible with the current language.




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Feature #11550: Current behaviour of super(...) is dangerous in the presence of more than one included modules.
https://bugs.ruby-lang.org/issues/11550#change-54424

* Author: Ronald Fischer
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
Consider a class

~~~
   class C <P
     include M1
     include M2
     def initialize
       ....
     end
   end
~~~ 

If P, M1 and M2 all provide a initialize method, and C::initialize calls super(...), the first initialize module in the chain, which has a formal parameter list matching the actual parameters in the super(....) call, is executed. The other ones are not executed. The following article demonstrates a clever way, how C::initialize can call all initializers of the included modules and of the parent class:

[[http://stdout.koraktor.de/blog/2010/10/13/ruby-calling-super-constructors-from-multiple-included-modules/]]

This solution works, but is complicated, and the reason is that the basic design of Ruby, with respect of initializing the base class, is flawed, for the following reason:

If we define an 'initialize' method, we certainly assume that its execution is necessary for the correct behaviour of our class or module. Therefore, the designer of a class or module should at least have the possibility to REQUIRE that initialize will be called (by a derived class), and the designer of a class which inherits from a parent class or includes a module, should have an EASY way to call all the parent initializers.

Here a first draft of how the language could be changed to meet this criterium; it's perhaps not the best design, but it might help clarifying my point:

(1) A class (or module) may define either a method initialize or a method initialize_strict (but not both). If it has initialize_strict defined, and the class where the module is included, respectively the class which inherits it, does NOT invoke this initializer, an exception is thrown, UNLESS initialize_strict can be called without parameters. In the latter case, it is always executed (even if no 'super' call is present). 

(2) A class method super_of is added to class Object, with the prototype super_of(klass,arg*), where klass is a symbol or String or instance of type Class or Module. The affect of invoking super_of(:Foo,x,y) is identical to invoking initialize(x,y) in the ancestor class Foo. If Foo is neither a direct ancestor nor an included module, an exception is thrown.




-- 
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