From: Igor Pirnovar <lists@...>
Date: 2012-10-27T00:44:02+09:00
Subject: Re: Name/symbol/object type clash? What is happening here?

Robert Klemme wrote in post #1081267:

> You are repeating that it is broken but you fail to explain what
> exactly is broken.  That's not a basis for discussions.

I thought my statement "I believe, we all can tolerate this glitch" was
clear enough. But if you insist to define what is broken, the reply is:
"Ruby grammar consistency when it comes to mixing classes with Structs".
Every solution that you come up with introduces more complications and
weird or even unacceptable run-arounds, like your underscore aliasing.
Try using your '_initialize' and then straight 'initialiye' and you'll
create two different behaviours both of whom are wrong. One
instantiating {{ s = S.new(0) }} with @num to 0, and the other adding 5
twice making @num 10, when using {{ self.num = n + 5 }} idiom rather
than {{ @num = n + 5 }}, which Struct does not recognize (this should be
fixed, namely, Struct should honour '@ semantics'; resorting to
'self#var' is not sufficient in all circumstances).

Struct does not honour Ruby's variable inheritance and class
initialization grammar with respect to inheritance, i.e.: all subclasses
have a single set of instance variables in the inheritance hierarchy. If
the idiom {{ class A < Struct.new(:num); end }} makes Struct a
superclass of A, then class A and indeed all its subclasses should
inherit @num instance method from Struct. Accessing Struct's @num via {{
self.num }} works only when you are using straight assignment, however
if you need to invoke any kind of computation, you have to resort to
tricks like aliasing which works only sometimes.

  S = Struct.new :num do
    alias _initialize initialize
  #  def _initialize(n)  #=> @num==0
    def initialize(n)  #=> @num==10
      super
      self.num = n + 5
    end

    alias _num= num=
    def num=(n) self._num= n + 5; end
  end

  s = S.new 0
  p s.num    #=> 10; ## with: '_initialize' #=>0
  s.num = 100
  p s.num    #=> 105

The trouble with Struct is that there is no way to implement
initialization of instance variables in base class (ie. in Struct) that
require more elaborate initialization skims than straight assignment.
You can accomplish this only with regular classes and their inheritance
hierarchies!

When you have to resort to tricks to accomplish things that are not out
of the ordinary, you better avoid those features when working outside of
your quick and dirty domain or "research lab", and Struct certainly
qualifies for that!

Cheers, igor

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