From: Rick DeNatale <rick.denatale@...>
Date: 2010-01-20T12:46:05+09:00
Subject: Re: help on the dummy module

On Tue, Jan 19, 2010 at 10:08 PM, Ruby Newbee <rubynewbee@gmail.com> wrote:
> module ShiftMeansAppend
>  def <<(x)  #1
>  end
> end
> class String
>  include ShiftMeansAppend
> end
> class Array
>  include ShiftMeansAppend
> end
> def append_to_self(x)
>  unless x.is_a? ShiftMeansAppend  #2
>    raise ArgumentError, "I can't trust this object's left-shift operator."
>  end
>  x << x
> end
>
>
> Hello,
>
> For the code above, I can't understand that:
>
> #1 why redefine "<<" and set its action to nothing?
> #2 why x.is_a? ShiftMeansAppend can be true when x is an array or a string?

Because, it seems to me, that the author of that code sees Ruby
through Java colored classes and doesn't understand Ruby all that
well.

I see that the code comes from a Ruby Cookbook book, and is discussing
how to not use duck-typing.

He's trying to do the equivalent of the technique in Java of using an
interface as a marker.

#1 is totally unnecessary, Since String and Array both have #<<
methods, and classes are searched for methods before any modules they
include, the method in the module will never be invoked.  It seems
that the author thinks that like Java a method needs to be defined in
an interface or a superclass to be considered the same method.   Not
so in Ruby which doesn't have interfaces.  Modules just add methods to
the repertoire of an objects instances.

#2 is_a? tests whether the argument is anywhere on the chain of
classes and modules which are on chain searched for an objects
methods.  So
x.is_a? y
is true if y is the class of x, or on the chain of superclasses of
that class, or a module included by one of those classes

ruby-1.9.1-p376 > a = Object.new
 => #<Object:0x00000101176068>
ruby-1.9.1-p376 > a.kind_of? Object
 => true
ruby-1.9.1-p376 > a.kind_of? Kernel
 => true
ruby-1.9.1-p376 > a.kind_of? Comparable
 => false

It also returns true for a  module if the singleton class of the
object includes the module, which can be accomplshed with the
Object#extend method

ruby-1.9.1-p376 > a.extend Comparable
 => #<Object:0x00000101176068>
ruby-1.9.1-p376 > a.kind_of? Comparable
 => true


The motivating example a function which takes an object and appends it
to itself is rather strained, IMHO. Personally I'd never be tempted to
write such a method.  Rather I'd implement it on String and Array

module SelfAppendable
  def append_to_self
      self << self
  end
end


class String
  include SelfAppendable
end

class Array
 include SelfAppendable
end

And I don't know if the Author really expects the result of appending
an Array to itself, inserting a element into an Array with << inserts
a reference to the element, so this will generate a recursive array,
which can cause problems.

ruby-1.8.6-p383 > [1, 2, 3].append_to_self
 => [1, 2, 3, [...]]

ruby-1.8.6-p383 > [1, 2, 3] << [1, 2, 3]
 => [1, 2, 3, [1, 2, 3]]
ruby-1.8.6-p383 > ([1, 2, 3] << [1, 2, 3]).flatten
 => [1, 2, 3, 1, 2, 3]
ruby-1.8.6-p383 > [1, 2, 3].append_to_self
 => [1, 2, 3, [...]]
ruby-1.8.6-p383 > [1, 2, 3].append_to_self.flatten
ArgumentError: tried to flatten recursive array


-- 
Rick DeNatale

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