From: Brian Candler <b.candler@...>
Date: 2011-01-19T06:47:29+09:00
Subject: Re: Calling by Reference - Two Questions

Mike Stephens wrote in post #975795:
>>Robert Klemme wrote
>> You cannot assign to self!
>> Calling self#[]= is something different - this is actually a method
>> that must be implemented for the object that self points to.
>
> Robert, can you expand on that?
>
> My example has an object of type Array. Array has a method =
> implemented, so in what way is []= different to = ? One works on self;
> the other raises an error.

Array doesn't have an '=' method. When you write

    foo[bar] = baz

this is just syntactic sugar for the following method call:

    foo.send(:[]=, bar, baz)

i.e. the name of the method being called is '[]=' and we are passing it 
two arguments, bar and baz.

So it's not an assignment. It's a method call; the implementation of 
this method may mutate the object (or may not).

Try this test program:

class Foo
  def []=(*args)
    puts "You sent #{args.inspect}"
  end
end
f = Foo.new
f["hello"] = "world"

Almost all things which look like operators in Ruby are actually method 
calls, and hence can be redefined. The exceptions are assignment (to 
local variables) and short-circuit boolean operators. Unary "!" can't be 
redefined in ruby 1.8, but I think it can in 1.9.

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