From: Jonathan Leighton <lists@...>
Date: 2006-01-02T21:05:11+09:00
Subject: Re: Can't change the value of self

Thanks very much for this explanation, the restriction makes a lot more
sense to me now.

Jon

On Mon, 2006-01-02 at 19:32 +0900, Ron Jeffries wrote:
> On Mon, 2 Jan 2006 09:37:45 +0900, Jonathan Leighton <lists@turnipspatch.com>
> wrote:
> 
> >Can't change the value of self (SyntaxError)
> >    self = round_to *args
> >
> >Why doesn't it work? What can I do instead?
> 
> One way of thinking about reassigning self is if you imagine in a numeric class
> saying
> 
> def double
>   self *= 2
> end
> 
> That would amount to saying, if we did 1.double,
>   1 becomes 2
> which would mean that 1+1 would thereafter be 4, and that would be bad.
> 
> What we need is not the value 1 to become 2, but to get the value 2 back when we
> say 1.double. So we say
> 
> def double
>   return self*2
> end
> 
> or, since "return" is implied
> 
> def double
>  self*2
> end
> 
> In general in OO, an object can change its contents, but it cannot change its
> "self". What should happen in this program?
> 
> one = 1
> another_one = 1
> assert_equal(2, one+one)
> assert_equal(2, another_one+another_one)
> one.double
> assert_equal(4, one+one) 
> assert_equal(???,another_one+another_one)
> assert_equal(???, 1+1)
> 
> When we double one, if the self*=2 thing worked, the ??? asserts would also find
> 4. We might be able to understand another_one being 2, but to have all
> occurrences of 1+1 in the program turn into 4 ... that would be bad.
> 
> Certain objects are called "value objects", and the idea is that like numbers,
> they never change value. For many objects it is a design choice whether or not
> to make them value objects, even if they are quite complex. For example, in the
> articles I'm currently writing on extended set theory on my site, I'm moving in
> the direction of making my sets value objects. They are essentially arbitrarily
> large collections of records (like relational database relations), and most DB
> apps think of relations as mutable. I'm going to make a different decision,
> namely that any set, once created, is a constant. It'll be interesting to see
> what happens.
> 
> 
-- 
Jonathan Leighton
http://turnipspatch.com/ | http://jonathanleighton.com/ | http://digital-proof.org/