From: MonkeeSage <MonkeeSage@...>
Date: 2007-12-30T10:19:58+09:00
Subject: Re: Add Array#first= and Array#last= to std lib

On Dec 29, 6:06 pm, Gary Wright <gwtm...@mac.com> wrote:
> On Dec 29, 2007, at 6:24 PM, MonkeeSage wrote:
>
> > I wasn't arguing for or against #last=, per se; I was only trying to
> > explain why it doesn't make sense, _to me_, "Unless the semantics [of
> > #last] change," to have #last=. I.e., #[] subscripts the array and
> > returns the value of the index, but #last returns the value of
> > subscripting the array at index #length-1; it is another level of
> > abstraction removed from #[]; so with those semantics, #last= would be
> > like #length= (or #pop=), and you end up with exceptions when the
> > value is immutable.
>
> Why do you shift your terminology when talking about last?  Using
> your terminology but my mental model of arrays:
>
> #[n] subscripts the array with n and returns the value of the index
> #[-1] subscripts the array with (length-1) and returns the value of
> the index
> #last subscripts the array with (length-1) and returns the value of
> the index
>
> Why do you rephrase that last case as:
>
> #last returns the value of subscripting the array at index #length-1
>
> Are you suggesting that #last does something distinctly different than
> subscripting via -1?  If not then it would seem your argument against
> last=
> would apply just as well to 'array[-1] = x'.  Are you suggesting that
> element assignment with negative subscripts also don't make sense to
> you?
>
> Objections to last= and first= based on redundancy is one thing, but
> you seem to be objecting based on semantics and I'm trying to understand
> that objection because the semantics seem to be quite natural to me.
>
> Gary Wright

As I understand it, #last is semantically equivalent to this code:

class Array
  def last
    self[self.length-1]
  end
end

Which means that #last is a specialized form of #[], which means that
it is another level of abstraction removed from #[]. So the result of
#last is the same as #[-1], but the semantics of #last are more like
#length or #pop, which are also a level of abstraction removed from
#[]. Once again, I'm not arguing against #last=, per se, but I fail to
see how it differs at the present time from, for example:

class Array
  def foo
    self[self.length-1]
  end
end

[1,2,3].foo = 25

Or more simply, [1,2,3].length = 20.

Of course, one could implement those methods where they so inclined,
but it would make the semantics of, e.g., #foo and #foo= very
different. In this case of #last / #first, I don't think it is very
confusing to have those different semantics; I was probably being too
much of a stickler for "purism".

Regards,
Jordan