From: Mark Hubbart <discordantus@...>
Date: 2005-10-08T06:33:55+09:00
Subject: Re: select! not present but reject! is

On 10/7/05, Jacob Fugal <lukfugl@gmail.com> wrote:
> On 10/7/05, David A. Black <dblack@wobblini.net> wrote:
> > Hi --
> >
> > On Fri, 7 Oct 2005, Rob Rypka wrote:
> >
> > > By your logic, reject! is also wrong.  johnny.reject! should reject
> > > the items, and then give them to you, which is the same as your
> > > select!.  That's not what it does, though...
> >
> > See my previous post -- it shows the difference (between reject! and
> > my sense of select!), which is in the return values.
>
> And that's where it confuses me. select and reject are complements, so
> why couldn't it be expected that select! and reject! be complements as
> well? Changing the semantics of select from converse of reject without
> bang to synonym for reject (differing only in return value) with bang
> makes no sense at all to me.
>
> > > In all honesty, I don't care what it's called, but reject! feels like
> > > it should have an opposite.
> >
> > Similarly (in case it's unclear), I have no underlying objection to
> > the idea of a reject! opposite :-)
>
> At least we're all agreed on that. :)
>
> I just fail to see why select should lose it's complementary
> relationship with reject when a bang is added. I suggest we retain
> that relationship and let select! and reject! remain complementary.
> With select and reject we have the following tautology[1]:
>
>   def test(ary)
>     selected = ary.select { |e| yield e }
>     rejected = ary.reject { |e| yield e }
>     rebuilt = selected + rejected
>     (rebuilt - ary).empty? and (ary - rebuilt).empty?
>   end
>
> I propose select! and reject! should have a similar tautology:
>
>   def test(ary)
>     selected = ary.dup.select! { |e| yield e }
>     rejected = ary.dup.reject! { |e| yield e }
>     rebuilt = selected + rejected
>     (rebuilt - ary).empty? and (ary - rebuilt).empty?
>   end

The english words "select" and "reject" are not opposites. "reject"
and "keep" might be, as someone else suggested.

Going back, say you have a box of apples, some of which are shiny, the
rest of which are dull. You don't want the dull ones. If you say:

  apple_box.reject! {|apple| apple.dull?}

...it sounds like you want to reject, or throw out, any dull apples,
leaving only the shiny ones. The opposite would be:

  apple_box.keep! {|apple| apple.shiny?}

...because you want to keep only the shiny ones. "keep" implies that
the rest are not kept, and are thrown out.

On the other hand, if you said:

  apple_box.select! {|apple| apple.shiny?}

It implies that you want to keep both; you remove the selected ones
from the box, and leave the unselected ones behind.

Note that your tautology (as written) for select!/reject! works for
this version also. "select!" would return an array of the values
removed from the apple_box.

Also, I'm not really advocating any action here. I'm just suggesting
that it would be a little weird for "select!" to be the exact opposite
of "reject!", since the Ruby definitions wouldn't stand up to the
English ones.

cheers,
Mark