From: "David A. Black" <dblack@...>
Date: 2008-04-05T19:20:26+09:00
Subject: Re: The ||= assignment operator

--1926193751-626661992-1207390825=:19443
Content-Type: MULTIPART/MIXED; BOUNDARY="1926193751-626661992-1207390825=:19443"

  This message is in MIME format.  The first part should be readable text,
  while the remaining parts are likely unreadable without MIME-aware tools.

--1926193751-626661992-1207390825=:19443
Content-Type: TEXT/PLAIN; charset=X-UNKNOWN; format=flowed
Content-Transfer-Encoding: QUOTED-PRINTABLE

HI --

On Sat, 5 Apr 2008, Joshua Ballanco wrote:

> Pe=C3=B1a, Botp wrote:
>> From: Chris Shea [mailto:cmshea@gmail.com]
>> # It's better to think of x ||=3D "ruby" as x || x =3D "ruby".
>> #
>> # --
>> # h =3D Hash.new('default value')
>> #
>> # h[:test] ||=3D 'assigned value'
>> # h # =3D> {}
>> #
>> # # same as:
>> # h[:test] || h[:test] =3D 'assigned value'
>> # h # =3D> {}
>> #
>> # # not:
>> # h[:test] =3D h[:test] || 'assigned value'
>> # h # =3D> {:test=3D>"default value"}
>>
>>
>> i'd say that's a bug in design (unless x+=3D1 is now x+x=3D1 ;)
>>
>> kind regards -botp
>
> Not at all. Rather, this is just a subtle misunderstanding of how hash
> is implemented. Consider the following:
>
>>> h =3D Hash.new
> =3D> {}
>>> h[:test] ||=3D 'testing without default'
> =3D> "testing without default"
>>> h
> =3D> {:test=3D>"testing without default"}
>>> h =3D Hash.new('default value')
> =3D> {}
>>> h[:test] ||=3D 'testing with default'
> =3D> "default value"
>>> h
> =3D> {}
>
> The only reason that Chris' example behaves like "x || x =3D stuff" is
> because he's defined a default value for the hash. If you set a default
> value, than you'll never have a keyed value be empty (i.e. nil).

The only expansion that accounts for all cases of the ||=3D operator is
x || x =3D stuff. If it were actually x =3D x || stuff, then this:

irb(main):005:0> h =3D Hash.new(1)
=3D> {}
irb(main):006:0> h[:x] =3D h[:x] || 2
=3D> 1
irb(main):007:0> h
=3D> {:x=3D>1}

would behave the same as this:

irb(main):008:0> h[:y] ||=3D 2
=3D> 1
irb(main):009:0> h
=3D> {:x=3D>1}

but it doesn't. But ||=3D does behave like this:

irb(main):010:0> h[:z] || h[:z] =3D 2
=3D> 1
irb(main):011:0> h
=3D> {:x=3D>1}

In other words, x ||=3D y =3D=3D x || x =3D y, or something :-)

I personally would like to see it act like x =3D x || y, though I think
hash defaults are the only case where the difference actually makes a
difference.


David

--=20
Rails training from David A. Black and Ruby Power and Light:
   ADVANCING WITH RAILS   April 14-17          New York City
   INTRO TO RAILS         June 9-12            Berlin
   ADVANCING WITH RAILS   June 16-19           Berlin
See http://www.rubypal.com for details and updates!
--1926193751-626661992-1207390825=:19443--
--1926193751-626661992-1207390825=:19443--