From: "David A. Black" <dblack@...>
Date: 2008-05-02T05:42:29+09:00
Subject: Re: Please explain nuances of ||=

HI --

On Fri, 2 May 2008, Phillip Gawlowski wrote:

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> Robert Klemme wrote:
> | On 01.05.2008 20:54, Simon Krahnke wrote:
> |> * David A. Black <dblack@rubypal.com> (18:56) schrieb:
> |>
> |>> Hi --
> |>>
> |>> On Fri, 2 May 2008, Ruby Freak wrote:
> |>>
> |>>> I am reading some of the ruby files in rails and I an seeing the ||=
> |>>> method used a lot.
> |>>> knowing ruby the way I do, I realize that she has lots of magical
> |>>> surprises and I really want to get to know this girl!
> |>> x ||= y means:  x || x = y
> |>
> |> No, it means: x = x || y
> |
> | I believe you are wrong.
> |
> | irb(main):001:0> h={}
> | => {}
> | irb(main):002:0> h=Hash.new true
> | => {}
> | irb(main):003:0> h[1]
> | => true
> | irb(main):004:0> h[1] ||= 10
> | => true
> | irb(main):005:0> h
> | => {}
> | irb(main):006:0>
> |
> | If you were right, h would look differently:
> |
> | irb(main):008:0> h=Hash.new true
> | => {}
> | irb(main):009:0> h[1] = h[1] || 10
> | => true
> | irb(main):010:0> h
> | => {1=>true}
> |
> | The same topic has been discussed exhaustively a few days ago.
> |
> | Cheers
> |
> |     robert
> |
> |
>
> You are wrong, too, though.
>
> I refer you to Ruby-Talk 297145[0]:
>
> Quoth Joshua Ballanco:
> 'The only reason that Chris' example behaves like "x || x = 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). '
>
> x = x || 'a value' works when x is nil or false. Once the Hash has a
> default value, it shouldn't be false, much less nil.
>
> See:
>
> irb(main):001:0> h = Hash.new
> => {}
> irb(main):002:0> h[1].nil?
> => true
> irb(main):003:0> h[1] = h[1] || 10
> => 10
> irb(main):004:0> h[2] = 'not nil'
> => "not nil"
> irb(main):005:0> h[2] = h[2] || 'a value'
> => "not nil"
> irb(main):006:0> h[3] = false
> => false
> irb(main):007:0> h[3] = h[3] || 'not false'
> => "not false"

The question, though, is what x ||= y expands to. Robert's point is
that if you say it expands to x = x || y, that doesn't account for
what happens with a hash that has a non-false default value.

Mind you, x || x = y doesn't account (as I mentioned) for the fact
that if x isn't defined, you can't use that syntax. x ||= y is really
its own thing, and doesn't expand 100% of the time to anything. But x
|| x = y, if you allow for the undefined x thing, describes all of the
behaviors, including the hash edge case.


David

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