From: Robert Klemme <shortcutter@...>
Date: 2007-09-07T06:10:09+09:00
Subject: Re: assigning to hash keys when there is a default value?

On 05.09.2007 00:15, dblack@wobblini.net wrote:
> 
> Hi --
> 
> On Wed, 5 Sep 2007, Rick DeNatale wrote:
> 
>> On 9/4/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
>>>>
>>>> I for one, am glad that it works this way.  The ruby idiom
>>>>
>>>>  x ||= y
>>>>
>>>> is heavily used for lazy initialization/caching.  While most often,
>>>> it's the rhs which is expensive to compute and therefore the thing we
>>>> want to short-circuit, since x= can in general be a method, and might
>>>> just be expensive, then optimizing the case where it boils down to x =
>>>> x as a nop, makes sense.
>>>
>>> I think The method always gets called, though:
>>>
>>>    class C
>>>      attr_reader :x
>>>      def x=(n)
>>>        puts "C#x="
>>>        true
>>>      end
>>>    end
>>>
>>>    c = C.new
>>>    c.x ||= 3    #   C#x=
>>>
>>> *Unless*, of course, the object is a Hash which has either (a) a key
>>> corresponding to the indicated value, or (b) a default value with
>>> boolean truth value.
>>
>> No, in the case you posited, the assignment happened and C#x= got
>> called because c.x returned nil.
> 
> Right; I got that wrong.
> 
>>> Sigh. I really wish it were otherwise. What an annoying exception to
>>> the rule.
>>
>> Except that the 'rule' wasn't as you thought.  The rule is that
>>
>>    x ||= y
>>
>> is the same as
>>
>>   x = y unless x
> 
> OK, then: What an annoying exception to what should be the rule :-)
> 
>>> Also, in the famous:
>>>
>>>    h = Hash.new(1)
>>>    h[5] ||= 10
>>>
>>> case, it definitely isn't doing the "x = x" equivalent, since that
>>> would set the 5 key to 1.
>>
>> And that's as expected because h[5] returns the default value and
>> doesn't affect the state of the hash a whit, it doesn't create a 5
>> key.  If you want the default to affect the hash you need something
>> like
>>
>>  hsh = Hash.new {|h,k| h[k] = 10}
> 
> That one I didn't get wrong :-) I didn't say that retrieving the
> default value creates a key (which it doesn't, since the default
> value, whether nil or what you set it to, is specifically the default
> value for keys that don't exist). My point was that this:
> 
>   h = Hash.new(1)
>   h[5] ||= 10
> 
> does not map to "x = x", assuming that x stands for h[5]. h[5] = h[5]
> *does* set a key; as I said, it would set the 5 key to 1.  In fact
> this whole thread is really about the fact that hash defaults, which
> don't set keys, can be true, which short-circuits the ||= thing. I do
> think it's the only such case, and probably fairly edge, though
> obviously I'd like to see it do otherwise.

I believe it's not the only case.  Although x= and []= are different, it 
seems in *both* cases assignment is not even invoked for ||=:

irb(main):001:0> class Foo
irb(main):002:1>   def x
irb(main):003:2>     p "x"
irb(main):004:2>     @x
irb(main):005:2>   end
irb(main):006:1>   def x=(v)
irb(main):007:2>     p "x="
irb(main):008:2>     @x=v
irb(main):009:2>   end
irb(main):010:1>   def [](k)
irb(main):011:2>     p "[]"
irb(main):012:2>     k
irb(main):013:2>   end
irb(main):014:1>   def []=(k,v)
irb(main):015:2>     p "[]="
irb(main):016:2>     k
irb(main):017:2>   end
irb(main):018:1> end
=> nil
irb(main):019:0> f=Foo.new
=> #<Foo:0x7ff6b5c4>
irb(main):020:0> f.x||=10
"x"
"x="
=> 10
irb(main):021:0> f.x||=20
"x"
=> 10
irb(main):022:0> f.x=30
"x="
=> 30
irb(main):023:0> f[10]||=20
"[]"
=> 10
irb(main):024:0> f[nil]||=30
"[]"
"[]="
=> 30
irb(main):025:0> f[20]=30
"[]="
=> 30
irb(main):026:0>

Kind regards

	robert