From: Justin Collins <collinsj@...>
Date: 2006-08-17T02:03:01+09:00
Subject: Re: block equality

Tom Jordan wrote:
> On 8/15/06, Justin Collins <collinsj@seattleu.edu> wrote:
>>
>>
>> Marshall T. Vandegrift wrote:
>> > "Tom Jordan" <tdjordan@gmail.com> writes:
>> >
>> >
>> >> Is there a way to compare two blocks with each other?
>> >>
>> >> If you promote a block into a Proc, how would you know that the
>> >> original block was the same?
>> >>
>> >
>> > I haven't dived into the code, but I'm pretty sure that (a) promoting
>> > a block to a Proc via &block in the argument list results in a new
>> > Proc each time and (b) ruby 1.8 provides no introspection into the
>> > actual contents of a block.
>> >
>> >
>> <snip>
>>
>> Yes, it's kind of like asking if two equations or two methods are equal.
>> On the other hand, you could probably use the SerializeableProc
>> (developed here: http://rubyquiz.com/quiz38.html), add an equality
>> method, and then you'd have it. :)
>
> Thanks Marshall and Justin.
>
> This http://ruby-doc.org/core/classes/Proc.html#M001161
>
> "Return true if prc is the same object as other_proc, or if they are
> both procs with the same body."
>
> led me to believe that Procs would evaluate to equality if they had
> the same body.
>
> So I would have expected that the following:
>
> s = lambda { x = 1 }
> t = lambda { x = 1 }
> s == t   # => true expected, false in reality
>
> So my question is; when would the body of Procs ever be the same?
>
> Thanks,
> -- Tom.
Hi Tom,

Well, I did find one instance where this works. If you use Proc#dup or 
Proc#clone, you will end up with a new Proc object, but they will 
evaluate to being equal:

irb(main):001:0> a = Proc.new { n = 1 }
=> #<Proc:0xb7d36224@(irb):1>
irb(main):002:0> b = a.dup
=> #<Proc:0xb7d36224@(irb):1>
irb(main):003:0> a.object_id
=> -605441814
irb(main):004:0> b.object_id
=> -605445814
irb(main):005:0> a == b
=> true
irb(main):006:0> b == a
=> true
irb(main):007:0> c = Proc.new { n = 1 }
=> #<Proc:0xb7d27ecc@(irb):7>
irb(main):008:0> d = c.clone
=> #<Proc:0xb7d27ecc@(irb):7>
irb(main):009:0> c.object_id
=> -605470914
irb(main):010:0> d.object_id
=> -605475424
irb(main):011:0> c == d
=> true
irb(main):012:0> d == c
=> true


Other than that, I don't see a way for two Procs to have the same block 
(and know it). Take a look at the source for Proc#==, it's somewhat 
helpful as well.

I realize this doesn't help you in the general case, though...

-Justin