From: Rick DeNatale <rick.denatale@...>
Date: 2009-02-26T00:23:28+09:00
Subject: Re: Where is splat implemented? / How does it work?

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On Wed, Feb 25, 2009 at 9:37 AM, David A. Black <dblack@rubypal.com> wrote:

> Hi --
>
> Mischa Fierer wrote:
>
>> Hi --
>>
>> After a good amount of digging, I'm still not sure how [*[1,2]] ==
>> [1,2]. I understand what it does, but I want to know why.
>>
>> I had originally assumed it was a normal method (like + or -), but it
>> appears to be implemented at a deeper level. It looks like rubinius does
>> something with cast_array. Ruby 1.8 does to_ary and ruby 1.9 does
>> to_splat. Where does this happen? to_ary says that it just returns self,
>> so how does self get exploded?
>>
>> Short of reading eval.c, does anyone have any pointers?
>>
>
> It ties itself to to_a:
>
> >> obj = Object.new
> => #<Object:0xa43db54>
> >> def obj.to_a; [1,2,3]; end
> => nil
> >> a = *obj
> => [1, 2, 3]
>
> That code works the same in 1.8 and 1.9.1. There may be some classes where
> it's optimized away so that you can't override it even by defining to_a. I'm
> not sure.
>

I'm pretty sure that implicit conversions to array use to_ary, not to_a.
This is the way it works in Ruby 1.8, and after an experimental period in
1.9 with to_splat, it appears to have gone back to using to_ary.  Grepping
the Ruby 1.9.1 source code for to_splat shows no hits.

To answer Mischa question about how it "ties itself," this is done
internally by the Ruby interpreter/VM when it needs to implicitly convert
the value of a *expression when it appears either as an actual parameter to
a method call, or on the right hand side of an assignment.

So it can't be explained completely at the ruby source code level, it's part
of the semantics of the language.

-- 
Rick DeNatale

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