From: ptkwt@... (Phil Tomson)
Date: 2006-02-17T07:38:27+09:00
Subject: Re: recontextualizing a block (looking for deep magic)

In article <5cd596d60602161330o67494818oadaad45229662f5c@mail.gmail.com>,
Jim Freeze  <jim@freeze.org> wrote:
>On 2/16/06, Phil Tomson <ptkwt@aracnet.com> wrote:
>> What I'm trying to do probably isn't possible, but maybe someone knows so=
>me
>> deep magic.
>>
>> I'd like something like:
>>
>>   def runit &b
>>     a =3D 42
>>     pnew =3D Proc.new &b, binding
>>     pnew.call
>>   end
>>
>>   runit { puts "a is: #{a}" }
>>
>>   #would print:
>>   a is: 42
>>
>> Or course, Proc.new currently doesn't take a Binding object second argume=
>nt.
>> What I'm wondering is if there is any way to 'rebind' or 'recontextualize=
>' a
>> block?
>
>This is interesting. For a while you had me thinking the Proc.new {} should
>rebind, but I couldn't get it to. Seems that the binding is based upon wher=
>e the
>block is defined inside the file.

Right. And 99% of the time that's what you want because they're closures.
.... but sometimes I find I want the block evaluation to be delayed and done in 
a different binding.

I think a Proc.new that took a binding would be useful sometimes, but I'm not 
hopeful that it could be implemented without mucking around with Ruby's 
internals (in C).

>
>This is the closest I could get:
>
>  def runit(b)
>    a =3D 42
>    pnew =3D lambda { eval(b,binding) }
>    pnew.call
>  end
>
>  runit %q{ puts "a is: #{a}" }
>

Yeah, I eventually came up with pretty-much the same thing.  This is probably 
what I'll end up doing since: 
  %q{ ... } 

doesn't look all that different from the regular block syntax:
    { ... }

....but as this is for a DSL, I'm sure users are going to be thinking "what the 
heck is the %q for?" and I don't blame them.  It hurts the aesthetics.

Phil