From: Gavin Sinclair <gsinclair@...>
Date: 2002-10-04T14:10:48+09:00
Subject: Re: Specifying local and external block parameters (that old chestnut)


From: "Hal E. Fulton" <hal9000@hypermetrics.com>


> From: "Gavin Sinclair" <gsinclair@soyabean.com.au>
> To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
> Sent: Thursday, October 03, 2002 8:58 PM
> Subject: Specifying local and external block parameters (that old chestnut)
> 
> 
> > I think it makes sense to explicitly mark *external* variables, as
> block-local
> > variables (parameters, in fact) would be the norm.
> >
> > Thus, if 'a' and 'c' are regular parameters and 'b' is an external
> variable,
> > then one could write
> >
> >   collection.each do |a,:b,c|
> >     ...
> >   end
> >
> > or
> >
> >   collection.each do |a,b,c|
> >     external b
> >     ...
> >   end
> >
> > I'm not sure which of the above I prefer.  One should optimise for the
> common
> > case.
> 
> One problem is that it's not just a simple variable. It
> can be anything that responds to assignment. For example,
> an accessor:
> 
>   0.upto(99) {|frame.scroll| sleep 0.1}

So could
  0.upto(99) {|:frame.scroll| sleep 0.1}
work?

Look quite good, I think, since we are used to :: as a scoping operator.

> > I'd like to put a question to the audience :)  Does anyone actually have a
> need
> > to use non-local block parameters?  Ever?
> 
> The above is an example.

Quite a nice example too, IMO.

> Also what if I want the last value(s) of a loop?
> 
>   x,i = nil
>   myobj.each_with_index do |x,i|
>     # process...
>     break if some_condition
>     # more processing...
>   end
> 
>   last_obj = x
>   last_index = i
> 
> Yes, I *could* save them off in different variables
> right before I did the break:
>   [...] 
> Two more variables and much ugliness. No, thanks.
> 
> Hal

I really dislike relying on finishing values like that, because:
 - you have to declare variables beforehand
 - code is harder to understand or be confident that it's right

Much better, IMO, is

  x, i = get_values(myobj)

  def get_values(obj)
    obj.each_with_index do |x, i|
      # process
      return x, i if some_condition
      # more processing
    end
  end

Gavin