From: ara.t.howard@...
Date: 2006-12-07T01:02:10+09:00
Subject: Re: invasive parameters as method mode variables

On Thu, 7 Dec 2006, Trans wrote:

> Working on a method where I would like one part of it's operations to
> be optional. Here's the crux of the code:
>
>  def parse_multicommand( line=nil, arity={} )
>    argv = parse_line(line)
>    argv = multi_flag(argv)  # optional?
>
>    # ...
>  end
>
> I would prefer not to add another parameter to the method signiture to
> signify this option b/c its rare and it would complexify the call to
> much, especially if there is more than one optional part --which is
> possible. But other tahn that I don't have much choice, I'll probably
> have to create an extra method called
> #parse_multicommand_without_multi_flag. Yuk!
>
> Not that helps me presently, but this did get me to thinking of using a
> "global" flag that can be set external to the method. That would work
> but it wouldn't be thread safe. Then it occured to me the
> Binding.of_caller could be helpful here. Using the Binding#[] extension
> for local variable access:
>
>  def parse_multicommand( line=nil, arity={} )
>    argv = parse_line(line)
>    argv = multi_flag(argv) unless Binding.of_caller[:no_multi_flag]
>
>    # ...
>  end
>
> Then to use:
>
>  no_multi_flag = true
>  parse_multicommand( line )
>
> I know this breaks the whole idea of function encapsulation, but it has
> it's advantages in certain cases like mine. It also might be of usedin
> a few other places like String#each's separator. So that got me to
> thinking of a cleaner way to provided this capability of invasive
> parameters as "method mode variables".
>
>  def parse_multicommand( line=nil, arity={} )
>    argv = parse_line(line)
>    argv = multi_flag(argv) unless --no_multi_flag
>
>    # ...
>  end
>
>  # '--' indicates a method mode variable
>  --no_multi_mode = true
>  parse_multicommand( line )
>
> Thouhgts?

method + state => object

     harp:~ > cat a.rb
     require 'rubygems'
     require 'attributes'

     class Parser
       attribute 'no_multi_flag'
       def parse_multicommand
         p [:parse_multicommand, no_multi_flag]
       end
       def parsing &b
         instance_eval &b
       end
     end

     parser = Parser.new

     parser.parsing{
       no_multi_flag true
       parse_multicommand
     }


     harp:~ > ruby a.rb
     [:parse_multicommand, true]


fun to clean up with prototype:

     harp:~ > cat a.rb
     require 'rubygems'
     require 'prototype'


     parser = prototype{
       no_multi_flag false

       def parse_multicommand
         p [:parse_multicommand, no_multi_flag]
       end

       def parsing &b
         instance_eval &b
       end
     }


     parser.parsing{
       parse_multicommand
     }

     parser.parsing{
       no_multi_flag true
       parse_multicommand
     }


     harp:~ > ruby a.rb
     [:parse_multicommand, false]
     [:parse_multicommand, true]


regards.

-a
-- 
if you want others to be happy, practice compassion.
if you want to be happy, practice compassion.  -- the dalai lama