From: John Carter <john.carter@...>
Date: 2004-11-29T13:59:24+09:00
Subject: Re: What is Ruby for "pimpl"?

On Mon, 29 Nov 2004, John Carter wrote:

> I trying to imagine a ruby translation of the pimpl idiom.

Let me be a bit more explicit about what I'm thinking about.

I ran a unit test that failed on a syntax error in another module/class.

Looking at the error message I realized that class A used B which used C 
which used D which under certain error conditions that could never occur 
while testing A, used E.

This left me feeling my program was too coupled. Why did the syntax of a 
module that I wasn't using block me?

In C++ I could have hidden the implementation of E behind a pimpl, and all 
would have been sweet.

So thinking out loud...
-------file A.rb------------------------------------------------------
require 'FixMe'

class A
   def foo
     dostuff()
   rescue BadThingsHappening => details
     b = FixMe.new( details, x, y, z)
     b.make_it_all_better
   end

   def bah
   end
end
----------------------------------------------------------------------
If I feed this to Ruby it insists that FixMe must be defined.

So if I do....
----------------------------------------------------------------------
require 'A'
class C
   def initialize
     a = A.new
     a.bah
   end
end
----------------------------------------------------------------------
Even though I never invoked foo, as soon as I "require 'A'", I must
have FixMe initialized.

So how about....
----------------------------------------------------------------------
class A
   def A.helper( b_class)
     @@helper = b_class
   end

   def foo
     dostuff()
   rescue BadThingsHappening => details
     b = @@helper.new( details, x, y, z)
     b.make_it_all_better
   end

   def bah
   end
end
----------------------------------------------------------------------
This way I could have

-File C.rb---------------------------------------------------------------------
require 'A'
class C
   def initialize
     a = A.new
     a.bah
   end
end
----------------------------------------------------------------------
and C can be used without FixMe being defined and another script that did use foo do something like...

-File other.rb--------------------------------------------------------
require 'FixMe'
require 'A'
A.helper( FixMe)
a = A.new
a.foo
----------------------------------------------------------------------
Or even more self contained...
----------------------------------------------------------------------
class SimpleHelper
   def initialize(details)
     @details = details
   end

   def make_it_all_better
     raise "I can't make '#{details}' better, I'm too dumb"
   end
end

class A
   @@helper = SimpleHelper

   def A.helper( b_class)
     @@helper = b_class
   end

   def foo
     dostuff()
   rescue BadThingsHappening => details
     b = @@helper.new( details, x, y, z)
     b.make_it_all_better
   end

   def bah
   end
end
----------------------------------------------------------------------

But this still requires a concrete definition of a Helper, so a simpler version might be..

----------------------------------------------------------------------
class A
   @@helper = nil

   def A.helper( b_class)
     @@helper = b_class
   end

   def foo
     dostuff()
   rescue BadThingsHappening => details
     raise details unless @@helper
     b = @@helper.new( details, x, y, z)
     b.make_it_all_better
   end

   def bah
   end
end
----------------------------------------------------------------------

That looks good, and has simplified my tests, but has complicated the
client code for A.

Sorry for thinking out loud, excuse me for existing.



John Carter                             Phone : (64)(3) 358 6639
Tait Electronics                        Fax   : (64)(3) 359 4632
PO Box 1645 Christchurch                Email : john.carter@tait.co.nz
New Zealand

The universe is absolutely plastered with the dashed lines exactly one
space long.