From: Mathieu Bouchard <matju@...>
Date: 2006-01-13T04:07:59+09:00
Subject: Re: Design contracts and refactoring (was Re: mathn: ugly warnings)

On Fri, 13 Jan 2006, mathew wrote:

> *Dean Wampler *<deanwampler gmail.com> writes:
> > Let me suggest an XP-style alternative; make thorough unit tests
> > required and make sure they "document" - and test! - the design
> > "contract".
> Unit tests are not an alternative. They are an additional requirement.

I find unit-tests to be often decomposable like this. Start with something 
like this:

  raise if Blah.new(666) != Blah.new(666)
  raise if Blah.new(747) != Blah.new(747)
  raise if Blah.new(242) != Blah.new(242)
  raise if Blah.new(69)  != Blah.new(69)
  raise if Blah.new(37)  != Blah.new(37)

then generalize it ("equality is defined based on the arg of .new"):

  for x in [666,747,242,69,37] do
    raise if Blah.new(x) != Blah.new(x)
  end

then extract a contract from it:

  class CheckedBlah < Blah
    def self.new(x)
      r = super(x)
      raise if r != super(x)
      r
    end
  end

so now all Blah object creation may be checked throughout actual uses of a 
program and not just unit tests. The unit test now reduces to:

  for x in [666,747,242,69,37] do Blah.new(x) end

so for many unit tests, all you have to do is just do things and discard 
the results, and the contract will do the job of checking.

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| Mathieu Bouchard - t�l:+1.514.383.3801 - http://artengine.ca/matju
| Freelance Digital Arts Engineer, Montr�al QC Canada