From: Pat Maddox <pergesu@...>
Date: 2006-03-06T08:48:55+09:00
Subject: Re: asserting in code vs unit tests

On 3/5/06, chiaro scuro <kiaroskuro@gmail.com> wrote:
> On 3/6/06, E. Saynatkari <none@none.net> wrote:
> > Or, instead, you could in many circumstances ascertain that the
> > method that produces the input does not produce negative numbers;
> > this is something that a unit test would do and it would eliminate
> > the need of a constraint :)
>
> We are on something interesting here.  I am tempted to agree with you,
> but for the sake of discussion and to explore the idea with you I will
> say:
>
>  * a test doesn't prove that it will never produce a negative number,
> it simply checks that it doesn't do it under certain very specific
> conditions
>  * a postcondition will always check that it doesn't produce a
> negative number and it will warn you if it does.
>
> In this sense pre and post conditions seems to serve different purposes.
>
> the precondition defines a scope of valid input and checks that that
> input is well formed.
> tests check a method, passing it valid input and expecting a 'specific' result.
> a postcondition defines a scope of valid output and checks that the
> output is well formed.
>
> pre/post alone are not enough because they just check the domain of variables.
>
> you need tests to verify that at least under some conditions the
> method yields meaningful results.
>
> tests alone however are never exhaustive and cannot prove correctness.
> assertions can catch some types of 'wrongness'.

Isn't that what exceptions are for?

def foo(a)
  raise 'Argument can not be negative' if a < 0

  b = something_that_should_always_be_positive

  raise 'Something strange happened and b was negative' if b < 0
  b
end

foo expects a to be positive, so when a negative number comes in
that's an exceptional case.  It also should always return a positive
number, so if b is negative then something really went wrong and it
should terminate.

Pat