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'.


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