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From: stimuli@my-dejanews.com
Subject: Re: Assertions
Date: 1999/05/20
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In article <7hv6bb$1l9@dfw-ixnews6.ix.netcom.com>,
  Richard D Riehle <laoXhai@ix.netcom.com> wrote:

>  The Require, Ensure, and Invariant conditions may be applied to
>  a subprogram, a type, or a even some variable.  The designer of
>  Eiffel (who did not invent these ideas but incorporated them into
>  the design of his language), calls this "design by contract."  It
>  is important to understand that this is a run-time contract.  Many
>  safety-critical software practitioners feel this is the wrong place
>  to evaluate assertions.  The SPARK language takes a more conservative
>  approach to the same idea.  Ada probably needs an approach somewhere
>  between SPARK and Eiffel.

This isn't entirely true, although I think I understand your point.
The Eiffel contract is not a run-time concept, it is a static property
of correct Eiffel systems (read: correct Eiffel programs). How an
Eiffel compiler goes about testing these is implementation dependant.
In fact, they needn't actually be checked as they have null semantics
within a correct program. It would be correct Eiffel for a compiler
to check the validity of the assertions at compile time and a correct
Eiffel program needn't check them at all. Now, given that a general
theorem checker for Eiffel is, in practical terms, very unlikely,
most Eiffel implementations perform runtime checks of the assertions.
This is a useful tool for testing and debugging, but it is not the
greater part of what assertions are.

--
Jeffrey L. Straszheim
stimuli@my-dejanews.com


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