[Haskell-cafe] Object Oriented programming for Functional Programmers

Mike Meyer mwm at mired.org
Wed Jan 2 12:28:14 CET 2013

On Wed, 2 Jan 2013 13:48:07 +0400
MigMit <miguelimo38 at yandex.ru> wrote:
> On Jan 2, 2013, at 10:52 AM, Mike Meyer <mwm at mired.org> wrote:
> > MigMit <miguelimo38 at yandex.ru> wrote:
> >> But really, "Design by Contract" — a theory? It certainly is a useful
> >> approach, but it doesn't seem to be a theory, not until we can actually
> >> prove something about it, and Eiffel doesn't seem to offer anything in
> >> this direction.
> > You just stated (briefly, and not very rigorously) the theory: DbC is a useful approach to programing. Note that it's a theory about *programming*, not the resulting program.
> Well, you can call that a theory, for sure. But I think it's usually called an "observation".

An "observation" is what you make to decide if a theory is true or
not. In order to make the observation (at least for theories about
helping programmers) you need an implementation so you can observe
people using it.

> I always thought the theory is something that allows us to develop
> some new knowledge.

Yup. Deciding whether or not the theory is true *is* a development of
new knowledge. I can say for a certainty that the documentation aspect
of DbC makes me more productive. The testing aspect of it needs more
testing (sorry).

> Just stating that "comfortable chairs make programmers more
> productive" doesn't constitute a theory.

Well, it's not very rigorous, and I can think of some
counterexamples. On the other hand, if you reparaphrased (sic) it as
"Chairs that encourage good posture make programmers more productive",
then you have a honest-to-goodness theory. Better yet, it's one that's
been thoroughly tested in ergonomics labs around the world.

At this point, we're arguing about the semantics of the word

On Wed, 2 Jan 2013 13:41:54 +0400
MigMit <miguelimo38 at yandex.ru> wrote:
> I don't know about DbC in general, but it's implementation in Eiffel
> seems to be nothing more than a few ASSERT macros, for some weird
> reason embedded into the language.

Either you used a particularly poor implementation of Eiffel, or you
didn't look at the implementation beyond writing them out. Every
Eiffel system I've used included tools that computed the contracts on
a method or class (remember, class invariants apply to subclasses,
etc.) and displayed them. Those are just as much part of DbC as the
"assert macros".

If you ignore that usage, you've correctly described things. At least
as well as saying that a function call implementation is a goto that
records a return address, for some weird reason embedded into the
language. Or <higher level construct> is just <implementation method>,
for some weird reason embedded into the language.

The "weird reason" is pretty much always the same: the construct in
question carries more semantic meaning than the implementation
method. Functions capture the notion of a distinct, reusable chunk of
code, that can have properties all it's own. This is a major step up
from just having a goto variant with an otog that undoes it.

Likewise, pre and post (and invariant) conditions capture the notion
of a contract. They express the terms of the contract implemented by
some specific bit of code. The contract is part of the interface to
that code. If you're actually doing DbC, it's no less important than
the rest of the interface. Like, for instance, the type signature.

Personally, I don't believe in turning off the conditions, for much
the same reason I don't believe in turning off array bounds
checking. I think it's better to get the right answer later than to
get the wrong answer now.

Mike Meyer <mwm at mired.org>		http://www.mired.org/
Independent Software developer/SCM consultant, email for more information.

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