Re: Interface/Implementation (was Re: Design by Contract)

["John G. Volan" <johnv@ac3i.dseg.ti.com>]

Nick Leaton wrote:
> 
>         x: MY_CLASS is
>            once
>               !!Result
>            end
> 
> The first time I use x, Result will be made. The second time I use x,
> the Result from the previous use will be returned. Think of it as a just
> in time singleton.

Suppose such a "once" function is referenced at several different points
in a program. If the compiler were able to predict which of these
references would be the first to occur at runtime, then perhaps it would
be able to perform some optimizations: It could inline the
object-creation code right at the first call, and then turn all the
subsequent calls into direct object references.  

But I don't think it's possible, in principle, for a compiler to make
such a prediction. (My guess is that it's equivalent to the halting
problem.)  So it seems to me that every reference to a "once" function
would have to incur the overhead of a functional call, plus a
conditional test to determine whether the object has already been
created "once" before.  Sounds like a pretty heavy-weight way to
implement global objects.

Contrast this with Ada's scheme: The compiler treats a reference to a
global object the same way it treats any other kind of object reference,
with no extra overhead inserted into the generated code.  But because of
Ada's elaboration-order rules, any such reference is guaranteed (more or
less) to occur after the elaboration of the object.

> Eiffel - Possibly the best language in the world - unless proven
> otherwise.

Hmm. To paraphrase -- who was it? Winston Churchill? -- perhaps you
should say "Eiffel is the worst language on Earth ... except for all the
others!" :-)

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