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

[Joachim Durchholz <joachim.durchholz@munich.netsurf.de>]

Tucker Taft wrote:
> Eiffel seems to have foregone even
> this bit of declaration-before-use -- there seems no need to
> declare the importing of one class prior to using it anywhere
> inside another.

There are two ways to "import" another class in Eiffel.

Variant 1 is inheriting a class. Inheritance links are declared right at
the start of a class, so here we have (incidentally) a rather strong
"import before use" policy.

Variant 2 is using a class, by declaring a parameter or local variable
of that class. Eiffel does not have an import clause in that case at
all!
This isn't even a disadvantage because an Eiffel module can contain only
one class (in fact there are no modules beyond classes in Eiffel).
Say we have a declaration
  feature dump (x: CONTAINER) is...
so we know x is of class CONTAINER. There is no module CONTAINER is in,
so we don't need an import clause to tell us (or the compiler) where to
find CONTAINER.
If I remember correctly, Ada import clauses are also used to resolve
name clashes, but that's unnecessary in Eiffel, too: anything in
CONTAINER will be used via x, so we always have a qualified call of the
form
  x.something
which automatically provides the necessary context.

Note 1: Note that x is much shorter than CONTAINER. If an Eiffel
"module" is so pervasive that typing the name everywhere becomes a
nuisance, we can choose entity names that are as short as we like to
access these features. In Ada, if we have a name clash, we're forced to
type the full module name wherever a clashing name is used (at least
that's what I remember, corrections welcome!).
Note 2: Inheritance gives access to the imported features without a need
for qualification, so here the possibility for name clashes exists. In
Eiffel you must rename such features in the inheriting class (which is
useful if the features are exported - inheriting two features with the
same name does not mean they are the same).

Regards,
Joachim
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