Split Capabilities: Making Capabilities Scale

[Norman Hardy]

At 10:08 -0700 00/07/27, Karp, Alan wrote:
>There seems to be some confusion as to what I'm talking about.  There is an
>object O with some interface.  There are facets of O that represent
>thinnings of that interface.  I agree there is no problem if the programmer
>of some other object knows the interface to O.  If the programmer only knows
>a facet of O, he may not be able to correctly code his object.
>
>In your example, the programmer knows the interface to the garage, in
>particular that it has an increment and a decrement method.  Could the
>programmer have produced a correct garage object knowing only about the
>decrement method?  Of course not.  Could two programmers, one knowing only
>about the increment method and the other knowing only about the decrement
>method produce an entrance object and an exit object, respectively, that
>someone else could combine into a correct garage?  Perhaps, but it is likely
>that the entrance and exit codes would be based on invalid assumptions.  For
>example, the entrance object may assume that a garage that becomes empty is
>an error because its programmer sees no way for the count to decrease.

I think that my example was too vague.
I had intended that there were just two programs GS (Garage System) 
and CCM (Car Counting Module). The GS program is responsible for 
ensuring the correctness of the value hidden in the counter object. 
The programmer of GS knows the entire semantics of the counter object 
and also the garage layout. The programmer of CCM knows only that his 
program will have access to a 0 argument procedure that his program 
is to invoke once per observed car. CCM and perhaps GS define 
instantiable objects and that several instances of CCM will be 
created, most likely by GS. Upon startup GS creates one counter 
object C. GS then creates an instance of CCM for each portal of the 
garage. If a particular CCM controls an entrance its creation is 
parameterized with the increment facet of C. Those controlling exits 
get the the decrement facet of C.

The remaining facet to learn the counter value is dispatched to the 
sign driving program. Note that the sign driving program will not 
come under suspicion if the count is in error.

Which programmer has insufficient information?

>   At any rate, the routine doesn't have any
>externally visible state, so it's not relevant to the present discussion.

Agreed.
-- 
Norman Hardy  <http://cap-lore/>