Re: Opacity considered harmful

[Sebastian Egner <sebastian.egner@xxxxxxxxxxx>]

Tony Garnock-Jones wrote:
> Michael Sperber wrote:
>> Without opacity and with reflection,
a client who
gets its hands on a
>> record can get at its fields,
thus breaking the
abstractions provided
>> by the maker of that record.
>
> I see this as an unqualified benefit.
[...]
> Sealedness and opacity have given
me nothing but
trouble, and have on
> occasion cost my clients real money.

This sort of dilemma usually tells me
that one should
think of
other alternatives. So how about this
one?:

Provide opaque records but also provide
clients with a
mechanism to get to the internal structure
by force.
Ideally, the mechanism is highly visible
(syntactically), and too inconvenient
to use 'by
accident'. 

A minimal design for this proposal is
adding
procedures 
    IGNORE-OPACITY-RECORD?
    IGNORE-OPACITY-RECORD-RTD

So the final choice is with the client:
Respect
opacity, or ignore it.

(The only compelling reason I see to
have the final
choice with 
the designer of the record type is security,
and I
agree with
Mike that SRFI 76 is not the beast to
send into that
arena.)

Sebastian.