Re: Syntax for tagged record types (was Re: Not intended for use in medical,)

["John G. Volan" <johnvolan@sprintmail.com>]

Robert A Duff wrote:
> 
> In article <mheaney-ya023680002105972302430001@news.ni.net>,
> Matthew Heaney <mheaney@ni.net> wrote:
> >I think that using discriminants as "constructor arguments" is a real
> >kludge, and obfuscates declarations. ...
> 
> Why?
> 
> - Bob

If the hypothetical constructor only took discrete or access-type
parameters, then faking those "parameters" as discriminants on a limited
type is okay. But if they have to be any other kind of type, you've got
a problem.  Faking a parameter of type "T" as a discriminant of type
"access T" only leaves the programmer with a dilemma: Who is responsible
for deallocating the designated T object?  The answers might be:

(1) The object with the discriminant. (It "owns" the designated T
object, so it should deallocate it on Finalize).

(2) The client who passed the T into the discriminant. (It may have
passed the same T access value as a discriminant to several objects, so
it should deallocate the designated T object only once all those other
objects are finalized).

(3) Nobody. The T object was never allocated on the heap in the first
place. It was aliased, and 'Access was passed to the discriminant.

This doesn't make for very clean abstractions. Choice (1) might be the
most attractive from an abstraction point a view, but it requires that
clients be highly circumspect in their behavior towards those access
discriminants (they must always do a "new" but never a 'Access, and they
must never do Unchecked_Deallocation) but this restriction is not
enforceable by the compiler.

Another issue with discriminants as constructor parameters is that there
is no provision for having multiple overloaded constructors with
different sets of parameters.

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