Re: Collective response to := messages

[dsr@hector.UUCP (David S. Rosenblum)]

In article <3702@hubcap.UUCP> wtwolfe@hubcap.clemson.edu writes:
>From article <10913@ulysses.homer.nj.att.com>, by dsr@hector.UUCP (David S. Rosenblum):
>
>   The LCD example is not mine, and I'll leave it to the author of the
>   example to make whatever counterpoints are appropriate.  But with regard
>   to the point concerning enforcement of type checking, I'd like to point
>   out that the example you gave was one in which you did NOT define an ADT;
>   all you did was define a local record type.

I constructed the example that way for clarity.  The arguments are just
as valid if I make the type private in some package.  The declared
objects would still be undefined, and the assignment of 0.0 would still
leave a component undefined.

Your suggestion of implementing ADTs as access types gets around the
problems I mentioned with parameter passing, but allowing overloading of
assignment then requires a great deal of faith (misguided in my view) in
the programmer to implement his or her ADTs in such a safe manner.  I
guess you could argue that many other language features have a similarly
implicit faith in the sanity of the programmer.

>   Another point: the ADT-handling package
>   could leave the denominator as an undefined integer if it jolly well
>   felt like it, as long as it met the requirements imposed by the package
>   specification.  I don't see how that would constitute any violation of
>   type checking.  

The ADT-handling package may feel like leaving a component undefined, but
Ada doesn't feel like leaving variables undefined after an assignment.
That's why such a user-defined assignment violates Ada's strong typing.


-------------------------------------------------------------------
David Rosenblum			UUCP: {ucbvax, decvax}!ulysses!dsr
AT&T Bell Laboratories		ARPA: dsr@ulysses.att.com
600 Mountain Ave.		      dsr%ulysses@att.arpa
Murray Hill, NJ 07974-2070
(201) 582-2906
-------------------------------------------------------------------