Re: One type for all

[Robert A Duff <bobduff@world.std.com>]

Robert Dewar <robert_dewar@my-deja.com> writes:

> In article <yeciu7hg0oo.fsf@king.cts.com>,
>   Keith Thompson <kst@cts.com> wrote:
> > ] Two types are said to be _compatible_ if any of the
> > following four conditions is true.
> > ]
> > ]     (a) They are the same type.
> > ]     (b) One is a subrange of the other, or both are
> subranges of the
> > ]         same host type.
> > ]     (c) Both are set types, their base types are compatible,
> and either
> > ]         both are packed or neither is packed.
> > ]     (d) Both are string types with the same number of
> components.
> 
> So thanks, it is indeed the case that Pascal does NOT have
> structural type equivalence, which is what I remembered (any
> other conclusion would have been a big surprise!)
>
> OK, so those are VERY restictive cases. (a) and (b) correspond
> closely to the Ada rules, and (c) and (d) are very special
> cases, similar in spirit indeed to the restricted Ada rules.

In the case of scalars, the rules are not at all like Ada -- in Pascal,
all integers may be freely intermixed (except in certain special cases
like passing a component of a packed array as a VAR parameter).  That
is, there is nothing in Pascal equivalent to Ada's "type T is range
1..10;".  Pascal only has the equivalent of "subtype S is Integer range
1..10;".

For records, you're right -- Pascal uses by-name equivalence.

I'm confused about pointers: I thought in:

    type T = ^Integer;
    var
        X: ^Integer;
        Y: ^Integer;

X and Y could be freely assigned to each other, and passed to a
parameter of type T.  No?

Also, note that Keith quoted from a book that came out after the Pascal
standard.  The original Jensen and Wirth book was rather unclear about
type equivalence / compatibility / etc, and was roundly criticized at
the time for it.  And of course compiler writers did what they liked;
the Pascal culture never developed a great deal of respect for standards
and compatibility.  (I even once used a Pascal compiler in which
identifiers were case sensitive!  The authors were C programmers, and
they did it the way they thought it ought to be done.)

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.