Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!unmvax!brainerd
From: brainerd@unmvax.unm.edu (Walt Brainerd)
Newsgroups: comp.lang.fortran
Subject: Re: dpANS Fortran 8x
Summary: The semantics are described
Message-ID: <147@unmvax.unm.edu>
Date: 15 Jun 89 03:56:26 GMT
References: <143@unmvax.unm.edu> <13938@lanl.gov>
Distribution: usa
Organization: University of New Mexico at Albuquerque
Lines: 61

In article <13938@lanl.gov>, jlg@lanl.gov (Jim Giles) writes:
> >> > REAL, DIMENSION (100, 100), TARGET :: A
> >> > REAL, DIMENSION (100), POINTER :: P
> >> >    . . .
> >> > P => A (50, :)  ! P IS AN ALIAS FOR ROW 50 OF A
> >> 
> >> In order to implement this, the pointer must actually be (what used to be
> >> called) a 'dope vector'.
> > 
> > Exactly right!  And the standard requires that this be implemented!
> 
> Since Walt is on the committee (or was - I don't know his present status),

He is still on it.

> I will assume he is right about the interpretation of the standard.  I was

Not necessarity a good assumption, in general!

> only pointing out that the present proposal, as written, does _not_ give
> any _semantic_ interpretation for the above example.  At least to my mind,
> a comment on a example statement does _not_ constitute an adequate definition
> of the semantics of a programming language feature.  If they intend for a
> POINTER value to actually be an entire array descriptor or 'dope vector',
> then they should say so explicitly and fully describe what that means.
> Failure to do so indicates to me that the meaning of POINTER is not well
> defined.
My example doesn't provide the semantics, but the standard does.
The syntax is
     pointer-name => target
An instance of a target, if you follow thru the syntax, is an array section,
as in the example above.  The semantics are given by:

    "A pointer assignment statement associates a pointer with a target."

The term "associated" is widely used in the standard; very roughly,
it means they are aliased.  In the example above, this means that I can
      PRINT *, A (50, 23)
or
      PRINT *, P (23)
with the same result.

The standard cannot suggest an implementation, such as a dope vector;
it can only describe what must happen.
> 
> Of course, if POINTER is supposed to be defined as Walt claims, it isn't
> really a pointer.  So it's misleading to people who know pointers from
> other languages.  Furthermore, it needlessly complicates the semantics
> of pointers when they are used in more conventional roles (recursive
> data structures, for example).

I don't follow this.  It simply means it can point to more complex
objects than in many languages.  I also don't see how it complicates
things when pointers are used to implement recursive data structures;
if you have "nodes" in a linked list consisting of an integer (say)
and a pointer to the next node, then, thinking truly of recursive
data structures, the pointer is just an alias for the rest of the list.
So a list of integers is either empty or a) an integer and
b) a list of integers (i.e., the pointer component).

Walt Brainerd, Unicomp, Inc. brainerd@unmvax.unm.edu