Ada question: access types

[seas.gwu.edu!nreich@uunet.uu.net (Norman Reich)]

Hi!  I am a new user to Ada (about 2 weeks), and new to
this newsgroup as well (about 2 minutes).  It was suggested
that I try out this newsgroup to seek help with a problem
that I was having.  I hope that I'm not misuing the group.
Anyway, here is my question:

I am using an IBM PS/2 with Meridian Ada, although I don't
think what I'm doing should be compiler dependent.

NOTE: Everything below is simplified to get my point across.

I have the following definitions:

     TYPE ptr IS ACCESS node;
     TYPE node IS
       RECORD
         name ...
         ...
         nextn :ptr;
       END RECORD;

     nl :ptr;

Within the program, I am doing a comparison, to see if the
list is empty or not:

     if (nl = NULL) then          --error at this line
     ...

The compiler is then giving me the following error at the
line above:

     type clash in parameter to "="

I have no idea why it would be doing this, and in examples
I have seen similar things done.  If anybody has any suggestions
that would be helpful, I would really appreciate it.

Thanks.

- Norman Reich
  nreich@seas.gwu.edu
  (301) 937-4858