Re: Procedure types and dynamic binding

[billwolf@hubcap.clemson.edu (William Thomas Wolfe,2847,)]

From article <5804@medusa.cs.purdue.edu>, by rjh@cs.purdue.EDU (Bob Hathaway):
>> In more complicated examples parameterizable operations for each Adt 
>> instance could simplify the program considerably, such as by avoiding
>> nested case statements.
>>
>>> [...] we must provide MAKE_*_PRINT, where * represents an indefinite set
>>> of cases.  We now see that our old friend, the case statement, is
>>> not gone; it has simply taken on a new form. 
> 
> This is not entirely correct, since I commented another more general solution.

     Then let's have a concrete example of this other, more general solution.

     Be generous with the comments.

% Also, it appears unrestricted pointers to subprograms, if thats what
% you propose, would provide dynamically checked operations at runtime.
% This mechanism is more powerful than statically checked procedural
% variables.  

     No, I haven't yet been convinced that subprograms should constitute
     a separate type; my current position is that subprograms should only
     be a means by which ADTs and tasks (think of a "main program" as an
     isolated task...) can be specified.  If you can convince us that there
     is a legitimate need for a subprogram type, then variables of the type
     and pointers to objects of the type will follow automatically.  

> Mats Weber writes:
>>I think there is one good reason why procedure types should not be included in
>>the Ada language: A secure implementation of procedure types must restrict
>>their use to global procedures (that is, procedures that are not nested in
>>other procedures or tasks), as is the case in Modula-2.

     Quick question: assuming the idea of a specification is expanded to
     include all externally accessible objects, what is the source of
     the insecurity?

% Graphical environments provide one example where dynamically parameterizable
% objects can be of great assistance.  One could parameterize graphical objects 
% with operations for displaying output, windows, etc., with each instance using
% the correct subprogram to implement its display operations.

     Specific examples, please...



                                           Bill Wolfe

                                    wtwolfe@hubcap.clemson.edu