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From: holden@cca.UUCP (Russ Holden)
Newsgroups: net.lang.ada
Subject: Re: ForTran-Ada + flamette + question
Message-ID: <8129@cca.UUCP>
Date: Fri, 30-May-86 08:46:42 EDT
Article-I.D.: cca.8129
Posted: Fri May 30 08:46:42 1986
Date-Received: Wed, 4-Jun-86 00:06:25 EDT
References: <4700034@ada-uts>
Organization: Computer Corp. of America, Cambridge
List-Id: <comp.lang.ada>

> 
> The following includes a suggestion for solving your problem, along
> with my own defense of the decision to omit procedure-pointers from Ada:
> 
> The first solution I can think of (and, I guess, the simplest?) would
> be to define an enumeration type, where each enumeral (is that a
> word?) corresponds to one of the functions you'd like to bind to a
> key-sequence.  Then just write a procedure that takes such an enumeral
> as an "in" parameter and then "case"s on that parameter to the
> appropriate call.
>
> Anyway, my point is that I think that one can do all that you can do in
> C in Ada by using the above enumerals-to-subprogram mapping method; I'd
> be interested if people can provide counter-examples (I realize that
> you'd have to do more if the procedures you call take different numbers
> of arguments, but then again, you'd have to be more fancy in C, too).


Rich, you are missing out on the beauty of procedure passing.  Simply put
it gives you the ability to dynamically bind actions without the invoker
of the procedure having to know about the procedure being called.  A 
couple of examples are:

1) Table-driven code - Using C all you have to do is add a procedure
   to a table.  Using the ADA enumerated-to-procedure mapping code
   modifications are involved (see below).

2) It provides the ability for higher levels of software to abstract
   procedures from lower levels such that lower levels call but do
   not "use" these procedures (in Parnas terminology).  For example,
   (and this is not hypothetical) is a DBMS which wants to perform
   qualification of records against user-specified predicates before
   retreiving them (copying them off their page essentially).  Since
   the code performing page level manipulations is generally at a much
   lower level in processing than the code which understands about
   the values in records a procedure made available to the page level
   is an appropriate mechanism.  The use of enumerated types does no
   good and there are no real good solutions that I know of in ADA
   (if anyone has any please send me mail).

   I have used procedure pointers in lots of other situations where
   a high-level wants or needs to provide a service to a lower level.
   The only requirement is that a "standard" invokation mechanism be
   defined.
-- 
Russell Holden
Computer Corporation of America
Four Cambridge Center
Cambridge, MA 02142