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From: richw@ada-uts
Newsgroups: net.lang.ada
Subject: Re: ForTran-Ada + flamette + question
Message-ID: <4700034@ada-uts>
Date: Wed, 28-May-86 11:54:00 EDT
Article-I.D.: ada-uts.4700034
Posted: Wed May 28 11:54:00 1986
Date-Received: Fri, 30-May-86 08:34:59 EDT
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Nf-From: ada-uts!richw    May 28 11:54:00 1986
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.

Abstractly, this new function is similar to "eval" in Lisp.  Now, instead
of binding characters sequences to function-pointers (as in C), just bind
the character sequences to enumerals.

Other comments about why I think the ability to pass procedures around
was omitted from Ada: (note that these are just opinions -- I'm not
quoting one of the design documents for Ada or anything like that)

In a language like Lisp, you can actually create procedures AT RUN-TIME
since they're just lists (i.e. data).  However, in C and Ada this isn't
true.  In C, all procedures that you can point to at run-time are defined
at compile time.  I think this last fact is an important one to consider;
the above method of mapping enumerals to subprogram calls works for Ada.
It wouldn't work in Lisp because you can't list all of the subprograms
that will exist when you run the program BEFORE you run it (because you
can build Lisp procedures "on the fly"); thus, you can't define an
enumeration type since that type has a finite number of compile-time
enumerals.

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).

If so, then omitting the ability to pass procedures around in Ada saved
the language designers from adding more syntactic and semantic and
run-time constructs to the language to support this.  Considering
the fact that Ada is already big, I think this was a wise decision --
especially since, with a minimal amount of work, you CAN do what you
want (using the enumeral-to-subprogram method).

Think about it -- strict type-checking becomes much more messy...
It is possible to add this to your language (e.g. CLU), but I personally
don't think it's worth complicating your language.  Even though C lets
you do it, C BY NO MEANS does type-checking of calls to procedures which
were passed in as arguments (it doesn't even check that the number of
arguments is correct).  Yes, C is an "unsafe" language...

Comments?  Remember, these are casual remarks and opinions -- please
think before you flame  :-)

Rich