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From: ok@goanna.cs.rmit.edu.au (Richard A. O'Keefe)
Subject: 'Digit is generic package
Date: 1996/08/23
Message-ID: <4vj3e6$hi1@goanna.cs.rmit.edu.au>#1/1
X-Deja-AN: 175952630
organization: Comp Sci, RMIT, Melbourne, Australia
nntp-posting-user: ok
newsgroups: comp.lang.ada
summary: max precision of two generic formal types?
Date: 1996-08-23T00:00:00+00:00
List-Id: <comp.lang.ada>


Some time ago I posted a question but never saw a reply,
so here it is again, in somewhat less detail.

What I *want* to do is

	generic

	    type Coord is digits <>;
	    type Value is digits <>;

	package Foo_Pkg is

	    type Number is digits Max(Coord'Digits, Value'Digits);

	    ...

	end Foo_Pkg;

I _could_ handle this by making Number a generic type too, and then
checking that Number'Digits >= Coord'Digits and Number'Digits >= Value'Digits,
but heck, I could use C++ or Fortran 90, or better still, Common Lisp.

There is another rather horrible workaround that I just thought of,
which could be used for a procedure, but not for a package:

	generic
	    type Coord is digits <>;
	    type Value is digits <>;
	    type Number is digits <>;
	procedure Hidden_Foo(.....);


	generic
	    type Coord is digits <>;
	    type Value is digits <>;
	procedure Visible_Foo(...);

	procedure Visible_Foo(...) is
	    procedure X is new Hidden_Foo(Coord, Value, Coord);
	    procedure Y is new Hidden_Foo(Value, Coord, Coord);
	begin
	    if Coord'Digits >= Value'Digits then
		X(...);
	    else
		Y(...);
	    end if;
	end Visible_Foo;

but it's a bit like a Johnson's "dog walking on its hind legs".

I have read the annotated Ada 95 RM carefully, and I know that when Gnat
rejects the code I want to write it is doing exactly what the standard
requires.  Apart from the problem with Max, there is the unpleasant fact
that Coord'Digits is not a static something-or-other expression, much to
my surprise.  This has the consequence, which I still find, um, quirky,
that

	type Number is new Coord;

is legal inside a generic package like this, but

	type Number is new digits Coord'Digits;

is not.

(a) I would get some consolation from knowing whether this was an *intended*
    restriction for Ada 95 and if so what harm would have resulted had this
    restriction not been in the language.

(b) Is there any better approach for doing this with a procedure than the
    make-to-instances-and-pick-one-at-runtime one shown above?

(c) Is there any reasonable approach for doing what I want with a package?


For what it's worth, the least ugly and most robust workaround I could
come up with was just to do

	type Number is new Long_Float;

and comment it to say what I really want.
-- 
Australian citizen since 14 August 1996.  *Now* I can vote the xxxs out!
Richard A. O'Keefe; http://www.cs.rmit.edu.au/~ok; RMIT Comp.Sci.