Re: Disallowing Pre-Defined Operations

[Keith Thompson <kst@cts.com>]

claveman@cod.nosc.mil (Charles H. Sampson) writes:
>      During the deliberations that led to Ada 95, was a mechanism for 
> disallowing the pre-defined operations of a type considered?  By "disal-
> lowing" I mean some way of informing the compiler that an attempt to use 
> a certain pre-defined operation is a compile-time error.  Did anyone 
> even ask for it?  (Obviously I didn't, even though I've thought since 
> the mid-eighties that it would be a useful capability to have.)

That seems like a good idea, but it would cause problems for generics.
If you forbid "*" on a floating-point type, you can't use it to
instantiate a generic that takes a floating-point formal type.  This
includes Float_IO (which very likely uses "*" and/or "/" internally).
It also introduces some conceptual problems with the built-in
attributes like 'Image and 'Value, though I suppose the language
definition could just ignore these problems.

It's too bad you can't define literals for a private type, but you can
come close by overload the unary "+" operator.  For example:

    type Time_Type is private;
    type Delta_Time_Type is private;
    function "-"(Left: Time_Type; Right: Time_Type) return Delta_Time_Type;
    function "-"(Left: Time_Type; Right: Delta_Time_Type) return Time_Type;
    -- other appropriate operations
    
    function "+"(Right: Float) return Time_Type;
    function "+"(Right: Float) return Delta_Time_Type;

You can then use:

    T : Time_Type := +123.45; -- or whatever
    T_Minus_1 : Time_Type := T - (+1.0);
    
I haven't tried compiling this, so I'm not certain it will actually
work, and I may be missing some obvious flaw.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center           <*>  <http://www.sdsc.edu/~kst>
Welcome to the last year of the 20th century.