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From: mark_biggar@my-deja.com
Subject: Re: Disallowing Pre-Defined Operations
Date: 2000/03/10
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In article <8a9eeg$qtv$1@newpoisson.nosc.mil>,
  claveman@cod.nosc.mil (Charles H. Sampson) wrote:
>      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.)
>
>      As an example of what I'm talking about, consider a package that
> implements three distinct floating-points types for measuring length,
> area, and volume.  The pre-defined "+" and "-" are acceptable and
there
> are obvious redefinitions of "*" and "/" in some cases.  However, the
> pre-defined "/" for operands of the same type don't make sense and it
> would be nice to get a compile-time warning if one of them is used.
> Yes, a redefinition that raises an exception can be written, but
that's
> not the same thing.
>
>      In the case that's bothering me at the moment, my project has a
> type called Time_Type.  (We can't be assured that Ada's built-in time
> types are adequate for our purposes.)  We've already had one fairly
dif-
> ficult bug because a variable of this type was used to hold the
differ-
> ence between two values of the type, known in our environment as a
> delta-time.  Now defining a new type Delta_Time_Type and the set of
Cal- endar-like operations is no big deal, but I have no way of telling
the
> compiler to flag such things as the pre-defined
>
>           function "-" (L, R : Time_Type) return Time_Type;
>
> and the "*", "/", and "**" operations on values of these types.
>
>      Declaring the types as private doesn't work.  The types are
inher-
> ently (and obviously) numeric and nobody on my project would want to
> lose the ability to use numeric literals when appropriate.  (Every
time
> this issue has popped up for me over the last 15+ years, it's been in
> the context of numeric types.)  Whenever time I come up with a way of
> pluging one hole, it opens another.  For example, overloading "-" as
>
>     function "-" (L : Time_Type; R : Float) return Delta_Time_Type;
>
> would allow any value of type Float to be subtracted, not just
literals.
>
>      Does it tell you something about my project when I say that if
the
> above definition of "-" were used, I'm sure somebody would discover
the
> hole and figure out some "clever" way of exploiting it?

Easy: just define the operator as abstract.  E.g.,:

type Length is new Integer;

function "*"(Left,Right: Length) return Length is abstract;
function "/"(Left,Right: Length) return Length is abstract;

-- now we define what we really want

function "/"(left,Right: Length) return Integer;

type Area is new Integer;

function "*"(Left,Right: Length) return Area;

-- etc.

--
Mark Biggar
mark@biggar.org


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