From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Elimination of "use" clauses
Date: 1999/07/13
Date: 1999-07-13T00:00:00+00:00 [thread overview]
Message-ID: <wccr9mc5y47.fsf@world.std.com> (raw)
In-Reply-To: 7ltus1$ah1@dfw-ixnews19.ix.netcom.com
Richard D Riehle <laoXhai@ix.netcom.com> writes:
> package Sweet_Tooth is
>
> type Frozen_Dessert is private;
> type Lollipop is (OrangeIce, Fudgecicle, Chocolate_Cream);
>
> Procedure Create (F : in out Frozen_Dessert);
> -- more operations
> -- declare an Operators only package
> package Ops is
> function "=" (L, R : Lollipop) return Boolean
> renames Sweet_Tooth."=";
> function ">" (L, R : Lollipop) return Boolean
> renames Sweet_Tooth.">";
I used to like this idea, but I've seen nasty bugs caused by it.
Imagine if that last ">" were replaced with "=" due to a cut-and-paste
error. It's hard to notice such a bug by reading the code, because as
soon as you see "package Ops is" followed by "function ">"", you already
know (or *think* you know) what comes next, so you don't read it
carefully. And when you're reading someplace else that calls "<", you
obviously think it's calling something called "<".
The "use type" clause is safer.
> This package has the advantage of exporting, through a use clause, only
> the operators you want unless you decide to make others explicitly
> visible. The client of the package has an easy way to achieve the
> necessary visibility. It is a better option than use type because it
> only makes visible a restricted set of operators.
Good point. I wouldn't mind having a concise way of saying (eg) "T is
an integer type with just "+" and "-" operators. But I still prefer
"use type" to "package Ops".
>... Also, it allows one
> to design new behavior for some operators while implementing others
> through a simple renames clause.
It seems like a dangerous idea to use anything *but* a renaming, if the
operator in question is predefined. I mean, if Sweet_Tooth."=" and
Sweet_Tooth.Ops."=" do two different things, you're asking for trouble.
Maybe I misunderstood what you mean, here.
>... With Ada 95 you could create a child package for Sweet_Tooth.Ops
> to keep it extensible.
I don't understand that point.
> Ada's use clause, like the #include of the C family of languages,
> is somewhat analogous to wiring your home with uninsulated cable.
That analogy is a bit overblown! If you want to insulate something, put
it in a package body where it belongs. And surely "use" is not in the
same category as "#include".
- Bob
--
Change robert to bob to get my real email address. Sorry.
next prev parent reply other threads:[~1999-07-13 0:00 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
1999-07-01 0:00 Elimination of "use" clauses Dr. Neil C. Audsley
1999-07-01 0:00 ` Joe Wisniewski
1999-07-01 0:00 ` Samuel T. Harris
1999-07-01 0:00 ` czgrr
1999-07-01 0:00 ` Ted Dennison
1999-07-02 0:00 ` czgrr
1999-07-02 0:00 ` Ted Dennison
1999-07-02 0:00 ` Robert Dewar
1999-07-02 0:00 ` Samuel T. Harris
1999-07-02 0:00 ` Robert Dewar
[not found] ` <7ltl2q$mog$1@nnrp1.deja.com>
1999-07-08 0:00 ` Michael F. Yoder
1999-07-09 0:00 ` Robert Dewar
1999-07-09 0:00 ` Michael F. Yoder
1999-07-14 0:00 ` Tucker Taft
1999-07-09 0:00 ` Dale Stanbrough
1999-07-12 0:00 ` Robert Dewar
1999-07-12 0:00 ` Ted Dennison
1999-07-10 0:00 ` Simon Wright
1999-07-12 0:00 ` Robert Dewar
1999-07-09 0:00 ` Richard D Riehle
1999-07-09 0:00 ` Michael F. Yoder
1999-07-09 0:00 ` Marin David Condic
1999-07-08 0:00 ` R. Tim Coslet
1999-07-09 0:00 ` Robert Dewar
1999-07-09 0:00 ` tmoran
1999-07-02 0:00 ` Ted Dennison
1999-07-02 0:00 ` Stephen Leake
1999-07-02 0:00 ` Robert Dewar
[not found] ` <7ltus1$ah1@dfw-ixnews19.ix.netcom.com>
1999-07-13 0:00 ` Robert A Duff [this message]
1999-07-18 0:00 ` Richard D Riehle
1999-07-18 0:00 ` jerry
1999-07-19 0:00 ` Vladimir Olensky
1999-07-20 0:00 ` Richard D Riehle
1999-07-20 0:00 ` Opaque Types (was Elimination of "use" clauses) David C. Hoos, Sr.
1999-07-20 0:00 ` Elimination of "use" clauses jerry
1999-07-18 0:00 ` Dale Stanbrough
1999-07-20 0:00 ` David Kristola
1999-07-20 0:00 ` Richard D Riehle
1999-07-19 0:00 ` Brian Rogoff
1999-07-20 0:00 ` Robert Dewar
1999-07-20 0:00 ` Brian Rogoff
1999-07-21 0:00 ` Ted Dennison
1999-07-21 0:00 ` Robert A Duff
1999-07-21 0:00 ` Robert Dewar
1999-07-21 0:00 ` Robert Dewar
1999-07-21 0:00 ` Brian Rogoff
1999-07-22 0:00 ` Robert Dewar
1999-07-22 0:00 ` Brian Rogoff
1999-07-22 0:00 ` Robert Dewar
1999-07-22 0:00 ` Brian Rogoff
1999-07-21 0:00 ` Robert A Duff
1999-07-21 0:00 ` Michael F. Yoder
1999-07-21 0:00 ` Robert A Duff
1999-07-23 0:00 ` Tucker Taft
1999-08-03 0:00 ` Richard D Riehle
1999-07-19 0:00 ` Ted Dennison
1999-07-19 0:00 ` Tucker Taft
1999-07-19 0:00 ` Ted Dennison
1999-07-02 0:00 ` Ed Falis
1999-07-02 0:00 ` Ed Falis
1999-07-03 0:00 ` Joe Wisniewski
1999-07-03 0:00 ` Ed Falis
1999-07-03 0:00 ` Keith Thompson
1999-07-13 0:00 ` Peter Amey
replies disabled
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox