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Newsgroups: comp.lang.ada
Subject: Re: Rational for not making cursor tagged in Containers
From: Georg Bauhaus <bauhaus@futureapps.de>
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Date: Fri, 20 Apr 2007 12:56:23 +0200
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Date: 2007-04-20T11:54:45+02:00
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On Fri, 2007-04-20 at 09:54 +0200, Dmitry A. Kazakov wrote:

> In a better language any type could be able implement record interface:
> 
>    type Foo is interface record  -- This not an implementation!
>       Mumble : Integer;
>    end record;
> private
>    type Foo is new Float; -- Implements the record interface
>    function "Mumble" (X :  Foo) return Integer; -- Getter
>    procedure "Mumble" (X : in out Foo; Value : Integer); -- Setter

C# and Eiffel do it this way; I'm almost sure there is a bunch of
Lisp macros achieving the same. But I think approaching simple records
like this will be a huge mistake in a language like Ada, for the
following reason: When a user of your type see the above definition

   type Foo is interface record ...

it means that he or she will *have* to look at the implementation
(or hope for extensive documentation): There is no
way of knowing whether reading or writing a record component will
just move some bits, or try to compute the first prime number of
the 1969 lottery on the way.

Having programmers implement record abstractions looses all
predictability that the LRM can otherwise provide for simple
records. We need some simple, fixed, stupid things outside
the control of programmers. (More politely, leave some
things to a compiler.)
 And, considering Controlled, forcing *all* types to be derived
from this base suffers from the same effects for the same reason.

Not every programming team consists of real or even wannabe
language designers. But who else has even got a chance of knowing
what they are doing to a larger program when implementing the
concept of record?