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From: "Stanley R. Allen" <s_allen@hso.link.com>
Subject: Re: Ada enumerations
Date: 1997/12/10
Message-ID: <348EDDC9.794B@hso.link.com>#1/1
X-Deja-AN: 296986871
Organization: NASA, Kennedy Space Center
Newsgroups: comp.lang.ada
Date: 1997-12-10T00:00:00+00:00
List-Id: <comp.lang.ada>


Matthew Heaney wrote:
> >
> >    function Value_List return Value_List_Type is
> >        Tmp : Value_List_Type (Enum'Pos (Enum'First) ..
> >                               Enum'Pos (Enum'Last));
> >    begin
> If you really insist on such a function, then do this:
> 
> generic
>    type Enum is (<>);
>    type Enum_Rep is range <>;
>    type Enum_Rep_Array is array (Natural range <>) of Enum_Rep;
> function Value_List return Value_List_Type;

This is certainly not a general solution.  Don't
forget: this is part of an interface to be used by
dozens of programmers on a large project with various
kinds of enumeration types.  It's too much of a burden
to make the users choose between two different generics
to instantiate depending on whether their enum type has
special representation or not.

> No, Ada doesn't come with an attribute to return the representation of an 
> enumeration literal, because it doesn't need to!

There are a number of features in Ada that you could
argue with on this basis.  Some of the features are
included for convenience (like 'Max), completeness, or
portability.  My argument for an attribute like
'Representation (or 'Enum_Rep in GNAT) is based on
these factors.  At the risk of seeming shrill, I'll
repeat myself: what good reason can there be for
allowing me to *specify* enumeration representations
but denying me the ability to easily *query* them also?

> And besides, if you're converting between enumeration literals and 
> integers often, you probably shouldn't be using an enumeration type 
> anyway.  Keep things simple, and use an integer type and associated 
> constants of that type.
> 

I would take too long in this forum to explain the
details, but note that working on a large project
often leaves you without the luxury of making that
choice.  In my case, I am not the one who decides
to "use an integer type and constants of that type"
instead of a rep-spec'd enumeration type.  The users
of my interface make that kind of decision as part
of their design process; it's my job to provide an
interface that's general enough to support them,
and simple enough to avoid complex programming on
their part.

-- 
Stanley Allen
mailto:s_allen@hso.link.com