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From: db@lfcs.ed.ac.uk (Dave Berry)
Newsgroups: comp.lang.eiffel
Subject: Re: Multi-branch instructions and unique values (Version 2.2 preview)
Keywords: Case, Enumeration
Message-ID: <1638@etive.ed.ac.uk>
Date: 24 Mar 89 17:43:00 GMT
References: <122@eiffel.UUCP>
Sender: news@etive.ed.ac.uk
Reply-To: db@lfcs.ed.ac.uk (Dave Berry)
Organization: Laboratory for the Foundations of Computer Science, Edinburgh U
Lines: 45

In article <122@eiffel.UUCP> bertrand@eiffel.UUCP (Bertrand Meyer) writes:
>
>   For  test  constants  declared  explicitly  (not
>``unique''), a notational facility is offered: a sequence of
>consecutive explicit values, such as 3, 4, 5  or  'a',  'b',
>'c',  'd'  may be abbreviated as an interval, written in the
>form min..max, as in one of the following:
>
>   3..5
>
>   'a'..'d'

Would it not be possible to extend this notational fcility to unique
constants declared in the same declararion?  To use Bertrand's example:

>   blue, white, red: INTEGER is unique;

If this notation were taken to mean that the unique values given to
blue, white and red must be consecutive, tests could be made against the
ranges  blue..white,  white..red  and  blue..red.  In a small example
like this, it isn't too useful, but it might be useful with a larger case.

Alternatively, since all unique constants in a test must be defined in
the same class, one could define all unique constants defined in the
same class to be consecutive in the order in which they were defined.
Then the above declaration would just be an abbreviation, as before.


A further note on else clauses: If you have user-controlled exceptions,
one can use an else clause to raise an exception suitable to the
particular test statement which fails to match.  This can be more useful
than raising a system exception.

I know Bertrand has accepted Erland's arguments for an else clause;
but I think this use for them is worth mentioning anyway.

(The references I have on eiffel are strongly against exceptions,
so I presume they're out of date!  I'm currently trying to get hold of
the eiffel book; I apologise if I've made any inaccurate assumptions
in the above.)


Dave Berry,	Laboratory for Foundations of Computer Science, Edinburgh.
		db%lfcs.ed.ac.uk@nss.cs.ucl.ac.uk
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