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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Bug in Ada - Latin 1 is not a subset of UTF-8
Date: Wed, 19 Oct 2016 19:31:56 -0500
Organization: JSA Research & Innovation
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Date: 2016-10-19T19:31:56-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:nu4nee$18le$1@gioia.aioe.org...
...
> Numeric character is a constraint expressible in Ada:
>
>    subtype Numeric is Character range '0'..'9';
>
> Numeric string constraint is not expressible, but it still a constraint.

It's expressible as a predicate, though; that's the entire point of 
predicates (to act like user-defined constraints):

    subtype Numeric_String is String
        with Dynamic_Predicate => (for all E of Numeric_String => E in 
Numeric);

It's not 100% as good as a constraint (as modifications of individual 
components won't be checked), but it almost always will do the job.

You also could declare a new type with the proper constraint:
    type Numeric_String is array (Positive range <>) of Numeric;

That will have all of the string operations, but it (unfortunately) can't be 
converted to String (you'd have to write a function to do that).

Since both of these possibilities exist, I'd hardly call the constraint "not 
expressible". At worst, it's inconvinient to express it.

                           Randy.