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From: Jeffrey Carter <jrcarter@acm.org>
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Newsgroups: comp.lang.ada
Subject: Re: how to check if a string variable contains a number or string?
References: <a04a773e.0211181241.4e9e13b0@posting.google.com>
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 <mailman.1037681222.31134.comp.lang.ada@ada.eu.org>
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Date: Sat, 23 Nov 2002 20:05:52 GMT
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Date: 2002-11-23T20:05:52+00:00
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AG wrote:
> Tst, tsk... If you use an existing subprogram that may
> raise an exception - don't you have to write some new
> code to handle that exception? And don't you have to
> provide some sort of recovery perhaps to simulate the
> non-exception version?

Perhaps you've discovered some way to use an existing subprogram without 
writing any "new" code?

More to the point, which is a better use of your valuable resources:

with Parser;
...
begin
    Number := Parser.Parse (Image);
    -- process Number
exception
when Parser.Invalid_Image =>
    Error ("Invalid number: " & Image);
end;

or

type Parse_Result (Valid : Boolean := False) is record
    case Valid is
    when False =>
       null;
    when True =>
       Number : Numeric_Value;
    end case;
end record;

function Parse (Image : String) return Parse_Result is
    -- Many
    -- declarations
    -- for
    -- a
    -- complex
    -- parsing
    -- algorithm
    -- identical
    -- to
    -- Parser.Parse
    -- except
    -- for
    -- error
    -- handling
begin -- Parse
    -- a
    -- very
    -- long
    -- and
    -- complex
    -- algorithm
    -- for
    -- parsing
    -- the
    -- character
    -- representation
    -- of
    -- a
    -- number,
    -- identical
    -- to
    -- Parser.Parse
    -- except
    -- for
    -- error
    -- handling
end Parse;
...
Result : Parse_Result;
...
Result := Parse (Image);

if Result.Valid then
    -- process Result.Number
else
    Error ("Invalid number: " & Image);
end if;

?

-- 
Jeff Carter
"I'm a lumberjack and I'm OK."
Monty Python's Flying Circus