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From: Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Data_Error and Enumeration_IO
Date: Thu, 12 Jan 2012 16:02:23 -0700
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Date: 2012-01-12T16:02:23-07:00
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On 01/12/2012 03:41 PM, John McCormick wrote:
>
> However, when my students leave out the call to Skip_Line in the
> Data_Error exception handler the behavior depends on the actual value
> entered.  It still works as desired when the input is alphabetic
> characters such as �abc�.  But when the input consists of digits such
> as �123� the loop becomes infinite, repeating the prompt and error
> message.  It appears to me that the call to Choice_IO.Get consumes the
> alphabetic input but does not consume the digit input leaving it
> available for the next call to Get.  I could not find anything in the
> language reference manual about how Enumeration_IO consumes input
> characters when Data_Error is raised.  Can anyone give me an
> explanation?  Is it implementation defined?  I am running GNAT GPL
> 2011.

ARM A.10.10 says of Get, "After skipping any leading blanks, line terminators, 
or page terminators, reads an identifier according to the syntax of this lexical 
element (lower and upper case being considered equivalent), or a character 
literal according to the syntax of this lexical element (including the 
apostrophes)."

Ignoring the part about character literals, it reads characters as long as they 
could be an identifier ("abc", for example; "a123" could also be an identifier). 
If the identifier so read is not a value of the type, you get Data_Error, but 
the characters have been skipped in the input. Input that begins with something 
that could not be the 1st character of an identifier is left to be read again, 
so "123" gives you the infinite loop.

-- 
Jeff Carter
"My legs are gray, my ears are gnarled, my eyes are old and bent."
Monty Python's Life of Brian
81

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