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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Interval Comparisons
Date: Tue, 04 Jun 2024 14:04:15 -0700
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Mikko <mikko.levanto@iki.fi> writes:
> On 2024-06-04 08:58:53 +0000, David Brown said:
>> On 04/06/2024 09:14, Lawrence D'Oliveiro wrote:
>>> Would it break backward compatibility for C to add a feature like this
>>> from Python? Namely, the ability to check if a value lies in an interval:
>>> def valid_char(c) :
>>> "is integer c the code for a valid Unicode character." \
>>> " This excludes surrogates."
>>> return \
>>> (
>>> 0 <= c <= 0x10FFFF
>>> and
>>> not (0xD800 <= c < 0xE000)
>>> )
>>> #end valid_char
>> Do you mean, could C treat "a <= x <= b" as "(a <= x) && (x <= b)" 
>> without breaking existing code?  The answer is no, C treats it as
>> the expression "(a <= x) <= b".  So you would be changing the
>> meaning of existing C code.  I think it's fair to say there is
>> likely to be very little existing correct and working C code that
>> relies on the current interpretation of such expressions, but the
>> possibility is enough to rule out such a change ever happening in C.
>> (And it would also complicate the grammar a fair bit.)
>> 
>> <https://c-faq.com/expr/transitivity.html>
>
> That does not prevet from doing the same with a different syntax.
> The main difference is that in the current C syntax that cannot be
> said without mentioning c twice. In the example program C would
> require that c is mentioned four times but the shown Python code
> only needs it mentioned twice. An ideal syntax woult only mention
> it once, perhaps
>
>  return c in 0 .. 0xD7FF, 0xE000 .. 0x10FFFF ;
>
> or
>
>  return c : [0 .. 0xD800), [0xE000 .. 0x10FFFF] ;
>
> or something like that, preferably so that no new reserved word is
> needed.

Relatedly, gcc has case ranges as an extension, and there's a proposal
to add them to C2Y (Y=6?):
<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3269.htm>

The gcc feature uses the existing "..." token rather than "..".  I'm not
sure whether using ".." would have caused problems beyond the need to
introduce a new token.

One minor issue, whether the feature uses ".." or "...", is that "1...2"
is a valid preprocessing number (and not a valid literal) so
`c in 1...2` would result in a syntax error.  You just need to add
spaces: `c in 1 ... 2` (which I'd argue is a good idea anyway).

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */