Re: Interval Comparisons

On 04/06/2024 11:13, Mikko wrote:

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.
 

Sure, you can always add new things to a language if they would previously have been syntax errors or constraint errors.  But is there a use for it?
It is fine if you have a language that has good support for lists, sets, ranges, and other higher-level features - then an "in" keyword is a great idea.  But C is not such a language, and that kind of feature would be well outside the scope of the language.
It would be easy enough to write a macro "in_range(a, x, b)" that would do the job.  It is even easier, and more productive, that you simply write the "valid_char" function and use it, if that's what you need.