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Date: Fri, 30 Jul 2021 14:45:34 +0100
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Subject: Re: Fwd: Re[2]: [PHP-DEV] PHP 8.1 enum const expressions problem
From: rowan.collins@gmail.com (Rowan Tommins)

On 30/07/2021 13:20, Kirill Nesmeyanov wrote:
> But in addition to bit-masks, there are also at least cases of addition and subtraction (the most popular). Can't we adapt all math expressions existing in nature to specific cases, thereby building our own pseudo-AST?
>   
> (Some::A + Some::B) * Some::C | Some::D;


If you are performing arithmetic on a value, that value is not of an 
enum type, or at least not in the way the current feature defines "enum".

If you write an enum for days of the week, you might give them integer 
values, such that "Day::MONDAY->value === 1" and "Day::SUNDAY->value === 
7". However, expressions such as "Day::SATURDAY + Day::SUNDAY"  and 
"Day::MONDAY * Day::TUESDAY" are clearly meaningless.

At a stretch, a language with operator overloading might define 
"Day::MONDAY + 1" to return "Day::TUESDAY", and "Day::MONDAY + 10" to 
either error or wrap around to "Day::THURSDAY"; but PHP has covered that 
use case by allowing methods, e.g. "Day::MONDAY->advanceBy(10)", which 
is more flexible and arguably more expressive.

On the other hand, the expression "$weekend = Day::SATURDAY | 
Day::SUNDAY" seems reasonable; but we don't need the result to be an 
integer - it would have no meaning on its own - we just need to be able 
to ask things like "is $today in $weekend?" Returning some kind of 
EnumSet object means we retain the type information, and can keep our 
values of 1 to 7 (or have no values at all) rather than having to use 
powers of 2.

Regards,

-- 
Rowan Tommins
[IMSoP]