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Re: redeclaration of enumerators?

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On 29/09/2021 16:03, Bart wrote:
> On 29/09/2021 14:36, David Brown wrote:
>> On 29/09/2021 00:31, Bart wrote:
>>> On 28/09/2021 22:57, Keith Thompson wrote:
>>>> John Bode <jfbode1029@gmail.com> writes:
>>>>> On 9/27/21 12:39 PM, Thiago Adams wrote:
>>>>>> I was wondering if we have some reason to
>>>>>> disallow redeclaration of identical (name/value) enumerators.
>>>>>> enum E1 {  A = 1 };
>>>>>> enum E2 {  A = 1 };
>>>>>
>>>>> Unlike structs and unions, each enum type is not its own
>>>>> namespace and enumeration constants belong to the "ordinary
>>>>> identifiers" namespace.
>>>>>
>>>>> The question is how you would disambiguate the two definitions
>>>>> of A - there's no equivalent to the '.' or '->' operators for
>>>>> enums.  There's no way to easily say "I'm talking about the A
>>>>> defined for enum E1, not the A defined for enum E2".
>>>>>
>>>>> Can't rely on type inference in assignments; enums are just an
>>>>> incredibly weak abstraction in C. There's no range checking
>>>>> such that you can only assign one of the defined enumeration
>>>>> constants to an object of that enum type.
>>>>>
>>>>> You'd either need to introduce a C++-style scoping operator like
>>>>> E1::A or E2::A, or you'd need to radically revamp and strengthen
>>>>> enum semantics.  And in the process you'd likely lose some capability
>>>>> that people find incredibly useful, such as bitwise-ORing enumeration
>>>>> constants together.
>>>>>
>>>>> enums in C are little more than a way to create symbolic constants for
>>>>> integer values.  They are not really *enumerated types* like you
>>>>> find in other languages.
>>>>
>>>> I think the suggestion is to allow duplicate names only if both the
>>>> name
>>>> and the value happen to match.  Thiago's example:
>>>>
>>>>       enum E1 {  A = 1 };
>>>>       enum E2 {  A = 1 };
>>>>
>>>> would be valid, but changing one of the values:
>>>>
>>>>       enum E1 {  A = 1 };
>>>>       enum E2 {  A = 2 };
>>>>
>>>> would result in a constraint violation.  (I presume the same would
>>>> apply
>>>> if the values are chosen implicitly.)
>>>>
>>>> There's no fundamental reason this couldn't be done, but I don't think
>>>> it's useful enough to justify a language change.  If you want two enum
>>>> types to share values, you can define them as a single type.
>>>>
>>>> (C++ does have an "enum class" feature which, among other things,
>>>> allows
>>>> for duplicate names, but then you always need to refer to "E1::A"
>>>> whether it's ambiguous or not.)
>>>
>>> That's surprising. With stricter typing, you think it would figure out
>>> that the A here:
>>>
>>>      enum E2 x = A;
>>>
>>> needs to be E2::A.
>>>
>>
>> It is not remotely surprising - in order to be able to use identifiers
>> within a nested code structure (I'm using that as a general term, not
>> just "struct") of some sort, you need to be "inside" the structure, in
>> the scope of a "using" clause, or you need to give the qualified name.
> 
> Yeah, you're right. It would be tricky anyway, because it can't be
> sorted using normal name resolution rules, since it relies on type info
> which may not be dealt with until a subsequent stage.
> 
> But also, you could have this:
> 
>    enum E2 {A=1, B, C};
>    enum E2 A, B;
> 
>        A = E2::C
>        B = A;
> 
> Is the RHS the variable A (which contains 3), or is it intended to be
> E2::A, which is 1?
> 
> (This makes me feel better about my own stuff, which can have 'closed'
> enums like C, but they need qualifying too:
> 
>   enum colours = (red, green, blue)
>   enum lights  = (red, amber, green)
> 
>   colours A = colours.green        # can't use A = green)
> 
>> Maybe new parsing rules for C++ can be figured out that would allow "E2
>> X = A;", or at least "E2 x { A };".  I don't see how it could easily be
>> done, however, without a lot of complications and side-effects.  This
>> isn't an ad-hoc language where you can just make up the rules as you go
>> along without a care of how they affect other parts of the language.
> 
> 
> I don't make up ad-hoc languages; they have to work. You can only cut a
> few minor corners.
> 
> But I understand parsing C++ can requires some ingenuity anyway, because
> the language is so complex and with unexpected and ambiguous
> interactions between features.
> 

The aim is to /minimise/ the unexpected or ambiguous interactions (for
those that have learned the language) - that is one of the reasons why
changes are so hard to make in this area, as you have to get experts to
think of all the weirdest corner cases.

I checked on the "using enum" in C++.  It was added in C++20, and lets
you write:

	enum class colours { red, green, blue };
	colours my_favourite_colour;

	my_favourite_colour = colours::red;
	using enum colours;
	my_favourite_colour = blue;

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redeclaration of enumerators? Thiago Adams <thiago.adams@gmail.com> - 2021-09-27 10:39 -0700
  Re: redeclaration of enumerators? John Bode <jfbode1029@gmail.com> - 2021-09-28 16:21 -0500
    Re: redeclaration of enumerators? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-28 14:57 -0700
      Re: redeclaration of enumerators? Thiago Adams <thiago.adams@gmail.com> - 2021-09-28 15:23 -0700
        Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 15:23 +0200
        Re: redeclaration of enumerators? Mark Bluemel <mark.bluemel@gmail.com> - 2021-09-30 01:15 -0700
      Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-09-28 23:31 +0100
        Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 15:36 +0200
          Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-09-29 15:03 +0100
            Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 16:46 +0200
              Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-09-29 16:08 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 20:14 +0200
                Re: redeclaration of enumerators? Thiago Adams <thiago.adams@gmail.com> - 2021-09-29 13:37 -0700
                Re: redeclaration of enumerators? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-29 18:25 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-30 12:45 +0200
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-09-30 07:36 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 09:20 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 11:24 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 04:49 -0700
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 13:06 +0100
                Re: redeclaration of enumerators? Tony Oliver <guinness.tony@gmail.com> - 2021-10-01 05:24 -0700
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 13:32 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 05:32 -0700
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 13:43 +0100
                Re: redeclaration of enumerators? Mark Bluemel <mark.bluemel@gmail.com> - 2021-10-01 06:13 -0700
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 06:29 -0700
                Re: redeclaration of enumerators? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-10-01 15:30 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 08:28 -0700
                Re: redeclaration of enumerators? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-10-01 17:08 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 15:12 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 15:37 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 15:30 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 19:12 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 19:06 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 15:32 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 15:38 +0100
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 17:40 +0100
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-02 12:16 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-02 15:34 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-02 15:16 +0100
                Re: redeclaration of enumerators? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-01 13:58 -0700
                Re: redeclaration of enumerators? Richard Damon <Richard@Damon-Family.org> - 2021-10-01 17:21 -0400
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 14:56 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-02 12:21 +0200
                Re: redeclaration of enumerators? scott@slp53.sl.home (Scott Lurndal) - 2021-09-30 14:47 +0000
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-09-30 10:10 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 09:24 +0200
            Re: redeclaration of enumerators? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-09-29 12:43 -0700
    Re: redeclaration of enumerators? Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-29 03:40 +0000

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