Re: [epochs] Proposal for an epoch-based organization of Boost libraries

From: Edward Diener (eldiener_at_[hidden])
Date: 2020-05-25 13:18:16

On 5/24/2020 8:38 PM, Edward Diener via Boost wrote:
> On 5/24/2020 4:15 PM, Joaquin M López Muñoz via Boost wrote:
>> TL;DR: we basically agree on the same tenets, but there's been some 
>> mutual
>> misunderstanding around during our conversation.
>>
>> El 24/05/2020 a las 20:02, Edward Diener via Boost escribió:
>>> On 5/24/2020 12:38 PM, Joaquin M López Muñoz via Boost wrote:
>>>>
>>>> I'm not sure I'm parsing your sentences right, but does that mean 
>>>> you're ok
>>>> with a library using cxx_dual from scracth but not ok if a 
>>>> Boost-reliant lib is
>>>> later updated via cxx_dual *only* to reduce Boost dependencies in 
>>>> C++11?
>>>
>>> No.
>>>
>>> I am ok with a library changing itself to use C++ standard libraries 
>>> instead of their Boost
>>> equivalents in any way it wants, whether using cxx-dual, whether 
>>> creating its own
>>> interfaces for both Boost and the C++ standard equivalent, or whether 
>>> just switching
>>> from using Boost to the C++ standard equivalent. But I am aware that 
>>> such a change
>>> takes work, [...]
>>
>>
>> This work is up to authors: if they are ok with not being given the shiny
>> epoch badge, they can continue with the status quo.
>>
>>
>>> and will displease some end-users. 
>>
>>
>> I fail to see in which situations users would be displeased by 
>> internal dependencies
>> being reduced.
>>
>>
>>> If the Epoch proposal is merely to get a given Boost library to 
>>> create another version
>>> of itself where it uses C++ standard libraries instead of Boost 
>>> libraries so it can be part
>>> of a later Epoch I can see the value of your proposal, but again who 
>>> will do the work
>>> of this transformation ? 
>>
>>
>> Answered above.
>>
>>
>>> But if the Epoch proposal entails forcing a given library to use some 
>>> features of
>>> a C++ standard so that it can be part of an Epoch [...]
>>
>>
>> The proposal does not entail that.
>>
>>
>>> If we are just talking about library dependencies, then a Boost 
>>> library which has
>>> 0 or more dependencies on only C++ standard libraries of a given 
>>> Epoch belonging
>>> to that Epoch is actually fine with me. But this of course means that 
>>> a Boost library
>>> which has no dependencies on either Boost libraries or their C++ 
>>> standard equivalent
>>> libraries of a given Epoch belongs to all Epochs, which is the way it 
>>> should be IMO.
>>> So Boost PP, as an example, belongs to all Epochs.
>>
>>
>> Well, yes, exactly, Boost.PP belongs to all epochs. Another example 
>> (mentioned in
>> the proposal) is Boost.Mp11: no internal dependencies, requires C++11, 
>> not subsumed
>> by the standard yet --> it belongs to all epochs from Boost11 to 
>> Boost20. "Requires C++11"
>> implies "works in C++N" for N>=11.
>>
>>
>>>>> The gist of your proposal which seems to me to be highly flawed is 
>>>>> the idea
>>>>> that at a certain Epoch level all dependencies of a library must 
>>>>> also exist at that
>>>>> same Epoch level or higher. How realistic is such a goal ? If 
>>>>> library X at Epoch level nnn
>>>>> successfully uses library Y at Epoch level nnn-1, why should 
>>>>> library Y be arbitrarily updated
>>>>> [...]
>>>>
>>>>
>>>> I understand you meant "should library X be arbitrarily updated" here.
>>>
>>> No, I meant that library Y should not be arbitrarily updated so that 
>>> library X can be
>>> considered valid for Epoch nnn. The only case I see where library Y 
>>> would need to be
>>> updated, with perhaps another version at Epoch level nnn, would be if 
>>> library Y had
>>> a dependency on a Boost library where an equivalent C++ standard 
>>> library which works
>>> at Epoch level nnn was available.
>>
>>
>> I understand your reasoning now and don't think there's more to 
>> discuss about it
>> now we've clarified that belonging in BoostN implies belonging  in 
>> BoostM (M>N) so
>> long as the lib is not subsumed by the standard. (To be precise, 
>> there's also rejection
>> rule 2, which denies promotion when a new library subsumes the old, 
>> case in point
>> MPL and Mp11).
>>
>>
>>>>> [...]for no good practical reason to use C++ features of Epoch 
>>>>> level nnn ?
>>>>>
>>>>
>>>> The reason is to reduce internal Boost dependencies.
>>>
>>> Adding C++ features at a certain C++ level rarely has to do with 
>>> dependencies as
>>> opposed to the necessities of programming design. If adding a feature 
>>> at a certain
>>> C++ level was able to reduce a dependency of a library to another 
>>> library I would
>>> in general be all for it. 
>>
>>
>> The proposal says nothing about adding features to be granted epoch 
>> promotion.
>>
>>
>>> But just let's take a library that is designed in such a way that it 
>>> does not need
>>> any features of, let's say, C++11. Why would anybody want to add some 
>>> feature
>>> just for the sake of saying that the library is at Epoch 11, when 
>>> such a feature
>>> is unnecessary in the design of the library. This is what I mean by 
>>> "absurdity".
>>
>>
>> I agree this is absurd. This is not what the proposal says.
>>
>>
>>>> [...]
>>>>
>>>> Take for instance Boost.Beast, which depends on boost::string_view 
>>>> but can
>>>> be made to depend on std::string_view instead via 
>>>> BOOST_BEAST_USE_STD_STRING_VIEW.
>>>> Shall I take that you deem this ability absurd?
>>>
>>> Not at all. But take a library which works fine at the C++03 on up 
>>> level, such as TTI,
>>> and has no dependencies on other Boost libraries which can be dropped 
>>> by adding
>>> some arbitrary feature of C++ 11 on up. Why should not this library 
>>> be used by an Epoch
>>> library at the C++11 level if its usefulness is apparent to that 
>>> Epoch library. And why
>>> disqualify that Epoch library from its level just because TTI does 
>>> not use C++11 features.
>>
>>
>> I think the terms of promotion/rejection have been discussed in enough 
>> detail above.
>> As for the particular case of Boost.TTI, it wouldn't enter into 
>> Boost11 because it uses
>> Boost.MPL, which is subsumed by Boost.Mp11 in C++11.
> 
> The MPL library uses no Boost library where a C++11 on up standard 
> library is available. So your "Boost.MPL, which is subsumed by 
> Boost.Mp11 in C++11" does not make sense to me. While MP11 may be a C+11 
> library that is worthy to use, I see nothing wrong with a C++11 on up 
> library using MPL as far as dependencies go.

This is my error. The MPL library does use Boost type_traits where there 
is an equivalent C++ standard type_traits. So I concur that use of MPL 
would place a Boost library like TTI below the C11 Epoch.