Re: [boost] New dependency report

Subject: Re: [boost] New dependency report
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2014-06-09 05:31:48

El 09/06/2014 7:12, Stephen Kelly escribió:
> Ion Gaztañaga wrote:
>
>> El 09/06/2014 0:24, Andrey Semashev escribió:
>>> On Sunday 08 June 2014 23:53:41 Ion Gaztañaga wrote:
>>>>
>>>> TypeTraits would not be MPL
>>>> dependant. MPL could depend on TypeTraits and Core.
>>>
>>> mpl::bool_ is not the only thing TypeTraits uses from MPL, so this
>>> dependency won't go that easily. This is arguably true for other
>>> libraries as well. Pulling more things from MPL to Core will make it
>>> heavier. I'd like to keep MPL stuff localized and IMHO Core is not the
>>> right place for it.
>>
>> Sorry, I missed the rest of dependencies. The circular dependency
>> between MPL and TypeTraits doesn't sound very good from a a design point
>> of view.
>
> If separate modules are so tightly coupled that they are inseparable, they
> are "incidentally" a single module (think 'incidental data structures').
> Neither can exist without the other. Typeof and Core also participate in a
> closed dependency loop with MPL and TypeTraits. The four form one
> 'incidental module'.
>
> You could decide whether absence of incidental modules is a design goal. If
> it is, then you can merge those four together when it is no longer possible
> to remove edges between them.

That would remove the number of dependencies to one giant dependency. 
Not very useful to reduce coupling and compilation times. Using 
mpl::and_/mpl::not_ instead of "&&" and "!" in TypeTraits does seem 
avoidable. Only CommonType seems a bit more complicated to break from 
MPL. But it's relatively easy to implement without MPL. Ideally in my mind:

MPL --- depends on --> TTraits--- depends on --> (Traits)Core.

Or even:

MPL --- depends on --> TTraits--- depends on --> (Traits)Core.

I think all TypeTraits dependencies on MPL are avoidable and makes Boost 
more layered and maintainable. Makes a lot of Boost libraries directly 
or indirectly independent form MPL. I haven't studied Typeof source code 
so I can speak about its dependencies.

However, breaking dependencies will need code changes in several 
libraries, so we must do this with care.

Best,

Ion