Re: [boost-ext] DI questions

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2021-02-20 02:09:49

On 19/02/2021 22:20, Krzysztof Jusiak via Boost wrote:

> I hope that the examples and explanation helps a bit, but if not I'm also
> more than happy/available to have a follow-up (via zoom or similar) if that
> helps (feedback could be used to improve the documentation)?

Kris I speak from experience with Outcome that you're going to have to 
be much more hand holdy than that description you just gave. You'll need 
to explain things in small, baby steps, because DI is one of those 
things that nobody understands until the lightbulb switches on, and then 
they totally get it.

Given that Mr. Dimov and several others have indicated they don't 
understand the motivation behind DI, if you want to see DI get into 
Boost, you're going to have to write a lot of personalised responses to 
people and keep repeating yourself to each individual in turn until 
people twig what DI is and why it's useful. I did this for Outcome, and 
whilst it was laborious at the time, I think it paid off in the end for 
everybody here.

(Incidentally, I chose DI to review manage for a reason, I think this is 
the first vocabulary library we've seen presented here since Outcome. I 
am *very* much hoping its review isn't going to turn into thousands of 
emails ...)

Andrzej, I'll give you my description of DI (which is not that of 
Boost.DI). For certain kinds of problem solution, it can make sense to 
turn inside out the traditional design of a C++ program i.e. place the 
innards on the outside, and put what is normally outside into the 
innards. You thus get an "inverted design" program.

Normally that's a bad idea, because maintenance is harder, it's anti 
social to other engineers, and so on. Also, in C++, we typically use 
template techniques such as CRTP for DI, we don't typically use 
polymorphic techniques in this, but that's peculiar to C++ and not what 
most languages do because they don't have powerful enough compile time 
facilities.

For complex use cases, or where a fixed ABI is absolutely needed, 
template based techniques are too painful to use to do DI, and for this 
niche use case, a common runtime framework such as that of Boost.DI lets 
arbitrary pieces of DI based code interoperate with one another in a 
composable fashion.

This use case is probably even more niche than that for Outcome, which 
maybe 15% of C++ developers would ever need to consider using. For 
non-template DI, I would estimate less than 5% of C++ developers would 
ever need to consider using it.

*However*, for where you do actually need runtime polymorphic DI, there 
are very few good solutions to hand in C++, unlike in other major 
languages. I think proposed Boost.DI could really fill that empty gap, 
and for those few who need that gap filled, they _badly_ need it filled. 
A standardised solution in this space would be particularly valuable.

Just to be clear, I was talking about DI in general above, I don't know 
if Boost.DI is the correct, or sufficient, or best approach to solving 
non-template DI in C++. That's for the review here to decide.

Niall