[boost] About all these metaprogramming libraries

Subject: [boost] About all these metaprogramming libraries
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2017-03-18 20:32:05

Dear Boost,

We've seen two metaprogramming libraries ask for a formal review on the list
recently. I expect a third (Brigand) might join the party soon, and maybe
even a fourth (Eric Niebler's meta). I think it would be wise to have a plan
for how we're going to deal with this.

Before going further, I'd also like to clear something out. Some people have
asked whether "classic" metaprogramming libraries still had their place
given that we now have Hana. As the author of Hana, I think the answer is
yes. While Hana is (IMHO) easier to use and more powerful, it also has an
important shortcoming. Hana is too slow when dealing with very large inputs.
This is due to the fact that it can also handle values, which is much
heavier than dealing with types only. This can be alleviated to some extent,
but the truth is that our execution model (the compiler) simply has to do
more work when handling values.

Hence, I think Boost does need a modern classic TMP library to handle these
cases where a library writer needs to handle gigantic type lists. I think
marketing such a library as an advanced tool for library writers would be a
service to the overall C++ community, but that's an orthogonal concern.

However, Boost needs one such library, not four. I think we can't just do 4
reviews and include all the libraries that pass in Boost; that would be a
huge disservice to our users, who will be left with the burden of choosing.
Heck, if we can't even make our mind, how can they?

So, how do we pick? Have we ever been in a similar situation where we have
multiple competing libraries solving _exactly_ the same problem?

Louis

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