Re: [boost] "Simple C++11 metaprogramming"

Subject: Re: [boost] "Simple C++11 metaprogramming"
From: Peter Dimov (lists_at_[hidden])
Date: 2015-06-02 06:42:36

Bruno Dutra wrote:

> Still, laziness and SFINAE friendliness are the properties I deem most 
> fundamental on any metaprogramming library.

I meant to respond to these points too.

The eagerness of template aliases is an obvious problem in at least one 
place and that place is mp_if, where the naive mp_if<P<T>, F<T>, G<T>> 
evaluates both F<T> and G<T>. But I'm not sure if there are others, under 
the assumption that we've no higher-order constructs. Eric Niebler's meta 
has an entire lazy:: namespace with deferred copies of all its constructs; 
continuing the general theme, I wonder whether all this is strictly needed 
once we jettison the lambda part.

SFINAE friendliness can be quite a curse. It often transforms reasonably 
comprehensible error messages into full-scale Agatha Christie mysteries (the 
compiler output even being of comparable length). So I'm not convinced (at 
the moment) that a metaprogramming library should be SFINAE-friendly. I 
presently tend to lean towards the philosophy of static_assert'ing as much 
as possible, and leaving the primary templates undefined instead of empty.