CTFE vs. traditional metaprogramming

[Don]

Jarrett Billingsley wrote:
> On Fri, Oct 9, 2009 at 4:57 PM, Sean Kelly <sean at invisibleduck.org> wrote:
>> == Quote from Andrei Alexandrescu (SeeWebsiteForEmail at erdani.org)'s article
>>> Thanks!
>>> I plan to add more text at the end of the chapter that discusses the
>>> opportunities of CTFE. Walter revealed to me that CTFE, particularly now
>>> after it's been improved by leaps and bounds by Don and by Walter
>>> himself, could obviate a lot of the traditional metaprogramming
>>> techniques developed for C++.
>>> One question that bugs me is, where do you draw the line? Say there's a
>>> metaprogramming problem at hand. How to decide on solving it with CTFE
>>> vs. solving it with templates? It would be great to have a simple
>>> guideline that puts in contrast the pluses and minuses of the two
>>> approaches.
>> CTFE is great for working with values while template metaprogramming is
>> great for working with types.  String mixins make CTFE good at working
>> with types as well, but I wouldn't consider them a novice-level feature.
> 
> Throw templates in there. Boom! Goodbye, CTFE.
> 
> (.stringof and templates do not get along.)

That's just a bug. The more fundamental problem is that you can't 
instantiate a template from inside CTFE. IE, you can cross from the 
"compile-time world" to the "runtime world" only once -- you can never 
get back.