B Revzin - if const expr isn't broken (was Re: My Meeting C++ Keynote video is now available)

[Stefan Koch]

On Thursday, 17 January 2019 at 19:31:24 UTC, H. S. Teoh wrote:
> On Thu, Jan 17, 2019 at 06:03:07PM +0000, Paul Backus via 
> Digitalmars-d-announce wrote: [...]
>> [2] 
>> https://bartoszmilewski.com/2009/10/21/what-does-haskell-have-to-do-with-c/
> [...]
>
> Coming back to the D example at the end, I totally agree with 
> the sentiment that D templates, in spite of their significant 
> improvements over C++ syntax, ultimately still follow the same 
> recursive model. Yes, you can use CTFE to achieve the same 
> thing at runtime, but it's not the same thing, and CTFE cannot 
> manipulate template argument lists (aka AliasSeq aka whatever 
> it is you call them).  This lack of symmetry percolates down 
> the entire template system, leading to the necessity of the 
> hack that Bartosz refers to.
>
> Had template argument lists / AliasSeq been symmetric w.r.t. 
> runtime list manipulation, we would've been able to write a 
> foreach loop that manipulates the AliasSeq in the most readable 
> way without needing to resort to hacks or recursive templates.
>
For 2 years I have pondered this problem, and I did come up with 
a solution.
It's actually not that hard to have CTFE interact with 
type-tuples.
You can pass them as function parameters, or return them if you 
wish.
Of course a type-tuple returning ctfe function, is compile-time 
only.
This solved one more problem that ctfe has:
helper functions required for ctfe can only be omitted from the 
binary, if you use the trick of putting them into a module which 
is the import path but never explicitly given on the command line.
newCTFE has the facility to be extended for this, and 
implementing type-functions is at least on my personal roadmap.

At Dconf 2018 Andrei and Walter said, a DIP which is 
substantiated enough might make it.
However due to lack of time, (and productivity-reducing internal 
changes) it will take some time until I can get started on this.

Also I plan for newCTFE to be in shape before I add 
type-manipulation abilities.

Cheers,

Stefan

P.S. There is one caveat: because of how type-functions work they 
cannot, you cannot create a non-anonymous symbol inside a 
type-function, because there is no way to infer a mangle.
You can however create an anonymous symbol and alias it inside a 
template body, which gives it a mangle and it can behave like a 
regular symbol.