First life-signs of type functions

[Stefan Koch]

On Wednesday, 13 May 2020 at 19:22:36 UTC, Nick Treleaven wrote:
> On Sunday, 10 May 2020 at 08:39:42 UTC, Nick Treleaven wrote:
>> bool anySatisfy(alias Tem, alias[] S)
>> {
>>     bool found;
>>     alias E;
>>     while (S.length)
>>     {
>>         E = S[0];
>>         if (Tem!E)
>>             return true;
>>         S = S[1..$];
>>     }
>>     return false;
>> }
>
> I realized after posting that Tem!E can't work, because the 
> type function must be compiled before evaluating E. Tem!E would 
> be a constant expression, if it even compiles (e.g. passing E 
> as a template alias parameter, not passing the value of E).
>
> So as Stefan wrote, type functions are like CTFE functions 
> (maybe a superset?).

Well E is defined at interpretation time.
So I _could_ deferr the template instance until the value of E is 
known.
But the whole purpose of type functions is to discourage the use 
of templates in cases in which they are not needed.
In cases where a template is needed type functions will be of 
little use.
(you can possibly use them to remove internal instances).