My first email to Walter, ever

[John Colvin]

On Sunday, 7 July 2013 at 12:27:02 UTC, Peter Alexander wrote:
> On Sunday, 7 July 2013 at 03:03:03 UTC, Andrei Alexandrescu 
> wrote:
>> Terrible. If you have conditionals, iteration, functions, and 
>> objects
>> in D's straight programming support, you should have 
>> conditionals,
>> iteration, functions, and objects in D's metalanguage.
>
> :-(
>
> template allSatisfy(alias F, T...)
> {
>     static if (T.length == 0)
>     {
>         enum allSatisfy = true;
>     }
>     else static if (T.length == 1)
>     {
>         enum allSatisfy = F!(T[0]);
>     }
>     else
>     {
>         enum allSatisfy =
>             allSatisfy!(F, T[ 0  .. $/2]) &&
>             allSatisfy!(F, T[$/2 ..  $ ]);
>     }
> }
>
> Still looks like half-assed functional programming to me.
>
> Where's the iteration? Why can't I write this?
>
> template allSatisfy(alias F, T...) {
>     foreach(t; T)
>         if (!F!(t))
>             return false;
>     return true;
> }
>
> (Those are rhetorical questions btw, before anyone links me to 
> a D tutorial).
>
> We're almost there with CTFE, but CTFE can only run functions 
> that could run at runtime. In a crazy world where types were 
> first class objects, stuff like this would be feasible. Or 
> perhaps we just need a compile-time metalanguage that allows 
> things like this to be run with CTFE?

So what you're effectively asking for is for templates to be 
compile-time functions where types are first class variables.

I would love that.