Reimplementing the bulk of std.meta iteratively

[Stefan Koch]

On Saturday, 26 September 2020 at 21:42:19 UTC, Andrei 
Alexandrescu wrote:
> On 9/26/20 5:16 PM, Timon Gehr wrote:
>> On 26.09.20 18:18, Andrei Alexandrescu wrote:
>>>
>>> Most implementations are a few lines long because they get to 
>>> leverage algorithms as implemented in std. Here's a typical 
>>> one:
>>>
>>> alias MostDerived(Args...) = dereify!({
>>>      auto ids = reify!Args;
>>>      sort!((a, b) => is(dereify!a : dereify!b))(ids);
>>>      return ids;
>>> }());
>> 
>> I am pretty sure this one does not work though.
>
> Oops, Steve just told me it doesn't have any unittests. So it 
> may as well not be working. Research goes on.

This example on the other hand works:

---
/+
import std.array;
import std.range;
import std.algorithm;
doesn't work yet but only because the template connstraints need 
to be adjusted.
+/

alias alias_array = typeof(((alias T) => __traits(getAttributes, 
T))(Byte));

auto sort3(alias less, R)(ref /*alias_array*/ R r)
{
     alias tmp;

     if (r.length == 1)
         return r;
     if (r.length == 2)
     {
         if (less(r[1], r[0]))
         {
             tmp = r[0];
             r[0] = r[1];
             r[1] = tmp;
         }
     }
     if (r.length == 3)
     {
         if (less(r[2], r[1]))
         {
             tmp = r[1];
             r[1] = r[2];
             r[2] = tmp;
         }
         if (less(r[1], r[0]))
         {
             tmp = r[0];
             r[0] = r[1];
             r[1] = tmp;
         }
     }
     return r;
}

auto sortBySize(alias_array types ...)
{
     types.sort3!((alias a, alias b) => a.sizeof < b.sizeof);

     return types;
};

alias Int = int;
alias Ushort = ushort; // we need these aliases because the 
parser won't allow us to call a function with a reserved type 
identifier.
alias Byte = byte;
pragma(msg, sortBySize(Ushort, Int, Byte)); //Output:  [(byte), 
(ushort), (int)]

---