Named arguments via struct initialization in functions

[Michael Coulombe via Digitalmars-d]

On Wednesday, 9 March 2016 at 12:55:16 UTC, Idan Arye wrote:
> Allowing something like `auto params = [:y : 50, :x : 100]` 
> won't really solve anything. It works nicely in Ruby, because 
> Ruby has dynamic typing and with some syntactic sugar you get 
> elegant syntax for dynamic structs. But in a structured 
> language like D, you run into the problem that `[:y : 50, :x : 
> 100] is an associative array with a determined type for it's 
> values, so you can't do things like `[:y : 50, :x : "hello"]` - 
> which greatly limits the usability of this syntax.

I posted (in a previous thread about this issue) a library 
solution to this:

import std.typecons, std.meta;
private struct Named(string n, T) {
     enum name = n;
     T value;
}
template NamedToAS(N...) {
     static if (N.length == 0) {
         alias NamedToAS = AliasSeq!();
     }
     else {
         alias NamedToAS = AliasSeq!(typeof(N[0].init.value), 
N[0].name, NamedToAS!(N[1 .. $]));
     }
}
struct Tup {
     static auto opDollar(size_t i)() {
         return immutable KeyWordDollar();
     }
     static auto opIndex(NamedArgs...)(NamedArgs nas) {
         Tuple!(NamedToAS!NamedArgs) tup;
         foreach(i, ref na ; tup) {
             na = nas[i].value;
         }
         return tup;
     }
}
void overrideFrom(TupDef, TupOver)(ref TupDef defaultMap, ref 
TupOver overrideMap) {
     foreach(i, ref field ; overrideMap) {
         __traits(getMember, defaultMap, TupOver.fieldNames[i]) = 
field;
     }
}
auto println(KWA)(KWA kwa) {
     auto args = Tup[$.str = "abc", $.repeat = 2, $.sep = " "];
     args.overrideFrom(kwa);
     import std.stdio;
     bool first = true;
     foreach(i ; 0 .. args.repeat) {
         if (first) first = false; else write(args.sep);
         write(args.str);
     }
     writeln();
}
unittest {
     println(Tup[$.str = "xyz", $.repeat = 8]);
}