Re: sortOn: sorts range of aggregates by member name(s)

[via Digitalmars-d]

On Wednesday, 5 November 2014 at 20:50:00 UTC, Nordlöw wrote:

On Wednesday, 5 November 2014 at 16:54:38 UTC, Marc Schütz wrote:

My idea was something along these lines (untested):

   template extractorFun(alias extractor) {
       static if(is(typeof(extractor) : string)) {
           auto ref extractorFun(T)(auto ref T a) {
               mixin("with(a) { return " ~ extractor ~ "; }");
           }
       } else {
           alias extractorFun = extractor;
       }
   }

   alias fn = extractorFun!extractor;
   r.sort!((a, b) => (fn(a) < fn(b)));

Thanks, I'll use this!

Still, is it even possible to extend extractor to become variadic or do I have to write a number of explicit overloads

1: void sortBy(alias xtr, R)(R r)
2: void sortBy(alias xtrA, alias xtrB, R)(R r)
3: void sortBy(alias xtrA, alias xtrB, alias xtrC, R)(R r)
etc

?

Again untested:

private alias makePredicate(alias xtr) =
    (a, b) => (extractorFun!xtr(a) < extractorFun!xtr(b));

auto sortBy(extractors..., R)(R r) {
    alias preds = staticMap!(makePredicate, extractors);
    return r.sort!preds;
}

(Variadic template parameters also accept aliases.)