Andrej Mitrovic:
However the [k, aa[k]] expression will allocate an array for each key you iterate over regardless if you use join or joiner. I posted a solution with "only", hope that works. :)
Perhaps the "s" suffix (to define fixed-sized arrays) could avoid that problem:
string[] r = aa.byKey.map!(k => [k, aa[k]]s).join; Bye, bearophile