> This language gave more unalloyed joy than anything else I've done as a programmer, though I've yet to build anything practically useful in it.

Ain't that the truth. Scheme/Racket, ML and other primarily academic functional languages are fun, but rarely used in business/industry. It's great for learning programming language ideas like scope, thunks, closures, etc but very little opportunity beyond that. But maybe that's why it was a joy to program with racket. No pressure to create anything practical with it. Just tinker and learn using it.