Here's a #TypeScript API design challenge I'm working on: adding async support to #Optique (CLI argument parser) without breaking the existing sync API.
The tricky part is combinators—when you compose parsers with object() or or(), the combined parser should automatically become async if any child parser is async, but stay sync if all children are sync. This “mode propagation” needs to work at both type level and runtime.
I've prototyped two approaches and documented findings. If you've tackled similar dual-mode API designs, I'd love to hear how you approached it.