Hi devs, To invoke Haskell computation in C, we need to call one of rts_eval* functions, which enters the scheduler loop, and returns only when the specified Haskell thread is finished or killed. We'd like to enhance the scheduler and add async variants of the rts_eval* functions, which take C callbacks to consume the Haskell thread result, kick off the scheduler loop, and the loop is allowed to exit when the Haskell thread is blocked. Sync variants of RTS API will continue to work with unchanged behavior. The main intended use case is async foreign calls for the WebAssembly target. When an async foreign call is made, the Haskell thread will block on an MVar to be fulfilled with the call result. But the scheduler will eventually fail to find work due to empty run queue and exit with error! We need a way to gracefully exit the scheduler, so the RTS API caller can process the async foreign call, fulfill that MVar and resume Haskell computation later. Question I: does the idea of adding async RTS API sound acceptable by GHC HQ? To be honest, it's not impossible to workaround lack of async RTS API: reuse the awaitEvent() logic in non-threaded RTS, pretend each async foreign call reads from a file descriptor and can be handled by the POSIX select() function in awaitEvent(). But it'd surely be nice to avoid such hacks and do things the principled way. Question II: how to modify the scheduler loop to implement this feature? Straightforward answer seems to be: check some RTS API non-blocking flag, if present, allow early exit due to empty run queue. Thanks a lot for reading this, I appreciate any suggestions or questions :) Best regards, Cheng