[mrtracy]

Co-routines (when used as an async primitive) are an an alternative to callbacks.

Callbacks exist to avoid the overhead of marshalling async results back to the original calling thread: instead of having the calling thread poll until the child thread(s) pass their results back, the calling function returns after kicking off the children, and one of the children directly resumes the work directly when the results are present. This is a significant performance gain.

However, callbacks have famously poor ergonomics, so many structured alternatives have been created: Promises/Futures, Async/Await, and CPP Coroutines.