An Exception_filter.t is a value which indicates to Lwt what exceptions to catch and what exceptions to let bubble up all the way out of the main loop immediately.

handle_all is the default filter. With it the all the exceptions (including Out_of_memory and Stack_overflow) can be handled: caught and transformed into rejected promises.

handle_all_except_runtime is a filter which lets the OCaml runtime exceptions (Out_of_memory and Stack_overflow) go through all the Lwt abstractions and bubble all the way out of the call to Lwt_main.run.

Note that if you set this handler, then the runtime exceptions leave the Lwt internal state inconsistent. For this reason, you will not be able to call Lwt_main.run again after such an exception has escaped Lwt_main.run.

set sets the given exception filter globally. You should call this function at most once during the start of your program, before the first call to Lwt_main.run.