Enforcing SomeAsyncException for signals

[NicolasT]

In Erlang, one can deliver a so-called signal to a process, which in Haskell could be represented as an asynchronous exception.

There are two ways a process can currently receive a signal:

• From the troupe runtime, e.g., an Exit signal when a linked process exited, or
• From another process which used exit to send SomeException to another process.

The built-in exceptions are already SomeAsyncException in the exception hierarchy, however, there's no enforcement of this for callers of exit.

Would it make sense to restrict exit (and, in general, the signal handling code) not to allow for SomeException to be injected, but always enforce SomeAsyncException? Of course, the exitReason field of an Exit message (or exception) should remain Maybe SomeException to support processes exiting due to some synchronous exception.

When using safe-exception's throwTo, if given a synchronous exception, it will be wrapped by an asynchronous one, which is a viable approach: https://hackage.haskell.org/package/safe-exceptions-0.1.7.3/docs/Control-Exception-Safe.html#v:throwTo

Alternatively, exit could raise when given a non-asynchronous exception.