Implement custom printer methods about missionary HOT 1 CLOSED

What happens here : hello-world is run, immediately completes with nil, then the canceller is returned (an instance of Fiber). The REPL tries to find a way to print the canceller, the canceller doesn't know how to print itself, but as it happens to implement IDeref the REPL tries to deref it. derefing a sp Fiber is a garbage operation so an exception is thrown and then the REPL prints the exception.

Quick workaround : don't print a canceller in a REPL, just define it in a var and maybe call it later.
Long-term solution : implement custom printer methods for cancellers to prevent the REPL to fallback to deref.

I file it as a bug since it's a major REPL inconvenience.

from missionary.