Wait For

This draft contains a new wait for %Executable% effect. This is not to be confused with the existing wait %Duration% effect.

The wait for ... effect is designed for creating advanced multi-process code.
It runs an executable on a background thread but waits for its completion, preserving the execution order.

wait for a new runnable:
  print "1"
  wait 1 second
  print "2"
print "3"

This behaviour is semi-equivalent to a regular run.

run a new runnable:
  print "1"
  wait 1 second
  print "2"
print "3"

The key difference between these two is that the background process can be branched off by cancelling the wait with a wake effect.

set {thread} to the current thread
wait for a new runnable:
  print "1"
  wake {thread} // the wait ends here
  wait 1 second
  print "3"
print "2"

This is a lot safer than using sleep/wake with a regular run ... in the background since a hidden monitor preserves the happens-before relationship and makes sure the code executes in the expected order.

This draft also covers a minor inconsistency that recycled background processes could keep the thread variables that were previously set.