Module Thread_safe_pipe.If_closed

Functions that write elements to the pipe take an If_closed.t argument to specify how to deal with the possibility that the pipe is closed.

The alternatives are to Raise on a closed pipe, or Return a variant indicating whether the pipe is closed. This allows lightweight syntax for calls that want to raise if the pipe is closed:

  write t a ~if_closed:Raise 

It also allows lightweight syntax for calls that want to match on whether the pipe was closed:

  match write t a ~if_closed:Return with
  | Closed  -> ...
  | Written -> ... 

Returning a variant is essential when one wants to distinguish a closed pipe from other errors. Also, since pipe-writing functions acquire the Async lock, it would be incorrect (due to races) to check is_closed prior to the lock acquisition.