(* filter-evt.sig *)

(* Copyright (C) 2006 Alley Stoughton

   This file is part of crypto, a cryptogram encoder/decoder.  See the
   file COPYING.txt for copying and usage restrictions *)

signature FILTER_EVT =
sig

(* filterEvt f ev starts an internal thread, creates an 'a channel
   valCh and a unit channel flushCh, and returns the pair of events
   (recvEvt valCh, sendEvt(flushCh, ()).

   the thread's main loop maintains a queue of 'a values, initially
   empty; each iteration works as follows, using selective
   communication

     it tries to synchonize on ev, discarding the resulting value if it
     doesn't satisfy f, and queuing it otherwise

     if the buffer is nonempty, it tries to synchronize on
     sendEvt(valCh, v), where v is the head of the queue; if this
     succeeds, it dequeues v

     it tries to synchronize on recvEvt flushCh; if this succeeds, it
     empties the queue

   if the consumer(s) of recvEvt valCh are responsive, the queue won't
   become very long *)

val filterEvt : ('a -> bool) -> 'a CML.event -> 'a CML.event * unit CML.event

end;