Sometimes you may need threadsafe, mutable variables. Concurrent ML provides them in two flavours.
An ivar may be in one of two states: empty, or full. An ivar may be written once, and provides synchronisation on reads. Once written, the ivar is full, and further writes are an error.
As a value may not be put yet, there is a nonblocking operation that returns an option, iGetPoll.
I-variables work with the event framework of CML, and thus may be an event source for threads to select upon, if accessed via iGetEvt.
(* examples/ivars.sml *)