Blocking vs Nonblocking Reads

Problems

• deref on some IDerefs can block.
  • futures
  • promises
• problem - If you can't accept blocking indefinitely you have no alternative
• some reference type might not yet have a value
  • futures
  • promises
  • delays
• problem - if you need to make a decision based upon whether or not such a reference has a value you can't, without forcing/blocking

Possible solutions

• categorize references that can block
  • e.g. IMightBlock
• offer a deref with timeout
  • e.g. (deref blocking-ref timeout timeout-val)
• categorize references that might not yet have a value
  • e.g. IMightNotHaveAValue
• offer a query fn for that
  • (has-value? aref)

Issues

• which (if any) of these make sense for other refs?
  • timeout seems really confusing for non-blocking
  • has-value? could be trivially true for other refs
    • but causes paranoia?
• overload deref for timeout, or new fn name?

Wants

• I want to know if a potentially blocking deref has a value
  • future-done? for Future
  • (.hasValue foo) for Promise

• I want to know if something has finished
  • await for Agent
  • future-done? for Future
  • Thread.isAlive (rare)

• I want to wait for something to finish
  • await for Agent
  • deref for Future, Delay, and Promise

• I want to wait a specific amount of time for something to finish
  • await-for for Agent

• I want to prevent something from happening
  • future-cancel for Future
  • ThreadPoolExecutor.remove for a custom thread pool
  • Avoid deref on a Delay

Example Problem

• printing at the REPL looks through a deref
  • this can block (possibly forever) if a data structure contains e.g. a promise
• The REPL should not perform the deref for a print request unless there is a value
  • option 1: never deref possible blockers
  • option 2: have a bindable val that controls how REPL prints blocking derefs, analogous to the controls for lazy data structures
• in any case, there needs to be a way to discover which derefs have a value

Facts

• deref can block
  • Never for Var, Ref, Agent, and Atom
  • Sometimes for Future, Delay, and Promise

• Java APIs
  • Future
    • .get() blocks until complete
      • interruptible, throws InterruptedException
    • .get(long, TimeUnit) blocks for specified duration
      • throws TimeouotException if it times out
      • interruptible, throws InterruptedException
    • .isDone() asks if finished
    • .cancel(bool mayInterrupt) attempts to cancel
      • if already finished or cancelled, returns false
      • if already running, tries to interrupt if argument is true
    • .isCancellled() asks if cancelled
  • CountDownLatch
    • .await() blocks until latch reaches zero
      • interruptible, throws InterruptedException
    • .await(long, TimeUnit) blocks until latch reaches zero
      • returns false if it times out
      • interruptible, throws InterruptedException
    • .getCount() returns current count
  • Semaphore
  • CyclicBarrier