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<h3>Problems</h3>

<ul>
	<li>deref on some IDerefs can block.
	<ul>
		<li>futures</li>
		<li>promises</li>
	</ul>
	</li>
	<li>problem - If you can't accept blocking indefinitely you have no alternative</li>
	<li>some reference type might not yet have a value
	<ul>
		<li>futures</li>
		<li>promises</li>
		<li>delays</li>
	</ul>
	</li>
	<li>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</li>
</ul>

<h4>Possible solutions</h4>

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

<h4><strong>Issues</strong></h4>

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

<hr />
<h3>Wants</h3>

<ul>
	<li>I want to know if a potentially blocking deref has a value
	<ul>
		<li><code>future-done?</code> for Future</li>
		<li><code>(.hasValue foo)</code> for Promise</li>
	</ul>
	</li>
</ul>

<ul>
	<li>I want to know if something has finished
	<ul>
		<li><code>await</code> for Agent</li>
		<li><code>future-done?</code> for Future</li>
		<li><code>Thread.isAlive</code> (rare)</li>
	</ul>
	</li>
</ul>

<ul>
	<li>I want to wait for something to finish
	<ul>
		<li><code>await</code> for Agent</li>
		<li><code>deref</code> for Future, Delay, and Promise</li>
	</ul>
	</li>
</ul>

<ul>
	<li>I want to wait a specific amount of time for something to finish
	<ul>
		<li><code>await-for</code> for Agent</li>
	</ul>
	</li>
</ul>

<ul>
	<li>I want to prevent something from happening
	<ul>
		<li><code>future-cancel</code> for Future</li>
		<li><code>ThreadPoolExecutor.remove</code> for a custom thread pool</li>
		<li>Avoid <code>deref</code> on a Delay</li>
	</ul>
	</li>
</ul>

<h3>Example Problem</h3>

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

<h3>Facts</h3>

<ul>
	<li><code>deref</code> can block
	<ul>
		<li>Never for Var, Ref, Agent, and Atom</li>
		<li>Sometimes for Future, Delay, and Promise</li>
	</ul>
	</li>
</ul>

<ul>
	<li>Java APIs
	<ul>
		<li><a href="http://download.oracle.com/javase/6/docs/api/java/util/concurrent/Future.html">Future</a>
		<ul>
			<li><code>.get()</code> blocks until complete
			<ul>
				<li>interruptible, throws InterruptedException</li>
			</ul>
			</li>
			<li><code>.get(long, TimeUnit)</code> blocks for specified duration
			<ul>
				<li>throws TimeouotException if it times out</li>
				<li>interruptible, throws InterruptedException</li>
			</ul>
			</li>
			<li><code>.isDone()</code> asks if finished</li>
			<li><code>.cancel(bool mayInterrupt)</code> attempts to cancel
			<ul>
				<li>if already finished or cancelled, returns false</li>
				<li>if already running, tries to interrupt if argument is true</li>
			</ul>
			</li>
			<li><code>.isCancellled()</code> asks if cancelled</li>
		</ul>
		</li>
		<li><a href="http://download.oracle.com/javase/6/docs/api/java/util/concurrent/CountDownLatch.html">CountDownLatch</a>
		<ul>
			<li><code>.await()</code> blocks until latch reaches zero
			<ul>
				<li>interruptible, throws InterruptedException</li>
			</ul>
			</li>
			<li><code>.await(long, TimeUnit)</code> blocks until latch reaches zero
			<ul>
				<li>returns false if it times out</li>
				<li>interruptible, throws InterruptedException</li>
			</ul>
			</li>
			<li><code>.getCount()</code> returns current count</li>
		</ul>
		</li>
		<li><a href="http://download.oracle.com/javase/6/docs/api/java/util/concurrent/Semaphore.html">Semaphore</a></li>
		<li><a href="http://download.oracle.com/javase/6/docs/api/java/util/concurrent/CyclicBarrier.html">CyclicBarrier</a></li>
	</ul>
	</li>
</ul>