cats.effect.testkit

Usage for simulating an ExecutionContext):
{{{
implicit val ec = TestContext()

ec.execute(new Runnable { def run() = println("task1") })

ex.execute(new Runnable {
def run() = {
println("outer")

 ec.execute(new Runnable {
   def run() = println("inner")
 })

}
})

// Nothing executes until tick gets called
ec.tick()

// Testing the resulting state
assert(ec.state.tasks.isEmpty)
assert(ec.state.lastReportedFailure == None)
}}}

Our TestContext can also simulate time passage, as we are able
to builds a cats.effect.Timer instance for any data type that
has a LiftIO instance:

{{{
val ctx = TestContext()

val timer: Timer[IO] = ctx.timer[IO]
}}}

We can now simulate actual time:

{{{
val io = timer.sleep(10.seconds) *> IO(1 + 1)
val f = io.unsafeToFuture()

// This invariant holds true, because our IO is async
assert(f.value == None)

// Not yet completed, because this does not simulate time passing:
ctx.tick()
assert(f.value == None)

// Simulating time passing:
ctx.tick(10.seconds)
assert(f.value == Some(Success(2))
}}}

Simulating time makes this pretty useful for testing race conditions:

{{{
val never = IO.async[Int] (_ => {})
val timeoutError = new TimeoutException
val timeout = timer.sleep(10.seconds) *> IO.raiseErrorInt

val pair = (never, timeout).parMapN(_ + _)

// Not yet
ctx.tick()
assert(f.value == None)
// Not yet
ctx.tick(5.seconds)
assert(f.value == None)

// Good to go:
ctx.tick(5.seconds)
assert(f.value, Some(Failure(timeoutError)))
}}}