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copied!<p>If you create a <code>Future</code> (using <code>scala.concurrent.future</code>) it <strong>will</strong> be calculated, no matter what you do. So you need a totally different strategy.</p> <p>Further, your interface does not even remotely allow to figure out what data you will actually use. How should the calculation of <code>myCalc</code> know that in the <code>onComplete</code> you'll only use <code>result1</code>?</p> <p>You could:</p> <ul> <li><p>Use only lazy vals:</p> <pre><code>val calculator = new Calculator { lazy val hardToCalculate1 = { println("calculating 1") 1 } // ... } </code></pre> <p>Pro: Simple<br /> Con: Not asynchronous</p></li> <li><p>Encapsulate the <code>Future</code> to allow requesting the calculation:</p> <pre><code>class ReqFuture[T](body: () => T) { lazy val fut = future { body() } } </code></pre> <p>But now you still have the problem that <code>myCalc</code> will request and wait for all of them. So you'll have to introduce <code>ReqFutures</code> in <code>Calculator</code>:</p> <pre><code>trait Calculator { def hardToCalculate1: ReqFuture[Int] // ... def result1 = for { h1 <- hardToCalculate1.fut h2 <- hardToCalculate2.fut } yield h1 + h2 } </code></pre> <p>Pro: When you call <code>result1</code>, only what you need is calculated (but still only once).<br/> Con: <code>result1</code> is a <code>Future[Int]</code> now. So <code>Futures</code> have fully penetrated your <code>Calculator</code>.</p></li> </ul> <p>If you cannot influence <code>Calculator</code> (what I am suspecting) and cannot change the code of <code>result1,2,3</code> there is unfortunately nothing in my knowledge that you can do to make execution lazy and asynchronous.</p>

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