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POLate evaluation in c#?
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  1. POLate evaluation in c#?
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    copied!<p>I am working on an application in c#. To make this application work, I have found myself doing some things that feel quite unnatural for the language I have selected. After going through many refactorings and refinements, I have come to the realization that the functionality that I am trying to implement is really a form of ‘lazy evaluation’ (I think). This is what I would like...</p> <pre><code>// Criteria 1: Lazy evaluation of expressions int x = LazyEvaluated.New&lt;int&gt;(); int y = LazyEvaluated.New&lt;int&gt;(); int z = LazyEvaluated.New&lt;int&gt;(); z.Assign(x + y); Assert.ThrowsException(z.Evalutate()); x.Assign(1); Assert.ThrowsException(z.Evalutate()); y.Assign(2); Assert.Equals(3, z.Evaluate()); x.Assign(3); Assert.Equals(5, z.Evaluate()); // Criteria 2: Referencing relative to parent object Car myCar = LazyEvaluated.New&lt;Car&gt;(); Engine engineInMyCar = LazyEvaluated.New&lt;Engine&gt;(); double displacementOfMyEngine = LazyEvaluated.New&lt;double&gt;(); engineInMyCar = myCar.Engine; displacementOfMyEngine = engineInMyCar.Displacement; Car subaru = new Car(new FlatFourEngine()); Car falcon = new Car(new InlineSixEngine()); myCar.Assign(subaru); Assert.IsTypeOf&lt;FlatFourEngine&gt;(engineInMyCar.Evaluate()); Assert.IsEqual(2.0, displacementOfMyEngine.Evaluate()); myCar.Assign(falcon); Assert.IsTypeOf&lt;InlineSixEngine&gt;(engineInMyCar.Evaluate()); Assert.IsEqual(4.0, displacementOfMyEngine.Evaluate()); </code></pre> <p>And these are the simple class definitions that I have used for illustration...</p> <pre><code>public class Car { private readonly Engine engine; public Car(Engine engine) { this.engine = engine; } public Engine Engine { get { return engine; } } } public abstract class Engine { public abstract double Displacement { get; } } public class FlatFourEngine : Engine { public override double Displacement { get { return 2.0; } } } public class InlineSixEngine : Engine { public override double Displacement { get { return 4.0; } } } </code></pre> <p>The reason I want this functionality is mostly for separation-of-concerns. To illustrate, let’s take the car analogue a bit further. My car is made up of lots of bits; engine, tyres, interior etc. My tyres need to be changed every 2 years by a tyre shop. My engine needs to have an oil change every 6 months by a mechanic. The interior needs to be vacuumed every 3 months by a detailer. It would be nice to able to do something along the lines of...</p> <pre><code>tyreShop.ReplaceTyres(myCar.Tyres, every2Years); mechanic.ChangeOil(myCar.Engine, every6Months); detailer.VacuumInterior(myCar.Interior, every3Months); </code></pre> <p>Some reasons for this approach;</p> <ul> <li>I would like to set-and-forget. Once I schedule a part of my car in for a service, I want it to happen on an ongoing basis without any further input from me.</li> <li>I don’t want to have to call each of my service centres and notify them the instant that I purchase a new car. In fact, they don’t need to know that I have a new car until it comes time for a service. If they try to service my car and it doesn’t exist, then it’s ok for them to throw an exception.</li> <li>I want each of my services to be as simple and focussed as possible. Why give the tyre shop a reference to my entire car (including engine, interior, etc), when the only thing they really care about is the tyres?</li> <li>I don’t want the tyreshop to give me a new set of tyres and force me to fit (assign) them to the car myself.</li> </ul> <p>I do not have any formal education in CS, and my only programming experience is with c &amp; c# languages. I’ve just spent the last 6 months getting familiar with c# &amp; .Net. For the most part, I really like the language and early-evaluation in general. But I am now questioning whether I would have been better off tackling my particular problem in a different language with inbuilt support for lazy evaluation.</p> <p>My questions are;</p> <ol> <li>If I were to start again, what would be the best language to tackle such a problem in?</li> <li>Are there any existing platforms for c# that provide support for lazy evaluation?</li> <li>What are the alternatives for making this work in c#? Are there any particular design patterns I should read up on?</li> </ol>
 

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