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PO"Is almost a" relationships in Scala
 

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  1. PO"Is almost a" relationships in Scala
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    <p>I'm trying to design a class hierarchy with a bunch of similar classes that don't quite share a "is a" relationships. Let's call these <code>Model</code> classes. These classes are meant to be paired with a collection similar algorithms which make use the <code>Model</code> classes but do not identically have the same requirements. Let's call these <code>Strategy</code> classes. The trick is that the <code>Strategy</code> classes require many of the same things from the <code>Model</code> classes, but not all <code>Model</code> classes may be able to implement those required methods. I'd like to not have empty "stub" methods that will just throw <code>UnsupportedOperationException</code>s and instead have a type-safe mixin-based way of dealing with this -- is there a design pattern that I can apply?</p> <p>For example,</p> <pre><code>object Main extends App { trait A { def f(one: Int): Int def g(two: Int): Int def h(three: Int): Int } class A1 extends A { override def f(one: Int): Int = {one + 1} override def g(two: Int): Int = {two + 2} override def h(three: Int): Int = {assert(false); 0} } class A2 extends A { override def f(one: Int): Int = {assert(false); 0} override def g(two: Int): Int = {two - 2} override def h(three: Int): Int = {three - 3} } trait B { def combine(i: Int): Int } trait B1 extends B { this: A =&gt; override def combine(i: Int) = {f(i) + g(i)} } trait B2 extends B { this: A =&gt; override def combine(i: Int) = {g(i) + h(i)} } override def main(args: Array[String]): Unit = { val a11 = new A1 with B1 val a22 = new A2 with B2 println(a11.combine(3)) println(a22.combine(3)) val a12 = new A1 with B2 println(a12.combine(3)) } } </code></pre> <p>Here <code>A</code> is a <code>Model</code> class and <code>B</code> is the <code>Strategy</code> class. Notice that <code>A1</code> may not be able to implement <code>h()</code> and that <code>A2</code> may not be able to implement <code>f()</code>, and depending on the strategy class this may or may not be a problem. I'd like to be able to find out which implementation of A can work with which implementation of B at compile time.</p> <p>I've used <a href="http://www.scala-lang.org/node/124" rel="nofollow">self-types</a> to express a more "has a" than "is a" relationship that would normally go with extension.</p>
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    1. USduckworthd
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    1. CONote your `override` modifier isn't necessary.
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      1. USPeter Schmitz
    2. CO@PeterSchmitz I know, but I find it helps me keep track of which methods are implementing some sort of interface.
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