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POGeneralized structural type conformance in Scala
 

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  1. POGeneralized structural type conformance in Scala
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    <p>I'm interested in the problem of conforming a specific type to a more general structural type. Consider the following examples:</p> <pre><code>trait Sup trait Sub extends Sup type General = { def contra(o: Sub): Unit def co(): Sup def defaults(age: Int): Unit def defaults2(age: Int): Unit def defaults3(first: String): Unit } trait Specific { def contra(o: Sup): Unit // doesn't conform def co(): Sub // conforms def defaults(age: Int, name: String = ""): Unit // doesn't conform def defaults2(name: String = "", age: Int = 0): Unit // doesn't conform def defaults3(first: String = "", last: String = ""): Unit // doesn't conform } </code></pre> <p>In each of the non-conforming cases, a call to the method in <code>General</code> can safely be resolved to the corresponding method in <code>Specific</code>. A more interesting practical example can be found in <a href="https://stackoverflow.com/questions/3459630/scala-immutable-objects-and-traits-with-val-fields">this question</a>:</p> <pre><code>trait Versionable[T] { self: { def copy(version: Int): T } =&gt; val version = 0 def incrementVersion = copy(version = version + 1) } case class Customer(name: String, override val version: Int) extends Versionable[Customer] { def changeName(newName: String) = copy(name = newName) } </code></pre> <p>Here, the Customer's <code>copy</code> method does not conform to the signature in Versionable's self-type annotation. Note, however, that if the compiler allowed, <code>copy</code> could be invoked just as it is in <code>Versionable.incrementVersion</code>. Clearly, the actual signature of Customer's <code>copy</code> method is too specific for use in Versionable, since it carries the irrelevant knowledge that one can optionally supply a <code>name</code> parameter.</p> <p>Are there ways to work around these limitations? Are there reasons that such generalized conformance would be a bad idea?</p>
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    1. COAnother practical example: http://stackoverflow.com/questions/4410469/refactoring-copy-functionality
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