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copied!<p>Well, simple, does it make sense? Think of Liskov substitution.</p> <h2>Co-variance</h2> <p>If <code>A <: B</code>, does it make sense to pass a <code>C[A]</code> where a <code>C[B]</code> is expected? If so, make it <code>C[+T]</code>. The classic example is the immutable <code>List</code>, where a <code>List[A]</code> can be passed to anything expecting a <code>List[B]</code>, assuming <code>A</code> is a subtype of <code>B</code>.</p> <p>Two counter examples:</p> <p>Mutable sequences are invariant, because it is possible to have type safety violations otherwise (in fact, Java's co-variant <code>Array</code> is vulnerable to just such things, which is why it is invariant in Scala).</p> <p>Immutable <code>Set</code> is invariant, even though its methods are very similar to those of an immutable <code>Seq</code>. The difference lies with <code>contains</code>, which is typed on sets and untyped (ie, accept <code>Any</code>) on sequences. So, even though it would otherwise be possible to make it co-variant, the desire for an increased type safety on a particular method led to a choice of invariance over co-variance.</p> <h2>Contra-variance</h2> <p>If <code>A <: B</code>, does it make sense to pass a <code>C[B]</code> where a <code>C[A]</code> is expected? If so, make it <code>C[-T]</code>. The classic would-be example is <code>Ordering</code>. While some unrelated technical problems prevent <code>Ordering</code> from being contra-variant, it is intuitive that anything that can order a super-class of <code>A</code> can also order <code>A</code>. It follows that <code>Ordering[B]</code>, which orders all elements of type <code>B</code>, a supertype of <code>A</code>, can be passed to something expecting an <code>Ordering[A]</code>.</p> <p>While Scala's <code>Ordering</code> is not contra-variant, <a href="http://code.google.com/p/scalaz/" rel="noreferrer">Scalaz</a>'s <a href="http://scalaz.googlecode.com/svn/continuous/latest/browse.sxr/scalaz/Order.scala.html" rel="noreferrer">Order</a> is contra-variant as expected. Another example from Scalaz is its <a href="http://scalaz.googlecode.com/svn/continuous/latest/browse.sxr/scalaz/Equal.scala.html" rel="noreferrer">Equal</a> trait.</p> <h2>Mixed Variance?</h2> <p>The most visible example of mixed variance in Scala is <code>Function1</code> (and 2, 3, etc). It is contra-variant in the parameter it receives, and co-variant in what it returns. Note, though, that <code>Function1</code> is what is used for a lot of closures, and closures are used in a lot of places, and these places are usually where Java uses (or would use) Single Abstract Method classes.</p> <p>So, if you have a situation where a SAM class applies, that's likely a place for mixed contra-variance and co-variance.</p>

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