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POHow do I form the union of scala SortedMaps?
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  1. POHow do I form the union of scala SortedMaps?
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    copied!<p>(I'm using Scala nightlies, and see the same behaviour in 2.8.0b1 RC4. I'm a Scala newcomer.)</p> <p>I have two <code>SortedMap</code>s that I'd like to form the union of. Here's the code I'd like to use:</p> <pre><code>import scala.collection._ object ViewBoundExample { class X def combine[Y](a: SortedMap[X, Y], b: SortedMap[X, Y]): SortedMap[X, Y] = { a ++ b } implicit def orderedX(x: X): Ordered[X] = new Ordered[X] { def compare(that: X) = 0 } } </code></pre> <p>The idea here is the 'implicit' statement means <code>X</code>s can be converted to <code>Ordered[X]</code>s, and then it makes sense combine <code>SortedMap</code>s into another <code>SortedMap</code>, rather than just a map.</p> <p>When I compile, I get</p> <pre><code>sieversii:scala-2.8.0.Beta1-RC4 scott$ bin/scalac -versionScala compiler version 2.8.0.Beta1-RC4 -- Copyright 2002-2010, LAMP/EPFL sieversii:scala-2.8.0.Beta1-RC4 scott$ bin/scalac ViewBoundExample.scala ViewBoundExample.scala:8: error: type arguments [ViewBoundExample.X] do not conform to method ordered's type parameter bounds [A &lt;: scala.math.Ordered[A]] a ++ b ^ one error found </code></pre> <p>It seems my problem would go away if that type parameter bound was <code>[A &lt;% scala.math.Ordered[A]]</code>, rather than <code>[A &lt;: scala.math.Ordered[A]]</code>. Unfortunately, I can't even work out where the method 'ordered' lives! Can anyone help me track it down?</p> <p>Failing that, what am I meant to do to produce the union of two <code>SortedMap</code>s? If I remove the return type of combine (or change it to <code>Map</code>) everything works fine --- but then I can't rely on the return being sorted!</p>
 

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