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    copied!<p>So if I were in your position, I think what I'd really want to be writing is something like:</p> <pre><code>case elem: Elem =&gt; elem.copy(attributes= for (attr &lt;- elem.attributes) yield attr match { case attr@Attribute("attr1", _, _) =&gt; attr.copy(value=attr.value.text.toInt * 2) case attr@Attribute("attr2", _, _) =&gt; attr.copy(value=attr.value.text.toInt * -1) case other =&gt; other } ) </code></pre> <p>There are two reasons this won't work out of the box:</p> <ol> <li><code>Attribute</code> doesn't have a useful <code>copy</code> method, and </li> <li>Mapping over a <code>MetaData</code> yields an <code>Iterable[MetaData]</code> instead of a <code>MetaData</code> so even something as simple as <code>elem.copy(attributes=elem.attributes.map(x =&gt; x))</code> will fail.</li> </ol> <p>To fix the first problem, we'll use an implicit to add a better copy method to <code>Attribute</code>:</p> <pre><code>implicit def addGoodCopyToAttribute(attr: Attribute) = new { def goodcopy(key: String = attr.key, value: Any = attr.value): Attribute = Attribute(attr.pre, key, Text(value.toString), attr.next) } </code></pre> <p>It can't be named <code>copy</code> since a method with that name already exists, so we'll just call it <code>goodcopy</code>. (Also, if you're ever creating values that are <code>Seq[Node]</code> instead of things that should be converted to strings, you could be a little more careful with <code>value</code>, but for our current purposes it's not necessary.)</p> <p>To fix the second problem, we'll use an implicit to explain how to create a <code>MetaData</code> from an <code>Iterable[MetaData]</code>:</p> <pre><code>implicit def iterableToMetaData(items: Iterable[MetaData]): MetaData = { items match { case Nil =&gt; Null case head :: tail =&gt; head.copy(next=iterableToMetaData(tail)) } } </code></pre> <p>Then you can write code pretty much like what I proposed at the beginning:</p> <pre><code>scala&gt; val elem = &lt;b attr1 = "100" attr2 = "50"/&gt; elem: scala.xml.Elem = &lt;b attr1="100" attr2="50"&gt;&lt;/b&gt; scala&gt; elem.copy(attributes= | for (attr &lt;- elem.attributes) yield attr match { | case attr@Attribute("attr1", _, _) =&gt; | attr.goodcopy(value=attr.value.text.toInt * 2) | case attr@Attribute("attr2", _, _) =&gt; | attr.goodcopy(value=attr.value.text.toInt * -1) | case other =&gt; other | } | ) res1: scala.xml.Elem = &lt;b attr1="200" attr2="-50"&gt;&lt;/b&gt; </code></pre>
 

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