带有空格的 Scala XML 模式

发布于 2024-12-23 20:30:11 字数 124 浏览 1 评论 0原文

是否存在与以下模式匹配的 xml 值？

xml match { case <foo> { x } </foo> => 42 }

原文

Is there a value of xml which will match the following pattern?

xml match { case <foo> { x } </foo> => 42 }

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橘香 2024-12-30 20:30:11

是的。

scala> val x = 33
x: Int = 33

scala> val xml = <foo> { x } </foo>
xml: scala.xml.Elem = <foo> 33 </foo>

scala> xml match { case <foo> { x } </foo> => 42 }
res0: Int = 42

我认为您的困惑之处在于它与 不匹配。 33

scala> <foo> 33 </foo> match { case <foo> { x } </foo> => 42 }
scala.MatchError: <foo> 33 </foo> (of class scala.xml.Elem)

这是因为当您使用 {} 时，scala 会在 {} 前后的空格中插入额外的元素，因此您得到三个元素，而不是一个。您可以通过调用 unapplySeq （这是用于模式匹配的）来查看这一点：

scala> scala.xml.Elem.unapplySeq(<foo> 33 </foo>)
res4: Option[(String, String, scala.xml.MetaData, scala.xml.NamespaceBinding, Seq[scala.xml.Node])] =
    Some((null,foo,,,ArrayBuffer( 33 )))

scala> scala.xml.Elem.unapplySeq(<foo> { x } </foo>)
res5: Option[(String, String, scala.xml.MetaData, scala.xml.NamespaceBinding, Seq[scala.xml.Node])] =
    Some((null,foo,,,ArrayBuffer( , 33,  )))

请注意，在第二个示例中，您在 ArrayBuffer 中获得了三个元素，而在第一个示例中仅获得了一个元素。所以模式不正确匹配。

Yes.

scala> val x = 33
x: Int = 33

scala> val xml = <foo> { x } </foo>
xml: scala.xml.Elem = <foo> 33 </foo>

scala> xml match { case <foo> { x } </foo> => 42 }
res0: Int = 42

I think where your confusion comes in is it doesn't match against <foo> 33 </foo>

scala> <foo> 33 </foo> match { case <foo> { x } </foo> => 42 }
scala.MatchError: <foo> 33 </foo> (of class scala.xml.Elem)

This is because when you use the {}, scala inserts extra elements for the spaces before and after the {}, so you get three elements, not one. You can see this by calling unapplySeq (which is what is used for the pattern matching):

scala> scala.xml.Elem.unapplySeq(<foo> 33 </foo>)
res4: Option[(String, String, scala.xml.MetaData, scala.xml.NamespaceBinding, Seq[scala.xml.Node])] =
    Some((null,foo,,,ArrayBuffer( 33 )))

scala> scala.xml.Elem.unapplySeq(<foo> { x } </foo>)
res5: Option[(String, String, scala.xml.MetaData, scala.xml.NamespaceBinding, Seq[scala.xml.Node])] =
    Some((null,foo,,,ArrayBuffer( , 33,  )))

Notice in the second example, you're getting three elements in the ArrayBuffer, and only one in the first. So the pattern doesn't match correctly.

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