scala - 运行 reduceLeft 时类型不匹配

Question

我有一个 ConfigRule 特征，它有 transform 方法作为

trait ConfigRule {
  final def transform(doc: Node): Node = { .....}
}

然后我有 ConfigTransformer 看起来像有 transform 方法

trait ConfigTransformer {
    def transform(doc: Node, rules: Seq[ConfigRule]) = {
    rules.reduceLeft((x, y) => x.transform(doc).andThen(y.transform(_)))
  }
}

我尝试将其运行为

object Runner extends App {
  override def main(args: Array[String]): Unit = {
    val n1 = NewNodeRule(<foo/>, <bar/>, Seq.empty, "adding bar under foo")
    val n2 = NewNodeRule(<garage/>, <car/>, Seq.empty, "adding car under garage")

    ConfigTransformer.transform(<Root/>, Seq(n1, n2))
  }
}

编译错误为

Error:(18, 56) type mismatch;
 found   : PartialFunction[Int,scala.xml.Node]
 required: solutions.xml.ConfigRule
    rules.reduceLeft((x, y) => x.transform(doc).andThen(y.transform(_)))

我做错了什么？

Answer 1

我还不是 Scala 专家，但问题是你没有在 reduceLeft 语句中返回 ConfigRule 类型的对象(它是一个 Node，这就是你得到错误的原因)。我猜你要找的是 foldLeft。这是一个例子:

trait ConfigTransformer {
    def transform(doc: Node, rules: Seq[ConfigRule]) = {
        rules.foldLeft(doc)((d, x) => x.transform(d))
    }
}