使用隐式功能实现Cake Pattern

Question

我有一个场景,我想实现一个Cake Pattern的变体,但是向一个类添加隐式功能(一个Spark DataFrame).

所以,基本上,我希望能够运行如下代码:

trait Transformer {
  this: ColumnAdder =>

  def transform(input: DataFrame): DataFrame = {
    input.addColumn("newCol")
  }
}

val input = sqlContext.range(0, 5)
val transformer = new Transformer with StringColumnAdder
val output = transformer.transform(input)
output.show

并找到如下结果:

+---+------+
| id|newCol|
+---+------+
|  0|newCol|
|  1|newCol|
|  2|newCol|
|  3|newCol|
|  4|newCol|
+---+------+

我的第一个想法是仅在基本特征中定义隐式类:

trait ColumnAdder {
  protected def _addColumn(df: DataFrame, colName: String): DataFrame

  implicit class ColumnAdderRichDataFrame(df: DataFrame) {
    def addColumn(colName: String): DataFrame = _addColumn(df, colName)
  }
}

trait StringColumnAdder extends ColumnAdder {
  protected def _addColumn(df: DataFrame, colName: String): DataFrame = {
    df.withColumn(colName, lit(colName))
  }
}

和它的作品,但我不是这种方法完全满意,因为函数签名重复的.所以我想到了另一种方法,使用(已弃用？)implicit def策略:

trait ColumnAdder {
  protected implicit def columnAdderImplicits(df: DataFrame): ColumnAdderDataFrame

  abstract class ColumnAdderDataFrame(df: DataFrame) {
    def addColumn(colName: String): DataFrame
  }
}

trait StringColumnAdder extends ColumnAdder {
  protected implicit def columnAdderImplicits(df: DataFrame): ColumnAdderDataFrame = new StringColumnAdderDataFrame(df)

  class StringColumnAdderDataFrame(df: DataFrame) extends ColumnAdderDataFrame(df) {
    def addColumn(colName: String): DataFrame = {
      df.withColumn(colName, lit(colName))
    }
  }
}

(可在此处找到完整的可重现代码,包括额外的特征模块)

所以,我想问哪种方法是最好的,是否有另一种更好的方法来实现我想要的.

Answer 1

只有两个快捷方式，但没有什么真正令人惊讶的：

trait ColumnAdder {
  protected implicit def columnAdderImplicits(df: DataFrame): ColumnAdderDataFrame
  abstract class ColumnAdderDataFrame {
    def addColumn(colName: String): DataFrame
  }
}

trait StringColumnAdder extends ColumnAdder {
  override def columnAdderImplicits(df: DataFrame) =
    new ColumnAdderDataFrame {
      def addColumn(colName: String): DataFrame =
        df.withColumn(colName, lit(colName))
    }
}

如果您愿意启用-language:reflectiveCalls（请注意影响），那么您还可以编写：

trait ColumnAdder {
  protected implicit def columnAdderImplicits(df: DataFrame): ColumnAdderDataFrame
  abstract class ColumnAdderDataFrame {
    def addColumn(colName: String): DataFrame
  }
}

trait StringColumnAdder extends ColumnAdder {
  override def columnAdderImplicits(df: DataFrame) =
    new ColumnAdderDataFrame {
      def addColumn(colName: String): DataFrame =
        df.withColumn(colName, lit(colName))
    }
}