spark隐式转换 toDf_隐式转换是什么

Spark SQL 中，
	DF.select()

select 报错 不能导入
	spark sql Cannot resolve overloaded method 'select'

咨询大佬后，隐式转换的原因 ,导入Spark的隐式转换后即可
	import spark.implicits._

import java.io.File
import scala.io.Source

object implicit2 { 
   
  def main(args: Array[String]): Unit = { 
   

    //java.io.File 只封装了文件的元数据，文件内容必须通过IO
    //所以File 后无法直接获取context
    val context:String = new File("").readContext
  }

}

object implicit2 { 
   
  def main(args: Array[String]): Unit = { 
   
    //声明隐式转换
    implicit def file2RichFile(file: File):RichFile = new RichFile(file)
    

    //java.io.File 只封装了文件的元数据，文件内容必须通过IO
    //所以File 后无法直接获取context
    val context:String = new File("").readContext
  }

}

class RichFile(file:File){ 
   
  //自己封装一个，让File后能readContext
  def readContext:String = { 
   
    Source.fromFile(file).mkString
  }
}

java.io.File 无 readContext方法
	查找implicit函数
		传入参数为File ， 返回方法当中有没有一个方法为readcontext
		以上匹配关系必须唯一

implicit def int2Date(int: Int):RichDate = new RichDate(int)

    val ago:String = "ago"
    val later:String = "later"
    val day2 = 2.days(ago)

class RichDate(day:Int){
  def days(when : String) = {
    if("ago"==when)
      LocalDate.now().plusDays(-day).toString
    else if("later"==when)
      LocalDate.now().plusDays(day).toString
    else
      println("later or age error")
  }
}

object implicit3 {
  def main(args: Array[String]): Unit = {

    val ago : String = "ago"
    val later : String = "later"

    println(3.days(later))

    implicit class RichDate(day:Int){
      def days(when:String): Unit ={
        if ("ago" == when){
          LocalDate.now().plusDays(-day).toString
        }else if("later" == when){
          LocalDate.now().plusDays(day).toString
        }
      }
    }
  }

  /**
   * :: Experimental ::
   * (Scala-specific) Implicit methods available in Scala for converting
   * common Scala objects into `DataFrame`s.
   *
   * {
  
  {
  
  {
   *   val sparkSession = SparkSession.builder.getOrCreate()
   *   import sparkSession.implicits._
   * }}}
   *
   * @since 2.0.0
   */
  @Experimental
  @InterfaceStability.Evolving
  object implicits extends SQLImplicits with Serializable {
    protected override def _sqlContext: SQLContext = SparkSession.this.sqlContext
  }

  /**
   * Selects a set of column based expressions.
   * {
  
  {
  
  {
   *   ds.select($"colA", $"colB" + 1)
   * }}}
   *
   * @group untypedrel
   * @since 2.0.0
   */
  @scala.annotation.varargs
  def select(cols: Column*): DataFrame = withPlan {
    Project(cols.map(_.named), logicalPlan)
  }