隐式转换中的隐式参数

Question

我试图在语言规范中找到应该告诉我这些隐式转换不起作用的位置：

scala> implicit def listToAlternativeList[F,T](xs: List[F])(implicit conv: (F) => T) = xs map conv
listToAlternativeList: [F,T](xs: List[F])(implicit conv: (F) => T)List[T]

scala> implicit def int2string(i: Int) = i.toString
int2string: (i: Int)java.lang.String

scala> val l = List(1, 2, 3)
l: List[Int] = List(1, 2, 3)

scala> val l2: List[String] = listToAlternativeList[Int,String](l)
l2: List[String] = List(1, 2, 3)

scala> val l2: List[String] = listToAlternativeList(l)            
l2: List[String] = List(1, 2, 3)

scala> val l2: List[String] = l                     
<console>:10: error: type mismatch;
 found   : List[Int]
 required: scala.List[String]
       val l2: List[String] = l
                          ^

基本上，我想要的就是将某种类型的列表分配给声明为的变量另一种类型并开始隐式转换。这显然不起作用。我可以想出解决这个问题的方法，但我只是讨厌我不理解这里工作的一般规则。有人吗？

Answer 1

隐式视图本身可能需要隐式参数。例如：

scala> implicit def listToStringList[A](as: List[A])(implicit f: A => String) = as map f
listToStringList: [A](as: List[A])(implicit f: (A) => String)List[String]

scala> implicit def i2s(i: Int) = i.toString
i2s: (i: Int)java.lang.String

scala> val l = List(1)
l: List[Int] = List(1)

scala> l: List[String]
res0: List[String] = List(1)

您的情况会发生什么？好吧，让我们用这个脚本的 scala -Ytyper-debug -Xlog-implicits 来看看幕后：

implicit def listToList[A, B](as: List[A])(implicit f: A => B): List[B] = as map f
implicit def i2s(i: Int): String = i.toString
val l = List(1)
l: List[String]

编译器然后解释：

typing (l: List[String]), pt = ?, undetparams = List(), implicits-enabled = true, silent = true
    typing List[String], pt = ?, undetparams = List(), implicits-enabled = true, silent = true
      typing scala.package, pt = ?, undetparams = List(), implicits-enabled = true, silent = true
        typing scala, pt = ?, undetparams = List(), implicits-enabled = true, silent = true
        typed scala:type with underlying package scala, undetparams = List(), pt = ?
        adapted scala:package scala to ?, List()
      typed scala.package:type with underlying object package, undetparams = List(), pt = ?
      adapted scala.package:object package to ?, List()
      typing String, pt = ?, undetparams = List(), implicits-enabled = true, silent = true
      typed scala.this.Predef.String:String, undetparams = List(), pt = ?
      adapted scala.this.Predef.String:String to ?, List()
    typed List[String]:List[String], undetparams = List(), pt = ?
    adapted List[String]:List[String] to ?, List()
    typing l, pt = List[String], undetparams = List(), implicits-enabled = true, silent = true
    typed $anon.this.l:=> List[Int], undetparams = List(), pt = List[String]
Beginning implicit search for $anon.this.l expecting (List[Int]) => List[String] looking for a view
begin implicit search: ($anon.this.l,(List[Int]) => List[String],true,List())
typed impl for (List[Int]) => List[String]? listToList:(as: List[?])(implicit f: (?) => ?)List[?] orig info= [A,B](as: List[A])(implicit f: (A) => B)List[B]/List()/true/true/this.type/true
typedImplicit0 typing$anon.this.listToList with wildpt = (List[Int]) => List[String] from implicit listToList:[A,B](as: List[A])(implicit f: (A) => B)List[B]
  typing $anon.this.listToList, pt = ?, undetparams = List(), implicits-enabled = false, silent = false
    typing $anon.this, pt = ?, undetparams = List(), implicits-enabled = false, silent = false
    typed $anon.this:this.type with underlying java.lang.Object{}, undetparams = List(), pt = ?
    adapted $anon.this:java.lang.Object{} to ?, List()
  typed $anon.this.listToList:[A,B](as: List[A])(implicit f: (A) => B)List[B], undetparams = List(), pt = ?
  adapted $anon.this.listToList:[A,B](as: List[A])(implicit f: (A) => B)List[B] to ?, List(type A, type B)
  typing <argument>, pt = List[?], undetparams = List(), implicits-enabled = false, silent = false
  typed <argument>:List[Int], undetparams = List(), pt = List[?]
  adapted <argument>:List[Int] to List[?], List()
  typing <argument>, pt = List[Int], undetparams = List(), implicits-enabled = false, silent = false
  typed <argument>:List[Int], undetparams = List(), pt = List[Int]
  adapted <argument>:List[Int] to List[Int], List()
typed implicit $anon.this.listToList[Int, B](<argument>):(implicit f: (Int) => B)List[B], pt = (List[Int]) => List[String]
adapted implicit method listToList:(as: List[Int])(implicit f: (Int) => B)List[B] to (List[Int]) => List[String]
incompatible: (as: List[Int])(implicit f: (Int) => B)List[B] does not match (List[Int]) => List[String]
Implicit search yielded: SearchResult(<empty>, TreeTypeSubstituter(List(),List()))

我不太确定这是一个错误还是功能。但也许这个输出将帮助其他人进一步阐明这个问题。