scala - 隐式转换中的隐式参数

Question

我试图在语言规范中找到应该通知我这些类型的隐式转换不起作用的地方:

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，我想要的只是将某种类型的列表分配给声明为另一种类型的变量，并启动隐式转换。这显然行不通。我可以想出办法解决这个问题，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 View 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)

在你的情况下会发生什么？好吧，让我们看看幕后的 scala -Ytyper-debug -Xlog-implicits 和这个脚本:

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]

然后编译器解释:

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()))

我不太确定这是错误还是功能。但也许这个输出会帮助其他人进一步阐明这个问题。