隐式参数在隐式转换中的应用

Question

我试图找到语言规范中应该告诉我这些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，我想做的是将某种类型的列表分配给声明为另一种类型的变量，并启用隐式转换。显然这不起作用。我可以想出解决方法，但我只是讨厌我不理解这里起作用的一般规则。有人知道吗？

- Wilfred Springer

Answer 1

隐式视图可能需要自身的隐式参数。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)

你的情况会发生什么？好吧，让我们通过这个脚本使用 scala -Ytyper-debug -Xlog-implicits 看看幕后发生了什么：

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]

编译器随后解释道：

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特性。但也许这个输出结果能帮助其他人更好地理解这个问题。

- retronym