scala 2.13 自动隐式解析错误

Question

我正面临这个与 scala 隐式解析相关的奇怪问题

这是代码片段

import scala.collection.Factory
import scala.collection.immutable.Seq

sealed trait A

sealed trait B

case class BImpl() extends B

case class AImpl() extends A

object implicitsContainer {

  type AB = (A, B)

  implicit def toStringAnyTuples[C[X] <: Iterable[X], A <: AB]
  (col: C[A])
  (implicit factory: Factory[(String, Any), C[(String, Any)]])
  : C[(String, Any)] = {

    factory.fromSpecific(col.iterator.map(f => f._1.toString -> f._2))
  }
}

object Main extends App {

  import implicitsContainer._

  def a(f: Seq[(String, Any)]): Seq[(String, Any)] = f

  val w: Seq[(AImpl, BImpl)] = Seq(AImpl() -> BImpl())

  val x: Seq[(String, Any)] = a(w)

  //    Won't compile
  //    val y: Seq[(String, Any)] = a(Seq(AImpl() -> BImpl()))
}

Scala 自动获取隐式方法

implicit def toStringAnyTuples[C[X] <: Iterable[X], A <: AB](col: C[A])
  (implicit factory: Factory[(String, Any), C[(String, Any)]])
  : C[(String, Any)] = {

    factory.fromSpecific(col.iterator.map(f => f._1.toString -> f._2))
  }

为此：-

  val w: Seq[(AImpl, BImpl)] = Seq(AImpl() -> BImpl())

  val x: Seq[(String, Any)] = a(w)

但为此抛出错误

val y: Seq[(String, Any)] = a(Seq(AImpl() -> BImpl()))

错误是：-

Error:(44, 47) type mismatch;
 found   : (AImpl, BImpl)
 required: (String, Any)
    val y: Seq[(String, Any)] = a(Seq(AImpl() -> BImpl()))

还有一点，如果我从 w

中删除类型

  val w = Seq(AImpl() -> BImpl())

  val x: Seq[(String, Any)] = a(w)

那么这也可以正常工作。唯一的错误是

val y: Seq[(String, Any)] = a(Seq(AImpl() -> BImpl()))

我正在使用：-

SCALA -> 2.13.3

SBT -> 1.3.13

JAVA -> 14

Answer 1

这只是类型推断问题。未推断 Seq.apply 的类型参数。尝试

val y: Seq[(String, Any)] = a(Seq[(AImpl, BImpl)](AImpl() -> BImpl()))

或

val y: Seq[(String, Any)] = a(Seq[(A, B)](AImpl() -> BImpl()))

Answer 2

您遇到的是 Scala 编译器中推理工作方式的症状。这是一个显示相同问题的较小示例：

object Test {
  class C[T](x: T)
  implicit def conv(c: C[Int]): C[String] = ???
  def m(cs: C[String]) = 1
  val ci = new C(1)
  def t1: Int = m(ci)       // OK
  def t2: Int = m(new C(1)) // found: Int, expected: String
}

当type-checkingnew C(1)时，编译器将预期的类型String下推为type 检查表达式 1，但失败了。在上面的行中，类型检查 ci 与预期类型 C[String] 由于隐式转换而成功。我在这里的建议是定义一个执行转换的扩展方法，而不是使转换隐式。为清楚起见，也建议这样做 - 隐式转换如您的示例中定义的那个可能会导致令人惊讶的、难以诊断的问题。在我的示例中，它看起来像这样：

object Test {
  class C[T](x: T)
  implicit class CExt(private val c: C[Int]) extends AnyVal {
    def toCString: C[String] = ???
  }
  def m(cs: C[String]) = 1
  val ci = new C(1)
  def t1: Int = m(ci.toCString)
  def t2: Int = m(new C(1).toCString)
}

scala 2.13 自动隐式解析错误

scala 2.13 auto implicit resolution error

scala

scala-collections

implicit

scala-2.13