如何解决编译时的类型错误：Any to T？

Question

我有一个 json 元组序列化程序。它首先反映在 Tuple 上并构建一个函数，给定一个元组，它将 return (TypeAdapter, value) 对的列表。（TypeAdapter 是一种呈现值的特定于类型的东西。）看起来像这样：

def extractTuple(p: Product): (Product)=>List[(TypeAdapter[_], Any)] = {
  val reflected = reflectOnTuple(tupleClass)  // extract a bunch of reflected metadata
  val tupleFieldInfo = reflected.tupleFieldInfo
  tupleFieldInfos match {
    case 1 =>
      (p: Product) => 
        List( (getTypeAdapterFor(tupleFieldInfo(0)), p.asInstanceOf[Tuple1[_]]._1) )
    case 2 =>
      (p: Product) => 
        List( (getTypeAdapterFor(tupleFieldInfo(0)), p.asInstanceOf[Tuple1[_]]._1),
          (getTypeAdapterFor(tupleFieldInfo(1)), p.asInstanceOf[Tuple1[_]]._2) )
    //... and so on to Tuple23
  }
}

在 JSON 序列化程序中，我有一个 writeTuple() 函数，如下所示。理论上它应该按原样工作，但是......我在 fieldValue 上遇到编译错误，说它是 Any 类型，而预期的是 ?1.T.

TypeAdapter 看起来像：

trait TypeAdapter[T] {
  def write[WIRE](
      t:      T,
      writer: Writer[WIRE],
      out:    mutable.Builder[WIRE, WIRE]): Unit
}

class JsonWriter() {
  def writeTuple[T](t: T, writeFn: (Product) => List[(TypeAdapter[_], Any)], out: mutable.Builder[JSON, JSON]): Unit = {
    out += "[".asInstanceOf[JSON]
    var first = true
    writeFn(t.asInstanceOf[Product]).foreach { case (fieldTA, fieldValue) =>
      if (first)
        first = false
      else
        out += ",".asInstanceOf[JSON]
      fieldTA.write(fieldValue, this, out) // <<-- this blows up (compile) on fieldValue because it's type Any, not some specific field Type
    }
    out += "]".asInstanceOf[JSON]
  }
}

如何让我的 TypeAdapter 相信该字段是正确的类型？

Answer 1

尝试将类型模式与类型变量一起使用 tp (lower-case)

class JsonWriter() extends Writer[JSON] {
  def writeTuple[T](t: T, writeFn: (Product) => List[(TypeAdapter[_], Any)], out: mutable.Builder[JSON, JSON]): Unit = {
    ...
    writeFn(t.asInstanceOf[Product]).foreach { case (fieldTA: TypeAdapter[tp], fieldValue) =>
      ...
      fieldTA.write(fieldValue.asInstanceOf[tp], this, out)
    }
    ...
  }
}

或T代替它

class JsonWriter() extends Writer[JSON] {
  def writeTuple[T](t: T, writeFn: (Product) => List[(TypeAdapter[_], Any)], out: mutable.Builder[JSON, JSON]): Unit = {
    ...
    writeFn(t.asInstanceOf[Product]).foreach { case (fieldTA: TypeAdapter[T], fieldValue) =>
      ...
      fieldTA.write(fieldValue.asInstanceOf[T], this, out)
    }
    ...
  }
}

看来您实际上并没有在 writeTuple[T] 中使用 T。也许你可以尝试单态 def writeTuple(t: Product... 此外，如果你想指定 (TypeAdapter[_], Any) 实际上是 (TypeAdapter[U], U) 在元组的两个元素中具有相同的 U 你可以尝试存在类型 (TypeAdapter[U], U) forSome {type U}。所以试试

class JsonWriter() extends Writer[JSON] {
  def writeTuple(t: Product, writeFn: (Product) => List[(TypeAdapter[U], U) forSome {type U}], out: mutable.Builder[JSON, JSON]): Unit = {
    out += "[".asInstanceOf[JSON]
    var first = true
    writeFn(t).foreach { case (fieldTA, fieldValue) =>
      if (first)
        first = false
      else
        out += ",".asInstanceOf[JSON]
      fieldTA.write(fieldValue, this, out)
    }
    out += "]".asInstanceOf[JSON]
  }
}

Answer 2

Dmytro，你的答案非常接近！不幸的是，第一个或第二个选项都没有编译。我认为第三个会很好，除了......我实际上使用的是 Dotty 而不是 Scala，并且 Dotty 消除了存在类型。

所以我尝试了以下方法，它奏效了。它涉及修改 TypeAdapter，因为它知道它的类型：

trait TypeAdapter[T] {
  type tpe = T
  def write[WIRE](
      t:      T,
      writer: Writer[WIRE],
      out:    mutable.Builder[WIRE, WIRE]): Unit
  inline def castAndWrite[WIRE]( 
      v:      Any, 
      writer: Writer[WIRE], 
      out:    mutable.Builder[WIRE, WIRE]): Unit = 
    write(v.asInstanceOf[tpe], writer, out)
}

从 JsonWriter 调用 castAndWrite() 可以调用 correctly-typed write() 机制。

如何解决编译时的类型错误：Any to T？

How can I resolve type error on compile: Any to T?

scala

generics

existential-type