在Scala反射中，如何获取具体子类的泛型类型参数？ [英] In Scala Reflection, How to get generic type parameter of a concrete subclass?

问题描述

假设我有一个Generic超类：

pre $ class GenericExample [T]（
a：String，
b：T
）{

def fn（i：T）：T = b
}


和一个具体的子类：

  case类示例（
a ：字符串，
b：Int 
）扩展GenericExample [Int]（a，b）
  

我想通过scala反射来获取函数fn的类型参数，所以我选择并过滤其成员：

  import ScalaReflection.universe._ 
 
 val baseType = typeTag [示例] 
 
 val成员= baseType 
 .tpe 
 .member（methodName： TermName）
 .asTerm 
 .alternatives 
 .map（_。asMethod）
 .head 
 
 val paramss = member.paramss 
 val actualTypess：List [List [Type]] = paramss.map {
 params => 
 params.map {
 param => 
 param.typeSignature 
 
 
  

我期待scala给我正确的结果，它是 List（List（Int）），而我只有通用的 List（List（T））

在文档中琢磨我发现typeSignature是罪魁祸首：

  *该方法总是以最通用的方式返回签名，即使底层符号是从泛型类型的
 *实例获得的。 
  

这表明我使用了另一种方法：

  def typeSignatureIn（site：Type）：Type 
  

但是，由于类示例不再是通用的，我没有办法从typeTag [示例]获取网站，任何人都可以告诉我如何获得typeOf [Int]只给typeTag [示例]？或者有没有办法做到这一点，我不得不恢复到Java反射？

非常感谢您的帮助。

更新：经过一些快速测试后，我发现即使是 MethodSymbol.returnType 也无法按预期工作，如下代码：

  member.returnType 
  

T ，并且不能通过 asSeenFrom 更正，因为以下代码不会更改结果：

  member.returnType.asSeenFrom（baseType.tpe，baseType.tpe.typeSymbol.asClass）
  

解决方案

我发布了我的解决方案：我认为Scala的设计没有别的选择：

Scala反射& Java反射是currying：Scala方法由多对括号组成，调用一个带参数的方法首先构造一个匿名类，它可以带多个括号对，或者如果没有更多的括号，则构造一个NullaryMethod类（也称为call-按名称），可以解决该问题以产生该方法的结果。因此，只有在方法已经分解为Method& NullaryMethod签名。

结果很明显，结果类型只能使用递归获得：

 private def methodSignatureToParameter_ReturnTypes（tpe：Type）：（List [List [Type]]，Type）= {
 tpe match {
 case n：NullaryMethodType => 
无 - > n.resultType 
 case m：MethodType => 
 val paramTypes：List [Type] = m.params.map（_。typeSignatureIn（tpe））
 val downstream = methodSignatureToParameter_ReturnTypes（m.resultType）
 downstream.copy（_1 = List（ paramTypes）++ methodSignatureToParameter_ReturnTypes（m.resultType）._ 1）
 case _ => 
无 - > tpe 
} 
} 
 
 def getParameter_ReturnTypes（symbol：MethodSymbol，impl：Type）= {
 $ b $ val签名= symbol.typeSignatureIn（impl）
 val result = methodSignatureToParameter_ReturnTypes（签名）
结果
} 
  

哪里 impl 是拥有该方法的类，并且符号是您从 Type中获得的。会员 scala反射

Assuming that I have a Generic superclass:

class GenericExample[T](
                         a: String,
                         b: T
                       ) {

  def fn(i: T): T = b
}

and a concrete subclass:

case class Example(
                    a: String,
                    b: Int
                  ) extends GenericExample[Int](a, b)

I want to get the type parameter of function "fn" by scala reflection, so I select and filter through its members:

import ScalaReflection.universe._

val baseType = typeTag[Example]

val member = baseType
  .tpe
  .member(methodName: TermName)
  .asTerm
  .alternatives
  .map(_.asMethod)
  .head

    val paramss = member.paramss
    val actualTypess: List[List[Type]] = paramss.map {
      params =>
        params.map {
          param =>
            param.typeSignature
        }
    }

I was expecting scala to give me the correct result, which is List(List(Int)), instead I only got the generic List(List(T))

Crunching through the document I found that typeSignature is the culprit:

 *  This method always returns signatures in the most generic way possible, even if the underlying symbol is obtained from an
 *  instantiation of a generic type.

And it suggests me to use the alternative:

def typeSignatureIn(site: Type): Type

However, since class Example is no longer generic, there is no way I can get site from typeTag[Example], can anyone suggest me how to get typeOf[Int] given only typeTag[Example]? Or there is no way to do it and I have to revert to Java reflection?

Thanks a lot for your help.

UPDATE: After some quick test I found that even MethodSymbol.returnType doesn't work as intended, the following code:

member.returnType

also yield T, annd it can't be corrected by asSeenFrom, as the following code doesn't change the result:

member.returnType.asSeenFrom(baseType.tpe, baseType.tpe.typeSymbol.asClass)

解决方案

I'm posting my solution: I think there is no alternative due to Scala's design:

The core difference between methods in Scala reflection & Java reflection is currying: Scala method comprises of many pairs of brackets, calling a method with arguments first merely constructs an anonymous class that can take more pairs of brackets, or if there is no more bracket left, constructs a NullaryMethod class (a.k.a. call-by-name) that can be resolved to yield the result of the method. So types of scala method is only resolved at this level, when method is already broken into Method & NullaryMethod Signatures.

As a result it becomes clear that the result type can only be get using recursion:

  private def methodSignatureToParameter_ReturnTypes(tpe: Type): (List[List[Type]], Type) = {
    tpe match {
      case n: NullaryMethodType =>
        Nil -> n.resultType
      case m: MethodType =>
        val paramTypes: List[Type] = m.params.map(_.typeSignatureIn(tpe))
        val downstream = methodSignatureToParameter_ReturnTypes(m.resultType)
        downstream.copy(_1 = List(paramTypes) ++ methodSignatureToParameter_ReturnTypes(m.resultType)._1)
      case _ =>
        Nil -> tpe
    }
  }

  def getParameter_ReturnTypes(symbol: MethodSymbol, impl: Type) = {

    val signature = symbol.typeSignatureIn(impl)
    val result = methodSignatureToParameter_ReturnTypes(signature)
    result
  }

Where impl is the class that owns the method, and symbol is what you obtained from Type.member(s) by scala reflection

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