避免具有无形替代品的结构类型 [英] Avoid structural type with shapeless alternative

问题描述

我想知道在给定的 case class 中是否存在特定的类成员.

I want to find out if a particular class member is present in a given case class or not.

以下确实给了我那个答案，在编译时失败，这是正确的.(归功于 Travis Brown)

Following does give me that answer, failing at compile time which is right. (credit to Travis Brown)

scala>  def getIntId[A, R <: HList](a: A)(implicit
 |   gen: LabelledGeneric.Aux[A, R],
 |   sel: Selector.Aux[R, Witness.`'id`.T, Int]
 | ): Int = sel(gen.to(a))

 case class Foo(id: String)
 case class Bar(id: Int, name: String)

scala> getIntId(Bar(123, "bar"))
res3: Int = 123

scala> getIntId(Foo("12345"))
<console>:15: error: could not find implicit value for parameter sel:    shapeless.ops.record.Selector.Aux[R,Symbol with shapeless.tag.Tagged[String("id")],Int]
          getIntId(Foo("12345"))

现在如果我没有一个具体的类型但是 T 传递给 getIntId 方法，有没有办法让它与泛型类型 一起工作 ?

Now if I don't have a concrete type but T to pass into getIntId method, is there a way to get this to work with generic type T ?

更新假设有一个名为 findIdFromType 的方法，如下所述，它接受类型 T(其中 T 将始终是某个 case 类).有可能完成这项工作吗?

Update Say have a method called findIdFromType as described below which takes in type T (where T will always be a some case class). is it possible to make this work?

def findIdFromType[T](t:T) = {
  getIntId(t) //as expected doesn't compile
}

推荐答案

getIntId 需要两个隐式参数.

如果需要在本地方法的上下文中调用它，则需要证明这些参数存在于这样的上下文中.

If you need to call it in the context of a local method, you need to prove that those parameters exist in such context.

为了做到这一点，你必须像这样传播隐性证据

In order to do so, you have to propagate the implicit evidence, like this

def findIdFromType[A, R <: HList](a: A)(implicit
  gen: LabelledGeneric.Aux[A, R],
  sel: Selector.Aux[R, Witness.`'id`.T, Int]): Int = getIntId(a)

这当然是一个完全没用的例子，但如果你想在包装方法内部执行其他操作，这是要走的路:传播隐式证据.

This of course is a completely useless example, but in case you want to perform other operations inside the wrapping method, this is the way to go: propagate the implicit evidence.

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