如何使 Scala 类型推断足够强大以发现泛型类型参数? [英] How to make Scala type inference powerful enough to discover generic type parameter?

问题描述

假设我定义了一个采用隐式 TypeTag 的函数:

Assuming that I defined a function that takes implicit TypeTag:

  def andOptionFn[A: TypeTag](g: Int => Option[A]) = {
    val ttg = ScalaReflection.universe.typeTag[A]
    println(ttg)
    ...
  }

并调用它:

andOptionFn{
v =>
  Some(v)
}

我希望 Scala 类型推断能够获得正确的类型 TypeTag(Option(Int))，但我得到了:

I would expect scala type inference to get the correct type TypeTag(Option(Int)), but instead I got:

TypeTag(Option(Any)) (not even a higher kind)

为什么Scala无法在编译时自动推断?以及需要做哪些工作来改进它?

Why Scala is unable to infer it automatically in compilation time? and what works needs to be done to improve it?

推荐答案

正确的结果是 TypeTag[Int]，Scala 确实打印了它:http://scastie.org/20273(在您将 ScalaReflection 替换为 scala.reflect.runtime 之后；如果ScalaReflection 是你自己的类，那是你应该寻找问题的地方).

The correct result is TypeTag[Int], and Scala does print it: http://scastie.org/20273 (after you replace ScalaReflection with scala.reflect.runtime; if ScalaReflection is your own class, that's where you should look for the problem).

http://scastie.org/20293 不是错误，而是类型推断的准确行为按照设计.对 Seq.apply[Option[_]] 的调用为每个参数提供了 Option[_] 的预期类型.什么时候

http://scastie.org/20293 isn't a bug, it's type inference behaving precisely as designed. Call to Seq.apply[Option[_]] gives the expected type of Option[_] to each argument. When

andOptionFn {
  v =>
    if (v > 1) Some(v)
    else None
}

使用这种预期类型进行类型检查，推断出的R当然是Any.

is typechecked with this expected type, the inferred R is of course Any.

编辑 2:阅读 http://www.scala-lang.org/files/archive/spec/2.11/06-expressions.html#local-type-in​​ference 更仔细，它似乎应该推断 Int 毕竟，但我不确定.我认为 Option[_] 被简化为 Option[Any](因为 Option 是协变的)，在这种情况下 Any 肯定应该被推断出来.但是 http://scastie.org/20293 和 http://scastie.org/20295 应该表现相同，所以至少有一个错误(我认为):)

EDIT 2: After reading http://www.scala-lang.org/files/archive/spec/2.11/06-expressions.html#local-type-inference more carefully, it does seem like it should infer Int after all, but I am not certain. What I thought is that Option[_] gets reduced to Option[Any] (because Option is covariant), in which case Any should certainly be inferred. But http://scastie.org/20293 and http://scastie.org/20295 should behave the same, so there is at least one bug (I think) :)

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