为采用带有类型参数的方法的typeclass定义构造函数？

Question

abstract class IsTC[A[_], T] {
  // type S is an abstract type rather than type parameter, but must implement Valid[A]
  type S
  implicit val sIsValid: Valid[S]

  def get(self: A[T], i: Int): T
  def changeType[_T]: A[_T]
}
object IsTC {
  def apply[A[_], T, _S](
    fget: (A[T], Int) => T,
    fchangeType:  // what should this type annotation be?
  ): IsTC[A, T] { type S = _S } = new IsTC[A, T] {
    type S = _S
    def get(self: A[T], i: Int) = fget(self, i)
    def changeType[_T]: A[_T] = fchangeType[_T]
  }
}

Answer 1

Scala2不支持多态函数。多态性可以是方法，而不是值。和函数都是值。多态函数可以用包装器进行仿真

// for [A] => (a: A) => B[A]
trait Poly {
  def apply[A](a: A): B[A]
}

或

// for [A <: U] => (a: A) => B[A]
trait Poly[U] {
  def apply[A <: U](a: A): B[A]
}

比如shapeless.poly。它们推广了普通函数

trait Function[A, B] {
  def apply(a: A): B
}

object IsTC {
  def apply[A[_], T, _S](
                          fget: (A[T], Int) => T,
                          fchangeType: FchangeType[A]
                        ): IsTC[A, T] { type S = _S } = new IsTC[A, T] {
    override type S = _S
    override implicit val sIsValid: Valid[_S] = ???
    override def get(self: A[T], i: Int) = fget(self, i)
    override def changeType[_T]: A[_T] = fchangeType[_T]
  }
}

trait FchangeType[A[_]] {
  def apply[X]: A[X]
}