是否有一种通用的方法来反转curried函数的参数顺序？

Question

我想知道你是否可以编写一个泛型函数,它接受一个curried函数,然后反转参数,如下所示:

def foo(a: String)(b: Boolean)(c: Int): String
val bar = invert(foo _)
foo("baz")(false)(12) must be equalTo(bar(12)(false)("baz"))

只要您为要解决的特定情况添加隐式逆变器,以下操作就可以正常工作.但是我对更一般的情况更感兴趣 - 也就是说,处理任何数量的讨论论点的情况.

trait Inverter[V, W] {
  def invert(v: V): W
}

implicit def function2Inverter[X, Y, Z] = 
  new Inverter[(X, Y) => Z, (Y, X) => Z] {
    def invert(v: (X, Y) => Z) = {
      def inverted(y: Y, x: X) = v(x, y)
      inverted _
    }
  }

implicit def curried2Inverter[X, Y, Z] = 
  new Inverter[X => Y => Z, Y => X => Z] {
    def invert(v: (X) => (Y) => Z) = {
      def inverted(y: Y)(x: X) = v(x)(y)
      inverted _
    }
  }

def invert[V, W](v: V)(implicit inverter: Inverter[V, W]): W = 
  inverter.invert(v)

哦,我希望有一个适用于Scala 2.9的解决方案.

Answer 1

TL; DR:使用这个要点.解释如下:

首先,定义一个类型类(和个案)以部分应用具有最后一个参数的函数:

trait PopLast[A, Last, Rem] {
  def pop(f: A, v: Last): Rem
}

trait LowPrioPopLast {
  implicit def popEnd[A,B] = new PopLast[A => B, A, B] {
    def pop(f: A => B, v: A) = f(v)
  }  
}

object PopLast extends LowPrioPopLast {
  implicit def popOne[A, B, C, Last, IRem](
    implicit iPop: PopLast[B => C, Last, IRem]) = 
      new PopLast[A => B => C, Last, A => IRem] {
        def pop(f: A => B => C, v: Last) = { a: A => iPop.pop(f(a), v) }
      }
}

然后,创建逆变器类型类:递归地执行最后一个参数的部分应用并反转结果.

trait Inverter[A] {
  type Out
  def invert(f: A): Out 
}

trait LowPrioInverter {
  implicit def invertEnd[A,B] = new Inverter[A => B] {
    type Out = A => B
    def invert(f: A => B) = f
  }
}

object Inverter extends LowPrioInverter {
  implicit def invertStep[A, Last, Rem](implicit pop: PopLast[A, Last, Rem],
    inv: Inverter[Rem]) = new Inverter[A] {
      type Out = Last => inv.Out
      def invert(f: A) = { a: Last => inv.invert(pop.pop(f, a)) }
  }
}

最后,封装成一个函数:

def invert[A](f: A)(implicit inv: Inverter[A]) = inv.invert(f)

然后我们去:

def foo(a: String)(b: Boolean)(c: Int): String = "bar"
val bar = invert(foo _)
// bar: Int => Boolean => String => String

bar(1)(true)("foo")