scala - 是否可以改进 Scala 中部分应用类型的类型推断？

Question

我正在尝试改进 traverse_ 的类型推断下面代码中的函数:

import scala.language.higherKinds

trait Applicative[AF[_]] {

  def ap[A, B](a: AF[A])(f: AF[A => B]): AF[B]

  def pure[A](a: A): AF[A]

  def fmap[A, B](a: AF[A])(f: A => B): AF[B]

}

def traverse_[AP[_]: Applicative, A](xs: Iterable[A])(f: A => AP[Unit]): AP[Unit] = {
  val ap = implicitly[Applicative[AP]]
  (xs :\ ap.pure(())) { (x, acc) =>
    val apFunc = ap.fmap(f(x))(a => identity[Unit] _)
    ap.ap(acc)(apFunc)
  }
}

implicit def optionAp = new Applicative[Option] {

  def ap[A, B](a: Option[A])(f: Option[A => B]): Option[B] = f flatMap (a map _)

  def pure[A](a: A) = Some(a)

  def fmap[A, B](a: Option[A])(f: A => B) = a map f

}

implicit def eitherAp[L] = new Applicative[({type l[x]=Either[L, x]})#l] {

  def ap[A, B](a: Either[L, A])(f: Either[L, A => B]): Either[L, B] = f.right flatMap (a.right map _)

  def pure[A](a: A) = Right(a)

  def fmap[A, B](a: Either[L, A])(f: A => B) = a.right map f

}

// silly, but compiles
val x = traverse_(1 to 10) {
  case 5 => None
  case _ => Some(())
}
println(x)

// also silly, but does not compile
val y = traverse_(1 to 10) {
  case 5 => Left("x")
  case _ => Right(())
}
println(y)

运行上面给出:

/Users/lodea/tmp/traverse.scala:49: error: no type parameters for method traverse_: (f: Int => AP[Unit])(implicit evidence$1: this.Applicative[AP])AP[Unit] exist so that it can be applied to arguments (Int => Product with Serializable with scala.util.Either[String,Unit])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : Int => Product with Serializable with scala.util.Either[String,Unit]
 required: Int => ?AP

val y = traverse_(1 to 10) {
                 ^
/Users/lodea/tmp/traverse.scala:49: error: type mismatch;
 found   : Int => Product with Serializable with scala.util.Either[String,Unit]
 required: Int => AP[Unit]
val y = traverse_(1 to 10) {
                           ^
two errors found

为了让它编译，我必须指定类型参数 traverse_ :

val y = traverse_[({type l[x]=Either[String, x]})#l, Int](1 to 10) {
  case 5 => Left("x")
  case _ => Right(())
}

有什么方法可以重组 traverse_ ，或代码的任何其他部分，使类型推断工作？当类型开始变得越来越复杂时，这会很快变得烦人。

Answer 1

正如 Ben James 所指出的，您正在寻找 Miles Sabin 的 Unapply trick . Here它在scalaz repo 中。 Here's traverseU ，在它的帮助下实现。 Here是一些示例用法。这是我针对您的特定情况的粗略(希望是正确的)实现(注意:我已将您的 Applicative 重命名为 ApplicativeTest 以免干扰 Applicative ，在 scalaz 中定义):

scalaz> core/console
[warn] Credentials file /home/folone/.ivy2/.credentials does not exist
[info] Starting scala interpreter...
[info] 
Welcome to Scala version 2.9.2 (OpenJDK 64-Bit Server VM, Java 1.7.0_15).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :paste
// Entering paste mode (ctrl-D to finish)

import scalaz._

trait ApplicativeTest[AF[_]] {
  def ap[A, B](a: AF[A])(f: AF[A => B]): AF[B]
  def pure[A](a: A): AF[A]
  def fmap[A, B](a: AF[A])(f: A => B): AF[B]
}

def traverse_[AP, A](xs: Iterable[A])(f: A => AP)(implicit G: Unapply[ApplicativeTest, AP]): G.M[Unit] = {
  (xs :\ G.TC.pure(())) { (x, acc) =>
    val apFunc = G.TC.fmap(G(f(x)))(a => identity[Unit] _)
    G.TC.ap(acc)(apFunc)
  }
}

implicit def optionAp = new ApplicativeTest[Option] {
  def ap[A, B](a: Option[A])(f: Option[A => B]): Option[B] = f flatMap (a map _)
  def pure[A](a: A) = Some(a)
  def fmap[A, B](a: Option[A])(f: A => B) = a map f
}

implicit def eitherAp[L]: ApplicativeTest[({type l[x]=Either[L, x]})#l] =
  new ApplicativeTest[({type l[x]=Either[L, x]})#l] {
    def ap[A, B](a: Either[L, A])(f: Either[L, A => B]): Either[L, B] = f.right flatMap (a.right map _)
    def pure[A](a: A) = Right(a)
    def fmap[A, B](a: Either[L, A])(f: A => B) = a.right map f
  }

implicit def iterAp = new ApplicativeTest[Iterable] {
  def ap[A, B](a: Iterable[A])(f: Iterable[A ⇒ B]): Iterable[B] = f flatMap(a map _)
  def pure[A](a: A) = Iterable(a)
  def fmap[A, B](a: Iterable[A])(f: A ⇒ B) = a map f
}

// Exiting paste mode, now interpreting.

import scalaz._
defined trait ApplicativeTest
traverse_: [AP, A](xs: Iterable[A])(f: A => AP)(implicit G: scalaz.Unapply[ApplicativeTest,AP])G.M[Unit]
optionAp: java.lang.Object with ApplicativeTest[Option]{def pure[A](a: A): Some[A]}
eitherAp: [L]=> ApplicativeTest[[x]Either[L,x]]
iterAp: java.lang.Object with ApplicativeTest[Iterable]

scala> val x = traverse_(1 to 10) {
     |   case 5 => None
     |   case _ => Some(())
     | }
x: Option[Unit] = None

scala> val y = traverse_(1 to 10) {
     |   case 5 => Left("x"): Either[String, Unit]
     |   case _ => Right(())
     | }
y: Either[String,Unit] = Left(x)

我仍然不知道如何让它推断 Either[String, Unit]而不是 Product with Serializable with scala.util.Either[String,Unit]除了像我在这一行所做的那样在其中一种情况下严格指定类型: case 5 => Left("x"): Either[String, Unit] .