Scala FlatMap 和应用上下文边界产生编译错误

Question

我在 Scala 中做一些工作，遇到了 implicit 个实例的问题。让我们考虑以下示例：

import cats.{Applicative, FlatMap, Monad}
import cats.syntax.functor._
import cats.syntax.flatMap._
import cats.syntax.applicative._


class Test[F[_]: Monad] extends App{
  val t1 = ().pure[F]
  val t2 = ().pure[F]

  def testFlatApplicative: F[Unit] =
    for{
      _ <- t1
      _ <- t2
    } yield ()
}

编译正常。但是由于 cats.Monad[F[_]] 声明如下：

@typeclass trait Monad[F[_]] extends FlatMap[F] with Applicative[F]

我希望以下内容也能正常工作

import cats.{Applicative, FlatMap, Monad}
import cats.syntax.functor._
import cats.syntax.flatMap._
import cats.syntax.applicative._


class Test[F[_]: FlatMap : Applicative] extends App{
  val t1 = ().pure[F]
  val t2 = ().pure[F]

  def testFlatApplicative: F[Unit] =
    for{
      _ <- t1
      _ <- t2
    } yield ()
}

但是编译失败，报错：

Error:(16, 12) value map is not a member of type parameter F[Unit]
      _ <- t2

这很奇怪。 Apply 扩展 Functor...

谁能解释这种行为？

Answer 1

class Test[F[_] : FlatMap : Applicative] 被脱糖为

class Test[F[_]](implicit flatMap: FlatMap[F], applicative: Applicative[F])

如果你对理解和语法进行脱糖处理，你就会发现问题所在：

def testFlatApplicative: F[Unit] = 
  FlatMap[F].flatMap(t1)(_ => 
    Functor[F].map(t2)(_ => ())
  )

Error: ambiguous implicit values:
 both value applicative in class Test of type cats.Applicative[F]
 and value flatMap in class Test of type cats.FlatMap[F]
 match expected type cats.Functor[F]

因此您可以手动解决歧义：

def testFlatApplicative: F[Unit] = 
  FlatMap[F].flatMap(t1)(_ => 
    applicative.map(t2)(_ => ())
  )

或

def testFlatApplicative: F[Unit] = 
  FlatMap[F].flatMap(t1)(_ => 
    flatMap.map(t2)(_ => ())
  )

写class Test[F[_]: Monad]就没有这个问题，因为你有单隐式。

Scala FlatMap 和应用上下文边界产生编译错误

Scala FlatMap and Applicative context boundaries yield compile error

functional-programming

scala

scala-cats