Scala FlatMap和应用程序上下文边界产生编译错误

我在Scala做一些工作，遇到了

隐式

实例的问题。让我们考虑下面的例子：

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 ()
}

我希望下面的方法也能奏效

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 ()
}

导入猫。{Applicative，FlatMap，Monad}
导入cats.syntax.functor_
导入cats.syntax.flatMap_
导入cats.syntax.applicative_
类测试[F[3;]：FlatMap:Applicative]扩展了应用程序{
val t1=（）.pure[F]
val t2=（）。纯[F]
def TestFlatApplication:F[单位]=
为了{
_
类测试[F[\u]：FlatMap:Applicative]
被设计为

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

如果你对理解和语法进行分析，你会发现问题：

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

在编写
类测试[F[\u]：Monad]
时不存在这样的问题，因为您使用了单隐式

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]

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

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