scala – 类型参数推断高阶类型类型= :-(

import scalaz._; import Scalaz._

def foo[M[_]:MonadPlus,A](a:A) = a.point[M]
// foo: [M[_], A](a: A)(implicit evidence$1: scalaz.MonadPlus[M])M[A]

def bar1[M[_]:MonadPlus](i:Int): M[Int] = 
  foo(i) // <-- error: ambiguous implicit values

// this works, but why?  Isn't it just the same?
def bar2[M[_]:MonadPlus](i:Int): M[Int] = 
  foo(i)(implicitly[MonadPlus[M]])

def bar3[M[_]](i:Int)(implicit m:MonadPlus[M]): M[Int] = 
  foo(i)(m) // slightly less surprising that this works

def bar4[M[_]:MonadPlus](i:Int): M[Int] = 
  foo[M,Int](i) // this also works, but why?

build.sbt:

scalaVersion := "2.9.2"

libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.0.0-M5"

(虽然我在2.10.0-RC3中得到了相同的结果)

最佳答案 如果您只是写下,则会收到相同的错误消息:

import scalaz._; import Scalaz._

def foo[M[_]:MonadPlus,A](a:A) = a.point[M]

foo(1) // <-- error: ambiguous implicit values
       // both value listInstance ...
       // and value optionInstance ...

我的理解是编译器试图对bar1方法的“主体”进行类型检查,而不考虑范围内可能存在MonadPlus [M]实例(由条形定义引入),它已经找到2个特定的MonadPlus [M]实例(listInstance和optionInstance)适用于foo调用.在这个阶段,它只是宣布一个模棱两可.

然后在bar2和bar3中明确指定要使用的实例,并在bar4中给出类型参数,这些参数是M和Int,在foo调用中不明确,因为具有这些约束的唯一隐含范围是隐式[MonadPlus [M]] .

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