4
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:类型参数推断+高阶类型+型类= :-(
scalaVersion := "2.9.2"
libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.0.0-M5"
(虽然我得到2.10.0-RC3的结果相同)
哎呀,是的。一年后,答案似乎对我很明显。 'bar1'不指定传递给'foo'的是哪种'MonadPlus'; 'bar2','bar3','bar4',通过显式参数或类型参数。老实说,我不知道我在想什么。 – arya