我有一个类Foo
有两个参数,我正在尝试为第一个参数固定的 Foo 编写一个 Functor 实例,如下所示:
object Scratchpad {
trait Functor[F[_]] {
def fmap[A, B](f: A => B): F[A] => F[B]
}
case class Foo[X, Y](value: Y)
implicit def fooInstances[X]: Functor[Foo[X, _]] =
new Functor[Foo[X, _]] {
def fmap[A, B](f: A => B): Foo[X, A] => Foo[X, B] =
foo => Foo[X, B](f(foo.value))
}
}
但是上面的代码编译失败,产生如下错误:
Error:(9, 41) Scratchpad.Foo[X, _] takes no type parameters, expected: one
implicit def fooInstances[X]: Functor[Foo[X, _]] =
我知道 Scalaz 用他们的
\/
做了类似的事情类型,但检查他们的源代码发现一个奇怪的 ?
,它不会为我编译:implicit def DisjunctionInstances1[L]: Traverse[L \/ ?] with Monad[L \/ ?] with BindRec[L \/ ?] with Cozip[L \/ ?] with Plus[L \/ ?] with Optional[L \/ ?] with MonadError[L \/ ?, L] =
Scalaz 怎么样
?
工作,以及如何为 Foo
编写 Functor 实例?
最佳答案
but inspection of their source code reveals an odd ?, which doesn't compile for me
?
来自 kind-projector
项目,这是一个 Scala 编译器插件,您需要添加到您的 build.sbt
:resolvers += Resolver.sonatypeRepo("releases")
addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.4")
这为您美化了类型 lambda 的创建:
implicit def fooInstances[X]: Functor[Foo[X, ?]] =
new Functor[Foo[X, ?]] {
def fmap[A, B](f: A => B): Foo[X, A] => Foo[X, B] =
foo => Foo[X, B](f(foo.value))
}
请记住,我们还可以使用具有类型别名的部分类型应用程序:
implicit def fooInstances[X] = {
type PartiallyAppliedFoo[A] = Foo[X, A]
new Functor[PartiallyAppliedFoo] {
override def fmap[A, B](f: (A) => B): (PartiallyAppliedFoo[A]) => PartiallyAppliedFoo[B] = foo => Foo[X, B](f(foo.value))
}
}
编辑 (05/04/2020)
注意 kind 项目的语法从
?
改变了。至 *
对于部分应用类型,因此上面的示例应该是:sbt:
resolvers += Resolver.sonatypeRepo("releases")
addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.11.0")
代码:
implicit def fooInstances[X]: Functor[Foo[X, *]] =
new Functor[Foo[X, *]] {
def fmap[A, B](f: A => B): Foo[X, A] => Foo[X, B] =
foo => Foo[X, B](f(foo.value))
}
关于scala - Scala 中带有两个参数的类型构造函数的仿函数实例,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/45271911/