Scalaz `F[_] : Applicative` 类型约束如何暗示使用隐式参数？
我正在努力理解 Scalaz 中的遍历 特征：
def traverse[F[_] : Applicative, A, B](f: A => F[B], t: T[A]): F[T[B]]
我不明白的部分是 F[_] : Applicative。
现在，让我们看看 Applicative 是：
trait Applicative[Z[_]] extends Pointed[Z] with Apply[Z] {
override def fmap[A, B](fa: Z[A], f: A => B): Z[B] = this(pure(f), fa)
override def apply[A, B](f: Z[A => B], a: Z[A]): Z[B] = liftA2(f, a, (_:A => B)(_: A))
def liftA2[A, B, C](a: Z[A], b: Z[B], f: (A, B) => C): Z[C] = apply(fmap(a, f.curried), b)
}
在这里，为了使 traverse 适用于某种类型 F，需要引入一个 Applicative[F 类型的隐式对象] 在范围内。
我想了解几件事：
F[_] : Applicative到底是什么意思？- 为什么
F[_]与Applicative有关？我们需要Applicative[F]，而不是F[something]扩展Applicative，对吧？ - 为什么此方法使用
Applicative[F]类型的隐式值而不声明隐式参数？
I am struggling to understand the following function definition in Traverse trait in Scalaz:
def traverse[F[_] : Applicative, A, B](f: A => F[B], t: T[A]): F[T[B]]
The part I don't understand is F[_] : Applicative.
Now, let's see what Applicative is:
trait Applicative[Z[_]] extends Pointed[Z] with Apply[Z] {
override def fmap[A, B](fa: Z[A], f: A => B): Z[B] = this(pure(f), fa)
override def apply[A, B](f: Z[A => B], a: Z[A]): Z[B] = liftA2(f, a, (_:A => B)(_: A))
def liftA2[A, B, C](a: Z[A], b: Z[B], f: (A, B) => C): Z[C] = apply(fmap(a, f.curried), b)
}
Here, for traverse to work for some type F, one needs to bring an implicit object of type Applicative[F] in scope.
I'd like to understand several things:
- Wat exactly does
F[_] : Applicativemean? - Why does
F[_]has something to do withApplicative? We needApplicative[F], not F[something] extends Applicative right? - Why does this method use implicit values of type
Applicative[F]without declaring implicit parameters?
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评论（1）
我认为所有三个问题都可以用以下表示法来回答：
相当于：
第一个表示法称为 F[_] 的上下文绑定。
I think all three questions can be answered with the fact that this notation:
is equivalent to this:
The first notation is known as a context bound for
F[_].