如何在闭包上声明清单

Question

Is it possible to declare a manifest on a closure

instead of

def extract[A](v:JValue)(implicit m: Manifest[A]) : A = v.extract[A]

something like ?

def extract[A] = (v: JValue) => v.extract[A]

Answer 1

this declares the manifest on the method which returns the function returning you a function for a specific type:

def extract[A](implicit m: Manifest[A]) = (v: JValue) => v.extract[A]

other than that it is impossible for two reasons:

1. scala functions unlike methods don't support parametric polymorphism - there is nothing like

   [A](v: JValue) => (m: Manifest[A]) => v.extract[A]

2. scala functions unlike methods don't support implicit parameters. while this is accepted syntax:

   { implicit manifest: Manifest[String] => v.extract[String] }

   the implicit here does have a different meaning: it makes the value manifest available for implicit resolution within the body of the function but has no effect when calling the function (you will have to provide the manifest explicitly)