有没有一种方法可以在Scala中隐式转换隐式参数? [英] Is there a way to implicitly convert an implicit parameter in Scala?

问题描述

有没有办法使它工作(Scala 2.8.1):

Is there a way to make this work (Scala 2.8.1):

class A
class B
def f(implicit b: B) {}
implicit val a = new A
implicit def aToB(a: A) = new B
f(a) // works ok
f // error: could not find implicit value for parameter b: B

实际上，我的问题是Lift的(2.2)依赖注入，我试图将Vendor [T]转换为T，并隐式地在类构造函数中要求它，而无需在每个val之后添加导入:

Actually my problem is with Lift's (2.2) dependency injection, i'm trying to convert Vendor[T] to T and implicitly require it in a class constructor without adding imports after each val:

object DependencyFactory extends Factory {
  implicit def vendorToVal[T](vendor: Vendor[T]): T = vendor.vend

  implicit val db = new FactoryMaker[DbAccess](Model) {}
  //uncommenting the following line makes it work, but can we avoid it? 
  //import db._
  implicit val userStore = new FactoryMaker[UserStore](new UserStore) {}
}

UserStore所在的位置:

class UserStore(implicit db: DbAccess)

我做错什么了吗?

更新

感谢轻松天使回答了第一部分.但这并不能解决我的Lift DI问题，因为事实证明范围存在相反的转换(从T到Vendor [T])，并且两者都导致错误:分散隐式扩展".

Thanks to Easy Angel for answering the first part. But it doesn't solve my Lift DI problem because it turns out that there is an opposite conversion in scope (from T to Vendor[T]) and having those both leads to 'error: diverging implicit expansion'.

可以解决吗?

UPDATE2

在上一版本之后又提出了一个问题:将某个Container [T]转换为T，并在范围内包含Container [U]的隐式实例，而具有隐式参数U的函数也会导致发散的隐式扩展": >

Wow one more problem after previous: having a conversion from some Container[T] to T with implicit instance of Container[U] in scope and a function with implicit parameter U leads to 'diverging implicit expansion' too:

class A
case class Vendor[T](v: T)
def f(implicit a: A) {}
implicit val vendor = Vendor(new A)
implicit def vendorToVal[T](implicit v: Vendor[T]) = v.v
f

有任何提示吗?

推荐答案

您几乎做到了.您只需要声明a隐式:

You almost made it. You only need to declare a implicit:

implicit def aToB(implicit a: A) = new B

在这种情况下，编译器尝试为f的第一个隐式参数找到一些隐式的B，并找到aToB.超过编译器的关系以满足aToB的要求(implicit a: A)并找到您的implicit val a.

In this case compiler tries to find some implicit B for the first implicit argument of f and it finds aToB. Than compiler ties to satisfy aToB's requirement (implicit a: A) and finds your implicit val a.

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