缓存隐式解析 [英] Caching implicit resolution

问题描述

为了减少项目的编译时间，我正在缓存某些由隐式查找解决的类型类.不过，这看起来有些麻烦，因为直接实现无法正常工作:

To reduce compile times of my project, I'm caching certain type classes that are resolved by implicit lookups. This appears somewhat cumbersome though, because the straight forward implementation does not work:

scala> implicit val x: String = implicitly[String]
x: String = null

隐式查找将其自己的未初始化定义视为有效实现. lazy val将无限递归地炸毁堆栈.因此，我目前正在以这种方式处理它:

The implicit lookup considers its own, uninitialized definition as a valid implementation. A lazy val would blow the stack with infinite recursion. Therefore I'm currently handling it in this fashion:

implicit val x: String = cache.x

object cache {
   val x: String = implicitly[String]
}

但是，这使其变得过于复杂，并且缓存定义不能轻松利用其他缓存类型类(因为它们不是隐式的).

But this makes it overly complicated, and the cache-definitions can not make use of other cached type classes easily (since they are not implicit).

此外，不幸的是，将值本身隐藏在范围之外确实行不通.

Also, hiding the value itself from scope does unfortunately not work.

scala> :pas
// Entering paste mode (ctrl-D to finish)

object scope {
    implicit val x: String = {
        import scope.{ x => _ }
        implicitly[String]
    }
}

// Exiting paste mode, now interpreting.

defined object scope

scala> scope.x
res0: String = null

是否有更优雅的方法来实现隐式分辨率缓存?

Is there a more elegant way to achieve an implicit resolution cache?

推荐答案

Shapeless 提供了带有cachedImplicit宏="https://github.com/milessabin/shapeless/blob/shapeless-2.2.5/core/src/main/scala/shapeless/package.scala#L126">与您的实现非常相似的实现 (它使用阴影来避免递归，并且它是一个宏的事实意味着用法可以更简洁).

Shapeless provides a cachedImplicit macro with an implementation that's very similar to yours (it uses shadowing to avoid the recursion, and the fact that it's a macro means the usage can be cleaner).

有有一些局限性要注意，您可能不想对它进行新的依赖这种单一方法，但是实现起来非常简洁，并且至少是一个很好的起点.

There are some limitations to be aware of, and you may not want to take on a new dependency for this single method, but the implementation is pretty concise, and it's at least a good starting point.

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