获取隐式证据选择的运行时类型

本文介绍了获取隐式证据选择的运行时类型的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

假设我有一组类型为String的String转换器:

Suppose I have a set of converters to String, as a Type class:

import scala.reflect.runtime.universe._ abstract class ToStringConverter[T] { def convert(value: T): String } implicit object IntToStringConverter extends ToStringConverter[Int] { def convert(value: Int) = value.toString } implicit object DoubleStringConverter extends ToStringConverter[Double] { def convert(value: Double) = value.toString }

以及使用类型信息选择正确的转换器的convert方法:

and a convert method that uses the type information to pick right converter:

def convert[T](v: T)(implicit ev: ToStringConverter[T]): String = ev.convert(v)

如果我事先有具体的类型，这很好用，例如:

This works fine If I have the concrete type in advance, for example:

scala> convert[Double](12.2) res0: String = 12.2 scala> convert[Int](12) res1: String = 12

是否可以将上面的convert方法与运行时类型一起使用，例如，下面的类型为't'?

Is it possible to use the convert method above with a runtime type, for example, with a type 't' below?

scala> val t = typeOf[Double] t: reflect.runtime.universe.Type = Double

推荐答案

如果要执行解析运行时，则需要进行反射，因为隐式会解决编译时的问题.像这样的代码就可以完成这项工作:

If you want to do the resolution runtime, reflection is needed, as implicits are resolved compile time. A code like this should do the job:

import scala.reflect.runtime.universe._ abstract class ToStringConverterAny { def convertAny(value: Any): String } abstract class ToStringConverter[T] extends ToStringConverterAny { def convertAny(value: Any): String = convert(value.asInstanceOf[T]) def convert(value: T): String } implicit object IntToStringConverter extends ToStringConverter[Int] { def convert(value: Int) = value.toString } implicit object DoubleStringConverter extends ToStringConverter[Double] { def convert(value: Double) = value.toString } val converters: Map[Type, ToStringConverterAny] = Map( typeOf[Int] -> IntToStringConverter, typeOf[Double] -> DoubleStringConverter ) def convert(t: Type, v: Any) = { converters(t).convertAny(v) } def convert[T](v: T)(implicit ev: ToStringConverter[T]): String = ev.convert(v) convert[Double](12.2) convert[Int](12) val t = typeOf[Double] val v: Any = 1.23 convert(t, v)

如果要自动构建converters映射，也可以为此使用反射，但是枚举派生类需要令人吃惊的非平凡代码(包括类加载器-当您考虑时可以理解).

If you want to building converters map automatically, you could also use reflection for this, but enumerating derived classes requires surprisingly non-trivial code (including class loaders - which is understandable when you think about it).

如果可以将ToStringConverterAny密封起来，则在宏中枚举其子类应该更加容易.

If you can make the ToStringConverterAny sealed, enumerating over its subclasses in a macro should be a bit easier.