避免在通用方法中转换为Nothing

Question

scala> def foo[U](t: Any) = t.asInstanceOf[U]
foo: [U](t: Any)U

scala> val s: String = foo("hi")

scala> val n = foo("hi")
java.lang.ClassCastException: java.lang.String cannot be cast to scala.runtime.Nothing$
    at .<init>(<console>:6)
    at .<clinit>(<console>)
    at RequestResult$.<init>(<console>:9)
    at RequestResult$.<clinit>(<console>)
    at RequestResult$scala_repl_result(<console>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$18.apply(Interpreter.scala:981)
    at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$18.apply(Interpreter.scala:981)
    at scala.util.control.Exce...

有没有办法写#foo,如果'U'没有被推断或明确设置为"真实"类型,它会返回Any？

Answer 1

不,静态类型是U.如果推断为Nothing,则编译器将不允许返回值类型Any.

您可以改进运行时错误消息:

def foo[U: Manifest](t: Any): U = if (implicitly[Manifest[U]] == manifest[Nothing]) 
  error("type not provided") 
else t.asInstanceOf[U]

或者按照Arjan的建议.