“return” and “try-catch-finally” block evaluation

问题:

The following two code generate different result:

def x = try{
  true
} finally false

invoke x gets true

def y:Boolean = try{
  return true
} finally {
  return false
}

invoke y gets false

the return version behave same as Java.

Personally I never use 'return' in scala. But it's good to know how scala evaluate the value of a try-catch-finally block. Thanks.

回答1:

You should not have a return statement in a finally block (even though it is technically allowed, at least in Java, C# for example forbids it).

If the Scala finally block had an implicit return, that would always clobber the intended return value. So that makes no sense.

But I suppose it cannot help you if you explicitly write it that way.

回答2:

According to the Scala language spec:

A try expression try { b } finally e evaluates the block b. If evaluation of b does not cause an exception to be thrown, the expression e is evaluated. If an exception is thrown during evaluation of e, the evaluation of the try expression is aborted with the thrown exception. If no exception is thrown during evaluation of e, the result of b is returned as the result of the try expression.

This behaviour would seem to be in contradiction with that spec. I would guess that, since 'return' causes an immediate return from the function, this results in overriding the standard behaviour for a try block. An illuminating example is:

def z : Boolean = {
  val foo = try { true } finally { return false }
  true
}

Invoking z returns false.