无限循环似乎混淆了Scala的类型系统

Question

Here is an artificial toy example that demonstrates my problem:

def sscce(): Int = {
  val rand = new Random()
  var count = 0
  while (true) {   // type mismatch; found: Unit, required: Int
    count += 1
    if (rand.nextInt() == 42) return count
  }
}

How can I help the compiler understand that this method will always return an Int ?

I know the above toy example could easily be refactored to get rid of the infinite loop altogether, but I really want to have the infinite loop in my actual code. Trust me on this ;)

Answer 1

Always return an Int :

def sscce(): Int = {
  val rand = new Random()
  var count = 0
  while (true) {
    count += 1
    if (rand.nextInt() == 42) return count
  }
  count // <-- this
}

Answer 2

You can also do:

def foo: Int = {
  ...
  while(true) {
    ... return ...
  }
  throw new IllegalStateException  // unreachable
}

this will typecheck because the type of the throw is Nothing , which is a subtype of Int .

Answer 3

See this question . While loops don't return a value. ie they return Unit which is the last statement in your function. So, the definition says it returns an Int but it actually returns Unit thus the type error. @ionut's answer fixes the type error by returning count as the last statement or here is a recursive approach.

def sscce(): Int = {
  val rand = new Random()
  def ssccer(count: Int): Int = {
    if(rand.nextInt == 42) return count
    else ssccer(count + 1)
  }
  ssccer(0)
}

Answer 4

Per the SLS , a while loop is executed similarly to:

def whileLoop(cond: => Boolean)(body: => Unit): Unit  =
  if (cond) { body ; whileLoop(cond)(body) } else {}

ie., it returns Unit . So the compiler sees the while as the last statement in sscce() , and therefore assumes that you're trying to return Unit . I don't think it's smart enough to realize that return count will eventually always return an Int .

The simple solution is to follow the suggestion of @Brian or @IonutGStan, and force it to return count , whether it truly needs it or not.

Answer 5

From a code quality standpoint, it would be good to ditch the while(true) loop and replace it with something more readable. As a nice side effect, it also solves your problem:

def sscce(): Int = {
  val rand = new Random()
  var count = 1
  while (rand.nextInt() != 42) {
    count += 1
  }
  count
}