Scala尝试[单位]混乱

Question

I have this piece of code

import scala.util.Try
val t: Try[Unit] = Try(Try(1))

and 2 questions:

• What is happening here? How can the type Try[Try[Int]] match with Try[Unit] ? Is it because Scala chooses the return type for block Try(1) to be Unit to match with the desired type?
• Is there anyway to detect a nested Try ? Says I have a Try[A] , how do I know if A is another Try[_] ?

Answer 1

You are basically forcing compiler to assign Try as Unit .

For exmple doSomething method below is supposed to return Int as that is last statement, but return type Unit is forcing it to return () .

scala> def doSomething: Unit = 1 + 1
doSomething: Unit

In your example val t: Try[Unit] = Try(Try(1 / 0)) , you asking compiler to treat inner Try(1 / 0) as Unit ; which means

scala> val innerTry: Unit = Try(1 / 0)
innerTry: Unit = ()

Which means even if Try fails, its Unit which is always Success for another Try that you have.

scala> val t: Try[Unit] = Try(someOperation)
t: scala.util.Try[Unit] = Success(())

Better remove the specific type you are providing and let the compiler figure it out,

scala> val t = Try(Try(1 / 0))
t: scala.util.Try[scala.util.Try[Int]] = Success(Failure(java.lang.ArithmeticException: / by zero))

Also read: Scala: Why can I convert Int to Unit?

final abstract class Unit private extends AnyVal {
  // Provide a more specific return type for Scaladoc
  override def getClass(): Class[Unit] = ???
}

Answer 2

Try(1) returns Success(1) . Try(Try(1) returns Success(()) because of its assignment to Type Try[Unit] Similar to the following commands in Scala REPL where x is forced to take Unit type:

scala> val x = 5
x: Int = 5

scala> val x:Unit = 5
<console>:23: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
       val x:Unit = 5
                    ^
x: Unit = ()

scala>

So obviously the compiler is taking the return value and is forced to type () which is Unit . Because it is Unit may be it makes sense the way compiler interprets it.