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Scala Try[Unit] confusion

I have this piece of code

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


and 2 questions:

Try[Try[Int]]

Try[Unit]

Try(1)

Unit

Try

Try[A]

A

Try[_]

2 Answers
2

You are basically forcing compiler to assign Try as Unit.

Try

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 ().

doSomething

Int

Unit

()

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

val t: Try[Unit] = Try(Try(1 / 0))

Try(1 / 0)

Unit

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.

Try

Unit

Success

Try

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] = ???



Try[A]

A

Try[_]

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:

Try(1)

Success(1)

Try(Try(1)

Success(())

Try[Unit]

Unit

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.

()

Unit

Unit

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