相当于 Scala 中 Python 的 Pass

Question

If there is a function that you don't want to do anything with you simple do something like this in Python:

def f():
    pass

My question is, is there something similar to pass in Scala?

Answer 1

pass is a syntactic quirk of Python. There are some cases where the grammar requires you to write a statement, but sometimes you don't want a statement there. That's what pass is for: it's a statement that does nothing.

Scala never requires you to write a statement, therefore the way to not write a statement is simply to not write a statement.

Answer 2

I think () is similar.

scala> def f() = ()
f: ()Unit

scala> f              

scala>

Answer 3

As i understand in python pass is used for not yet implemented cases. If you need such thing in scala then use ??? it's similar to () , but is a function returning Nothing ( def ??? : Nothing = throw new NotImplementedError ) . Your code will compile, but if you call such a method it will crash with NotImplementedError

def foo: ResultType = ???

Answer 4

Generally you can substitute Unit for pass . You do nothing and the line evaluates to Unit , similar to Java's void , but itself an explicit type.

// implicit return value of type Unit
def showMsg(msg:Option[String]) = msg match {
    case None => Unit
    case Some(m) => println(m)
}

Answer 5

say you create a stub for a function that will return a string

you can add "" as a placeholder and your function will compile

Before

❯ cat TrainingSet.scala
def getCommitMessage(x: String): String = {
}

❯ ./TrainingSet.scala
cat: /Users/lgeoff/.sdkman/candidates/java/current/release: No such file or directory
/Volumes/workplace/migrate_ant_to_gradle/./TrainingSet.scala:59: error: type mismatch;
 found   : Unit
 required: String
  def getCommitMessage(x: String): String = {
                                            ^
one error found

After

❯ cat TrainingSet.scala
def getCommitMessage(x: String): String = {
    ""
}

❯ ./TrainingSet.scala
❯