Scala按名称调用-声明未使用的参数

Question

One of the benefits of call-by-name is that expensiveOperation() will not get run in the following examples:

Call-by-value:

def test( x: Int, y: Int ): Int = x * x

// expensiveOperation is evaluated and the result passed to test()
test( 4, expensiveOperation() )  

Call-by-name:

def test( x: Int, y: => Int ): Int = x * x

// expensionOperation is not evaluated
test( 4, expensiveOperation() ) 

My question though is why would you declare a function parameter (y in my case) when you aren't going to use it?

Answer 1

Your example is a bit contrived, consider this code

def test( x: Boolean, y: => Int ): Int = if(x) y else 0

// expensionOperation is not evaluated
test( false, expensiveOperation() ) 

When the first parameter is false you are saving a lot of time not evaluating expensive operation.

Answer 2

It is just a contrived example to illustrate the idea of call-by-name, namely that the argument passed in is never evaluated if never called.

Perhaps a better example would be the following:

trait Option[+T] {
    def getOrElse[B >: A](default: => B): B
}

If the Option is Some , then the wrapped value is returned and default is never evaluated. If it is None and only when it is None will default be evaluated (and consequently returned).

Answer 3

Using logging is a much better example:

def debug(msg: String) = if (logging.enabled) println(msg)

debug(slowStatistics()) // slowStatistics will always be evaluated

While in the case of call-by-name:

def debug(msg: => String) = if (logging.enabled) println(msg)

debug(slowStatistics()) // slowStatistics will be evaluated only if logging.enabled