Call by Name， Call by Value

1. Description

Typically, parameters to functions are by-value parameters; that is, the value of the parameter is determined before it is passed to the function. But what if we need to write a function that accepts a parameter as an expression that we don’t want evaluated until it’s called within our function? For this circumstance, Scala offers call-by-name parameters.

A call-by-name mechanism passes a code block to the call and each time the call accesses the parameter, the code block is executed and the value is calculated.

code example

package com.brown/**  * Created by BrownWong on 2016/9/29.  */object Hello {  def main(args: Array[String]): Unit = {    callByValue(func())  // func()先调用，其返回值作为实参传递给callByValue()函数，然后callByValue()函数调用    println("---------------------")    callByName(func())  // callByName()函数先调用,若函数体内部需要引用参数,才发生求值(func()被调用)  }  def func() = {    println("Calling func()")    1  }  def callByValue(x: Int) = {    println("X1 = " + x)    println("X2 = " + x)  }  def callByName(x: => Int) = {    println("X1 = " + x)    println("X2 = " + x)  }}

output

Calling func()X1 = 1X2 = 1---------------------Calling func()X1 = 1Calling func()X2 = 1

By-name parameters behave like functions; the expression is evaluated every time it is used. So, by-name parameters are in a sense lazy.

2. An example

Here is a simple implementation of a while-like loop construct, called continue:

code

package com.brown/**  * Created by BrownWong on 2016/9/29.  */object Hello {  def main(args: Array[String]): Unit = {    var count = 0    continue(count < 5) {      println(s"at $count")      count += 1    }  }  @annotation.tailrec  def continue(conditional: => Boolean)(body: => Unit) {    if (conditional) {      body      continue(conditional)(body)    }  }}

output:

at 0at 1at 2at 3at 4

Ref

《Programming Scala》
http://stackoverflow.com/questions/13337338/call-by-name-vs-call-by-value-in-scala-clarification-needed