Scala Call by Name && Call by Value

Scala Call by Name && Call by Value

如下scala代码，看一下这两者的区别。

def something() = {
  println("calling something")
  1 // return value
}

def callByValue(x: Int) = {
  println("x1=" + x)
  println("x2=" + x)
}

def callByName(x: => Int) = {
  println("x1=" + x)
  println("x2=" + x)
}

callByValue(something())

println("====================")

callByName(something())

看一下运行结果，

E:\test-scala>scala function_called.scala
calling something
x1=1
x2=1
====================
calling something
x1=1
calling something
x2=1

Call by Value

正如其名字一样，函数调用的过程中传值，比如C 语言中的值调用，something函数返回Int 值，函数调用传值。

Call by Name

首先看 wiki 上如何解释的：In call-by-name evaluation, the arguments to a function are not evaluated（ 求…的值（或数）） before the function is called — rather, they are substituted（代替，代换） directly into the function body (using capture-avoiding substitution（代替）) and then left to be evaluated whenever they appear in the function. If an argument is not used in the function body, the argument is never evaluated; if it is used several times, it is re-evaluated each time it appears. 

这段文字就很好的解释了callByName的效果。根据上边那个例子。

参考资料

http://blog.csdn.net/bobozhengsir/article/details/13023023

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

http://blog.sina.com.cn/s/blog_6d27562901019oxw.html

http://stackoverflow.com/questions/6297153/what-does-code-unit-mean-in-scala

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