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Scala | Functions Call-by-Name
  • Last Updated : 08 Apr, 2019

In Scala when arguments pass through call-by-value function it compute the passed-in expression’s or arguments value once before calling the function . But a call-by-Name function in Scala calls the expression and recompute the passed-in expression’s value every time it get accessed inside the function. Here example are shown with difference and syntax.

Call-by-value

This method uses in-mode semantics. Changes made to formal parameter do not get transmitted back to the caller. Any modifications to the formal parameter variable inside the called function or method affect only the separate storage location and will not be reflected in the actual parameter in the calling environment. This method is also called as call by value.

Syntax :

def callByValue(x: Int)




// Scala program of function call-by-value
  
// Creating object
object GFG 
{
    // Main method
    def main(args: Array[String]) 
    {
        // Defined function
        def ArticleCounts(i: Int) 
        {
            println("Tanya did article " +
                    "on day one is 1 - Total = " + i)
            println("Tanya did article " +
                    "on day two is 1 - Total = " + i)
            println("Tanya did article "
                    "on day three is 1 - Total = " + i)
            println("Tanya did article " +
                    "on day four is 1 - Total = " + i)
        }
  
        var Total = 0;
          
        // function call
        ArticleCounts 
        {
            Total += 1 ; Total
              
        }
    }
}
Output:
Tanya did article on day one is 1 - Total = 1
Tanya did article on day two is 1 - Total = 1
Tanya did article on day three is 1 - Total = 1
Tanya did article on day four is 1 - Total = 1

Here, by using function call-by-value mechanism in above program total count of article not increased.



Call-by-Name

A call-by-name mechanism passes a code block to the function call and the code block is complied, executed and calculated the value. message will be printed first than returns the value.
Syntax :

def callByName(x: => Int)

Example :




// Scala program of function call-by-name
  
// Creating object
object main 
{
    // Main method
    def main(args: Array[String])
    {
        // Defined function call-by-name
        def ArticleCounts(i: => Int) 
        {
            println("Tanya did articles "
                    " on day one is 1 - Total = " + i)
            println("Tanya did articles "
                    "on day two is 1 - Total = " + i)
            println("Tanya did articles "
                    "on day three is 1 - Total = " + i)
            println("Tanya did articles "
                    "on day four is 1 - Total = " + i)
        }
  
        var Total = 0;
          
        // calling function
        ArticleCounts 
        {
            Total += 1 ; Total
        }
}
}
Output:
Tanya did articles  on day one is 1 - Total = 1
Tanya did articles on day two is 1 - Total = 2
Tanya did articles on day three is 1 - Total = 3
Tanya did articles on day four is 1 - Total = 4

Here, by using function call-by-name mechanism in above program total count of article will increased.
Another program of function call-by-name.

Example :




// Scala program of function call-by-name
  
// Creating object
object GFG
{
    // Main method
    def main(args: Array[String])
    {
        def something() = 
        {
            println("calling something")
            1 // return value
        }
          
        // Defined function
        def callByName(x: => Int) = 
        {
            println("x1=" + x)
            println("x2=" + x) 
        }
          
        // Calling function
        callByName(something)
    }
}
Output:
calling something
x1=1
calling something
x2=1

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