Scala Closures

A closure is a function whose return value depends on one or more variables declared outside the function.

Generally speaking, a closure can be simply considered as another function that can access the local variables of a function.

For example, the following anonymous function:

val multiplier = (i:Int) => i * 10

There is a variable i inside the function body, which serves as a parameter of the function. As for the following code:

val multiplier = (i:Int) => i * factor

In the multiplier, there are two variables: i and factor. One of them, i, is a formal parameter of the function, and when the multiplier function is called, i is assigned a new value. However, factor is not a formal parameter but a free variable. Consider the following code:

var factor = 3  
val multiplier = (i:Int) => i * factor

Here we introduce a free variable factor, which is defined outside the function.

The function variable multiplier defined in this way becomes a "closure" because it references a variable defined outside the function, and the process of defining this function is to capture this free variable and form a closed function.

Complete Example

Example

object Test {  
   def main(args: Array[String]) {  
      println( "muliplier(1) value = " +  multiplier(1) )  
      println( "muliplier(2) value = " +  multiplier(2) )  
   }  
   var factor = 3  
   val multiplier = (i:Int) => i * factor  
}

When the above code is executed, the output is:

$ scalac Test.scala  
$  scala Test  
muliplier(1) value = 3  
muliplier(2) value = 6