Closures in Scala

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

In general, a closure can be simply understood as another function that can access the local variables of a function.

As follows, this anonymous function:

val multiplier = (i: Int) => i　*　10

There is a variable i in the function body, which is a parameter of the function. As shown in the following code snippet:

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

There are two variables in multiplier: i and factor. One of them, i, is a formal parameter of the function, which is assigned a new value when the multiplier function is called. 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 refers to variables defined outside the function. The process of defining this function is to capture the free variable and form a closed function.

Complete 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　　
}

Executing the above code, the output is:

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