Understanding Closures in Scala

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

In simple terms, a closure can be regarded as another function that can access the local variables of a function.

For example, the following anonymous function: val multiplier = (i:Int) => i * 10 The function body contains a variable i as a parameter. Another example: val multiplier = (i:Int) => i * factor In multiplier there are two variables: i and factor. i is a formal parameter receiving a new value on each call, while factor is a free variable defined outside the function: var factor = 3<br/>val multiplier = (i:Int) => i * factor Here we introduce a free variable factor that is defined outside the function.

Thus the function variable multiplier becomes a "closure" because it references the externally defined variable, capturing it to form a closed function.

Complete example:

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

Running the above code produces the following output:

$ scalac Test.scala<br/>$ scala Test<br/>muliplier(1) value = 3<br/>muliplier(2) value = 6