Implicit Classes in Scala

Question

Below is the example program for Scala Implicit Class:

object Run {
   implicit class IntTimes(x: Int) {
      def times [A](f: =>A): Unit = {
         def loop(current: Int): Unit =

         if(current > 0){
            f
            loop(current - 1)
         }
         loop(x)
      }
   }
}

There is an other class that calls " 4 times println("hello")" as following, but I can not understand " 4 times println("hello")" mean?

object Demo {
   def main(args: Array[String]) {
      4 times println("hello")
   }
}

stackoverflow.com/questions/2842540/…

Nilesh
– Nilesh

2016-11-30 06:57:33 +00:00
Commented Nov 30, 2016 at 6:57 — Nilesh
– Nilesh, Commented Nov 30, 2016 at 6:57
can this be called like ' times(4) println("hello") '

Pubudu Jayasanka
– Pubudu Jayasanka

2018-04-25 04:23:44 +00:00
Commented Apr 25, 2018 at 4:23 — Pubudu Jayasanka
– Pubudu Jayasanka, Commented Apr 25, 2018 at 4:23

StrikeR · Accepted Answer · 2018-06-18 11:26:03Z

10

4 times println("hello") roughly translates into:

val c = new IntTimes(4)
c.times(println("hello"))

That is, since there is an implicit class that takes an Int as its only argument, with a method times, doing 4.times implicitly instantiates the class with 4 as argument, and then invokes times on it.

edited Jun 18, 2018 at 11:26

StrikeR

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answered Nov 30, 2016 at 7:12

Buhb

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Comments

JasonY · Accepted Answer · 2018-08-21 12:55:09Z

6

I also found this example way overly complicated to demonstrate Implicit Classes.

In a nutshell Implicit Classes overwrite other classes with new methods and properties.

In this example, it is overwriting Int to give it a method times. The times method takes a call-by-name parameter with a generic return type:

def times [A](f: =>A): Unit

Means f is a function that returns a generic type A.

When a call-by-name variable is used, it calls the function and becomes the return value. Inside times method it is using recursion to loop the call of f the number of times the integer it's called on.

In scala you can call methods by using a dot or space, and no need for brackets for parameters, so object.method(param1, param2) could be object method param1 param2

Hence the final call 4 times println("hello") is actually 4.times(println("hello"))

answered Aug 21, 2018 at 12:55

JasonY

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2 Comments

user3103957 Over a year ago

I am little confused here. I see that the method "times" takes a functional argument which does not take any input and returns A as output. But in the invocation of "times", the function println is passed which takes a string as i/p and returns Unit. This is just opposite to the signature. But this is working. Can someone please help me to understand. Thanks in advance!

sherminator35 Over a year ago

It is useful to see that f has a call by name semantics so evaluation of f is delayed until the actual execution of the line in the body which calls f. The braces for the function body of loop have been skipped to show the body contains only the if statement. So, the method times just defines loop and calls it once.

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