Scala Tutorials Part #28 - Getting started with Implicits


Implicits

This is part 28 of the Scala tutorial series. Check here for the full series.

Index

Introduction

Implicit in the dictionary means predefined or understood. The implicit keyword is extensively used in Scala and its important to understand its usage. If used correctly, they can greatly reduce the amount of boilerplate code.

Implicit Variables & Method Parameters

Let’s declare a method that takes in an implicit parameter.

def multiply(implicit param1: Int) = param1 * value

value is a variable which the multiply method has scope to. So let’s just declare a simple variable as below.

val value = 10

We will be able to call the multiply method without any parameters since one of them is implicit and the other is global. But, it will lead to an error as we have not declared any implicit variables that is suitable for that method.

println(multiply) 

Implicit error

Defining an implicit variable fixes the error.

implicit val param1 = 2

Now,

println(multiply)

Will print 20.

An interesting thing to note here is that the implicit parameter can be of any name and the method would still work.

object RunExample extends App{

val value = 10

implicit val anotherValue = 2

def multiply(implicit param1: Int) = param1 * value

//Prints 20
println(multiply)

}

As you can see, only the variable type is more important rather than the name. At this point, an important question that would come up is what will happen if there are two implicit variables of the same type. Let’s test it out.

Two implicit variables error

The compiler is much more strict here. It could resort to taking the first defined implicit value but it leads to bad/ambiguous code, so it promptly complains here.

Once we declare the first parameter as implicit, anything that follows it is also implicit. The below example makes it more clear.

def multiply(implicit param1: Int,param2 : Int) = param1 * param2

If we call just multiply it prints 4 as it takes the value of anotherValue twice. Usually there will be only one implicit parameter for a method. We cannot call param2 directly here since it is implicit.

It also not possible to make the second parameter implicit. Such a syntax would lead to a compiler error.

Implicit second param error

Implicit Functions

Just like implicit method parameters, implicit functions are useful in places where we want the compiler to call the method automatically.

Let’s take the below example.

val x : Int = 20.23

This will obviously throw an error.

Variable type error

Let’s declare an implicit function that converts Double to Int.

implicit def doubleToInt(d: Double) = d.toInt

Now the the above code works as expected i.e it auto converts the Double to Int.

implicit def doubleToInt(d: Double) = d.toInt

val x : Int = 20.23

//Prints 20
println(x)

Very handy in situations where it needs this to be done by default

Taking a look at the decompiled code

The decompiled code for this is pretty simple.

public final void delayedEndpoint$com$testing$Runnable$1() {
this.value = 10;
this.anotherValue = 2;
.MODULE$.println(BoxesRunTime.boxToInteger(this.multiply(this.anotherValue())));
}

It directly replaces the references to the variables at compile time. If it seems confusing, the first thing is to look in the docs and take a look at the decompiled code.

Conclusion

Scala is not the first language to have this. C# has implicits which work very similar to Scala in terms of functionality and having a slightly different syntax. Scala libraries such as Akka make use implicits heavily. Many advanced patterns are possible with implicits which I’ll cover in later tutorials.

This design pattern tends to be over used and should be used with caution. With power comes responsibility.


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Scala


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