The Implicit Arguments

While exploring Scala, one of the features that caught my eyes was that functions can accept two sets of arguments. Typically, we write function name(argumentList) in most languages. Scala goes one step ahead and allows function name(argumentList)(implicitArgumentList). In this article, we shall explore implicit arguments from a Scala-newbie perspective. meaning that we won’t dig deep into architectural patterns and theoretical implications, rather, we explore them from a syntax and syntax-sugar perspective.

A First Dive

Let’s write a function to concatenate a list of strings with a separator. As a good first step, we write this as a function accepting two arguments and a return value

def concatStrings(strings: Seq[String], separator: String): String = {
  strings.foldRight("") {_ + separator + _}
}

Several things are going on here, let’s break them down one by one:

• Scala uses [] to denote generics; thus, Seq[String] is a Seq of Strings

• The return value is the value of the last line of the function; writing return is optional

• foldRight is similar to reduce function in JS, its first argument is the initial value ""

• foldRight takes a reducer function as a curried argument

• The reducer function is of type (String, B) => B, The type B comes from the initial value, which is also String now

• The funny _s are a shorthand to access function arguments; the First _ is of type String, and the second _ is of type B (= String). You can’t write a third _, that’s an error 😅

• {...} is a shorthand for the funciton body, otherwise you would have written strings.foldRight("")((a, b) => a + separator + b); Notice two set of arguments foldRight()(), don’t worry, we will address it right away 😄

Then, we invoke this function and get a return value from anywhere.

def main(args: Array[String]): Unit = {
  val strings = Seq("Hello", "World")
  val result = concatStrings(strings, "+")
  assert(result == "Hello+World+") // The extra + is because of foldRight
}

Hello Implicit!

Let’s make our function concatStrings to accept an implicit separator argument. The function body looks similar to the previous case, excpet the argument list:

def concatStrings(strings: Seq[String])(implicit separator: String): String = {
  strings.foldRight("")((a, b) => a + separator + b)
}

The second set of arguments begins with the implicit keyword and marks them as implicit. The implicit arguments are inferred from the scope at compile-time and passed to the function as an argument. To make it clear, let’s look at the invocation of our new concatStrings function.

def main(args: Array[String]): Unit = {
  val strings = Seq("Hello", "World")
  implicit val delimiter: String = "+"
  val result = concatStrings(strings)
  assert(result == "Hello+World+")
}

Here, we are creating a constant named delimiter of type String, which is also declared as implicit. When the compiler encounters the call to concatStrings, it looks for values of type String in the scope, which is marked as implicit. In this case, it is the constant delimiter. Then, it is passed as an argument to the function.

Let’s play with implicit arguments by modifying the code.

Missing Implicit

If the compiler can not find implicit values in the scope, it is an error.

def main(args: Array[String]): Unit = {
  val strings = Seq("Hello", "World")
  val result = concatStrings(strings)
  assert(result == "Hello+World+")
}

Here, the compiler complains with a readable, understandable error

could not find implicit value for parameter separator: String
	val result = concatStrings(strings)

Multiple Implicits

If we have multiple implicit values of the same type, that is also an error.

def main(args: Array[String]): Unit = {
  val strings = Seq("Hello", "World")
  implicit val delimiter: String = "+"
  implicit val anotherDelimiter: String = "+"
  val result = concatStrings(strings)
  assert(result == "Hello+World+")
}

The error is

ambiguous implicit values:
 both value anotherDelimiter of type String
 and value delimiter of type String
 match expected type String
    val result = concatStrings(strings)

Explicitly Implicit

We can also explicitly pass the implicit argument, as we did in foldRight()

def main(args: Array[String]): Unit = {
  val strings = Seq("Hello", "World")
  val result = concatStrings(strings)("+") // Notice the second argument list
  assert(result == "Hello+World+")
}

Implicit from Upper Scope

Implicit values can be from upper scope as well:

object ImplicitHelper {
  def concatStrings(): String = {
    // Same as before
  }
}

object Main {
  implicit val delimiter: String = "+"
  
  def main(args: Array[String]): Unit = {
    val strings = Seq("Hello", "World")
    val result = ImplicitHelper.concatStrings(strings)
    assert(result == "Hello+World+")
  }
}

Here, the object ... is a singleton object, which can define methods as well. Think of it as a static method. When we have implicit values present in multiple scopes, the closest one is picked first (as expected 😁).

object ImplicitHelper {} // Same as before

object Main {
  implicit val delimiter: String = "+"

  def main(args: Array[String]): Unit = {
    val strings = Seq("Hello", "World")
    implicit val delimiter: String = "_" // This one is picked
    val result = ImplicitHelper.concatStrings(strings)
    assert(result == "Hello_World_")
  }
}

Implicits can flow in from import as well, suppose we have an object named Config in the file Config

// Config.scala
object Config {
  implicit val delimiter: String = ","
}

In our Main.scala (in which we write the main function),

// Main.scala
import Config._

object ImplicitHelper {} // Same as before

object Main {
  def main(args: Array[String]): Unit = {
    val strings = Seq("Hello", "World")
    val result = ImplicitHelper.concatStrings(strings)
    assert(result == "Hello,World,")
  }
}

Since we imported every member of the object Config, the compiler can match the delimiter to the argument of contactStrings

Viewing Implicit Arguments

Since the implicit arguments can be inferred directly from the code without analyzing all the imports in all scopes, debugging others’ code can be hard. Fortunately, IntelliJ has an option to explicitly show the implicit variables 🎉. Just enable it from View -> Show Implicit Hints (you might need Scala Plugin to do this). With this option enabled, the editor window looks like this,

Now you can follow the definition of the value as usual.

Implicit Conversions

Since functions are the first-class citizens of Scala, they can be an implicit argument as well, here is a sample:

import scala.language.implicitConversions

object ImplicitHelper {
  def toInt[A](argument: A)(implicit function: (A) => Int): Int =
function(argument)
}

object Main {
  implicit def length(string: String): Int = string.length

  def main(args: Array[String]): Unit = {
    val string = "Hii"
    val result = ImplicitHelper.toInt(string)
    assert(result == 3)
  }
}

Here, the object method Main.length is the implicit parameter to the toInt function. Note that we also needed to import scala.language.implicitConversions. To quote the Scala Docs, we need this,

Because implicit conversions can have pitfalls if used indiscriminately the compiler warns when compiling the implicit conversion definition.

So, we are making it explicit that we are using implicit conversions 😄 (BTW, conversion = function).

Implicitly Implicit 😎

We have already seen that we can bring in implicit values in the scope using import. If we can have multiple implicit values defined in the imported classes, we have some interesting effects.

Let’s take the earlier example of concatStrings(). We have a DefaultConfig object setting delimiter to ",", and a SpecialConfig object setting it to "+". Let’s also have these files:

1. ImplicitHelper.scala — Defines concatStrings method
2. Main.scala — Invokes the contactStrings method
3. DefaultConfig.scala, and
4. SpecialConfig.scala — as defined earlier

Now,

// DefaultConfig.scala
object DefaultConfig {
  implicit val delimiter: String = ","
}

And,

// SpecialConfig.scala
object SpecialConfig {
  implicit val delimiter: String = "+"
}

And,

// ImplicitHelper.scala
object ImplicitHelper {} // As before

At last,

import ImplicitHelper._
import DefaultConfig._

object Main {
  def main(args: Array[String]): Unit = {
    val strings = Seq("Hello", "World")
    val result = concatStrings(strings)
    assert(result == "Hello,World,")
  }
}

Since we import DefaultConfig._, the delimiter is now ",". We can quickly swap that for SpecialConfig._ to have a different delimiter.

import ImplicitHelper._
import SpecialConfig._

object Main {
  def main(args: Array[String]): Unit = {
    val strings = Seq("Hello", "World")
    val result = concatStrings(strings)
    assert(result == "Hello+World+")
  }
}	

And, the import order does not matter.

Concluding Thoughts

The implicit arguments, especially through import provide a nice way to implement composability and dependency injection patterns. Thanks to the tooling, we still get the benefits of following arguments to their definitions. Sometimes, like in implicit conversions, we still need to be careful not to misuse them.