Infix functions in DSLs in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of infix function call in Kotlin DSL

What is the output of this Kotlin code using an infix function in a DSL style?

Kotlin

class Builder {
    private val items = mutableListOf<String>()
    infix fun add(item: String) {
        items.add(item)
    }
    fun build() = items.joinToString(", ")
}

fun main() {
    val builder = Builder()
    builder add "apple"
    builder add "banana"
    println(builder.build())
}

A [apple, banana]

B apple, banana

C apple banana

D apple; banana

💡 Hint

Remember that the build function joins items with a comma and space.

The infix function 'add' adds items to the list. The build function joins them with a comma and space, so the output is 'apple, banana'.

❓ Predict Output

intermediate

Result of chained infix calls in Kotlin DSL

What is the output of this Kotlin code with chained infix function calls?

Kotlin

class SentenceBuilder {
    private val words = mutableListOf<String>()
    infix fun add(word: String): SentenceBuilder {
        words.add(word)
        return this
    }
    fun build() = words.joinToString(" ")
}

fun main() {
    val sentence = SentenceBuilder() add "Hello" add "world" add "!"
    println(sentence.build())
}

A Hello world !

B Hello, world, !

C Hello world!

D Hello world

💡 Hint

Check how the words are joined in the build function.

Each 'add' call adds a word and returns the builder, allowing chaining. The build joins words with spaces, so output is 'Hello world !'.

🔧 Debug

advanced

Identify the error in this infix function usage

What error does this Kotlin code produce when run?

Kotlin

class Config {
    infix fun set(key: String) {
        println("Setting $key")
    }
}

fun main() {
    val config = Config()
    config set "mode" to "dark"
}

A Unresolved reference: to

B No error, prints 'Setting mode to dark'

C SyntaxError: Unexpected token 'to'

D TypeError: 'to' is not a function

💡 Hint

Check if 'to' is defined or imported in this context.

The code tries to use 'to' as an infix function or operator, but it is not defined or imported, causing an 'Unresolved reference: to' error.

🧠 Conceptual

advanced

Understanding infix function constraints in Kotlin DSLs

Which of the following is NOT a requirement for a function to be used as an infix function in Kotlin?

A It must be a member function or an extension function

B It must have exactly one parameter

C It must return Unit

D It must be marked with the 'infix' keyword

💡 Hint

Think about the return type flexibility of infix functions.

Infix functions can return any type, not necessarily Unit. The other options are required for infix functions.

❓ Predict Output

expert

Output of nested infix function calls in Kotlin DSL

What is the output of this Kotlin code using nested infix functions in a DSL style?

Kotlin

class Node(val name: String) {
    private val children = mutableListOf<Node>()
    infix fun add(child: Node): Node {
        children.add(child)
        return this
    }
    fun printTree(indent: String = ""): String {
        val current = "$indent$name"
        val childStrings = children.joinToString("\n") { it.printTree(indent + "  ") }
        return if (children.isEmpty()) current else "$current\n$childStrings"
    }
}

fun main() {
    val root = Node("root") add Node("child1") add Node("child2") add Node("child3")
    println(root.printTree())
}

A

root
  child1
    child2
  child3

B root child1 child2 child3

C

root
  child1
    child2
      child3

D

root
  child1
  child2
  child3

💡 Hint

Consider how the infix 'add' returns 'this' and how chaining affects the tree structure.

Each 'add' adds a child to the current node and returns the same node, so all children are added to 'root'. Chaining means the second 'add' is called on the result of the first, which is 'root', so children are siblings. So output is option D.