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.

Practice

1. What is the main benefit of using infix functions in Kotlin DSLs?

easy

A. They allow calling functions without dots or parentheses, making code more readable.

B. They improve the performance of the program.

C. They enable functions to have multiple return types.

D. They automatically generate UI components.

Solution

1. Step 1: Understand the purpose of infix functions

   Infix functions let you call a function with one argument without using dots or parentheses, making code look cleaner.

2. Step 2: Identify the benefit in DSLs

   This style makes domain-specific languages (DSLs) easier to read and write, resembling natural language.

3. Final Answer:

   They allow calling functions without dots or parentheses, making code more readable. -> Option A

4. Quick Check:

   Infix functions improve readability = C [OK]

Hint: Infix means no dots or parentheses for one-argument calls [OK]

Common Mistakes:

• Thinking infix improves performance
• Believing infix allows multiple return types
• Assuming infix auto-generates UI

2. Which of the following is the correct syntax to declare an infix function in Kotlin?

easy

A. infix fun add(x: Int) { }

B. fun infix add(x: Int) { }

C. fun add infix(x: Int) { }

D. fun add(x: Int) infix { }

Solution

1. Step 1: Recall infix function declaration rules

   In Kotlin, the keyword infix must appear before fun when declaring an infix function.

2. Step 2: Match the correct syntax

   infix fun add(x: Int) { } correctly places infix before fun and declares the function with one parameter.

3. Final Answer:

   infix fun add(x: Int) { } -> Option A

4. Quick Check:

   Keyword order: infix before fun = A [OK]

Hint: Remember: 'infix' keyword comes before 'fun' in declaration [OK]

Common Mistakes:

• Placing 'infix' after function name
• Putting 'infix' after parameter list
• Omitting 'infix' keyword entirely

3. What will be the output of the following Kotlin code?

class Builder {
    infix fun add(value: String) {
        println("Added: $value")
    }
}

fun main() {
    val b = Builder()
    b add "Kotlin"
}

medium

A. No output

B. b add Kotlin

C. Syntax error

D. Added: Kotlin

Solution

1. Step 1: Understand infix function call

   The infix function add is called on object b with argument "Kotlin" without dots or parentheses.

2. Step 2: Check the function behavior

   The function prints "Added: Kotlin" to the console.

3. Final Answer:

   Added: Kotlin -> Option D

4. Quick Check:

   Infix call prints 'Added: Kotlin' = A [OK]

Hint: Infix calls print inside function as expected [OK]

Common Mistakes:

• Expecting syntax error due to missing parentheses
• Thinking it prints the call expression
• Assuming no output without explicit print

4. Identify the error in this Kotlin code using an infix function:

class Config {
    infix fun set(key: String, value: String) {
        println("$key = $value")
    }
}

fun main() {
    val c = Config()
    c set ("color", "blue")
}

medium

A. Missing return type declaration.

B. Infix functions cannot have more than one parameter.

C. The function name 'set' is reserved and cannot be infix.

D. Parentheses are not allowed in infix calls.

Solution

1. Step 1: Check infix function parameter count

   Kotlin infix functions must have exactly one parameter, but here set has two parameters.

2. Step 2: Understand the error cause

   This causes a compilation error because infix functions cannot have multiple parameters.

3. Final Answer:

   Infix functions cannot have more than one parameter. -> Option B

4. Quick Check:

   One parameter only for infix = D [OK]

Hint: Infix functions accept exactly one parameter [OK]

Common Mistakes:

• Trying to use multiple parameters in infix
• Assuming parentheses are forbidden in all calls
• Thinking 'set' is a reserved infix name

5. Given the following Kotlin DSL code, what will be the output?

class HtmlTag(val name: String) {
    private val children = mutableListOf<HtmlTag>()
    infix fun add(child: HtmlTag) {
        children.add(child)
    }
    fun render(): String {
        val inner = children.joinToString("") { it.render() }
        return "<$name>$inner</$name>"
    }
}

fun main() {
    val html = HtmlTag("html")
    val body = HtmlTag("body")
    val p = HtmlTag("p")
    html add body
    body add p
    println(html.render())
}

hard

A.

B.

C.

D. Compilation error due to infix usage

Solution

1. Step 1: Understand the infix add function

   The infix function add adds child tags to the parent's children list, building a tree structure.

2. Step 2: Trace the tag hierarchy and rendering

   html adds body, body adds p, so the structure is html > body > p. The render function outputs nested tags accordingly.

3. Final Answer:

   <html><body><p></p></body></html> -> Option C

4. Quick Check:

   Nested tags render correctly = B [OK]

Hint: Infix builds nested structure; render outputs nested tags [OK]

Common Mistakes:

• Mixing tag closing order
• Assuming infix causes compile error
• Ignoring nested children in render