Infix functions in DSLs in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an infix function named to.

Kotlin

infix fun String.[1](other: String): Pair<String, String> = Pair(this, other)

Drag options to blanks, or click blank then click option'

Afrom

Band

Cto

Dwith

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that is not declared as infix.

Choosing a name that is not descriptive or conventional.

The infix function to allows you to write expressions like "a" to "b" to create pairs.

2fill in blank

medium

Complete the code to call the infix function to to create a pair.

Kotlin

val pair = "key" [1] "value"

Drag options to blanks, or click blank then click option'

Ato

Bwith

Cfrom

Dand

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation instead of infix call.

Using a wrong function name.

The infix function to is called without dots or parentheses, like "key" to "value".

3fill in blank

hard

Fix the error in the infix function declaration by completing the code.

Kotlin

infix fun [1].to(other: String): Pair<String, String> = Pair(this, other)

Drag options to blanks, or click blank then click option'

AInt

BAny

CPair

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong receiver type like Int or Pair.

Forgetting the receiver type.

The infix function to is declared as an extension on String to pair two strings.

4fill in blank

hard

Fill both blanks to create an infix function connect that joins two strings with a dash.

Kotlin

infix fun String.[1](other: String): String = this [2] "-" + other

Drag options to blanks, or click blank then click option'

Aconnect

B+

C*

Djoin

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong operator like * which is invalid for strings.

Choosing a function name that is not descriptive.

The infix function connect concatenates two strings with a dash using the + operator.

5fill in blank

hard

Fill all three blanks to create a DSL that uses infix functions to build a map.

Kotlin

val map = mapOf(
    "one" [1] 1,
    "two" [2] 2,
    "three" [3] 3
)

Drag options to blanks, or click blank then click option'

Ato

Bfrom

C->

Dwith

Attempts:

3 left

💡 Hint

Common Mistakes

Using operators like -> which is not an infix function here.

Mixing different infix functions in the same map.

The to infix function is used to create pairs inside mapOf to build a map.

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