Infix functions in DSLs in Kotlin - Mini Project: Build & Apply

Using Infix Functions in Kotlin DSLs

📖 Scenario: Imagine you are creating a small Kotlin program to build a simple shopping list using a Domain Specific Language (DSL). You want to use infix functions to make the code easy to read and write, like a natural language.

🎯 Goal: Build a Kotlin program that uses infix functions to add items with quantities to a shopping list in a clear and readable way.

📋 What You'll Learn

Create a data class called Item with name and quantity properties

Create an infix function called qty to set the quantity of an item

Create a list called shoppingList to hold Item objects

Add items to the shoppingList using the infix qty function

Print the shopping list with item names and quantities

💡 Why This Matters

🌍 Real World

Infix functions help make Kotlin DSLs more readable and expressive, useful for configuration files, build scripts, and custom mini-languages.

💼 Career

Understanding infix functions and DSLs is valuable for Kotlin developers working on clean APIs, build tools like Gradle, or domain-specific applications.

1

Create the Item data class

Create a Kotlin data class called Item with two properties: name of type String and quantity of type Int.

Kotlin

# Your code here

Hint

Use data class Item(val name: String, val quantity: Int) to define the class.

2

Create the infix function qty

Create an infix function called qty that extends String. It should take an Int parameter called amount and return an Item with the string as the name and amount as the quantity.

Kotlin

data class Item(val name: String, val quantity: Int)

# Your code here

Hint

Define the infix function with infix fun String.qty(amount: Int): Item and return Item(this, amount).

3

Create the shoppingList and add items

Create a mutable list called shoppingList of type Item. Add these items using the infix qty function: "Apples" qty 5, "Bananas" qty 3, and "Carrots" qty 7.

Kotlin

data class Item(val name: String, val quantity: Int)

infix fun String.qty(amount: Int): Item {
    return Item(this, amount)
}

# Your code here

Hint

Use mutableListOf() to create the list and add items with shoppingList.add("Apples" qty 5).

4

Print the shopping list

Use a for loop with variables item to iterate over shoppingList. Print each item's name and quantity in the format: "Apples: 5".

Kotlin

data class Item(val name: String, val quantity: Int)

infix fun String.qty(amount: Int): Item {
    return Item(this, amount)
}

val shoppingList = mutableListOf()
shoppingList.add("Apples" qty 5)
shoppingList.add("Bananas" qty 3)
shoppingList.add("Carrots" qty 7)

# Your code here

Hint

Use for (item in shoppingList) and println("${item.name}: ${item.quantity}") to print each item.

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