Infix functions in DSLs in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an infix function in Kotlin?

An infix function is a special kind of function that can be called using infix notation, meaning you can write it between its receiver and argument without dots or parentheses, making code more readable and natural.

beginner

How do you declare an infix function in Kotlin?

You declare an infix function by adding the keyword infix before the function name. It must be a member function or an extension function with a single parameter and no varargs or default values.

intermediate

Why are infix functions useful in DSLs (Domain Specific Languages)?

Infix functions make DSL code look cleaner and more like natural language, improving readability and expressiveness by allowing calls without dots or parentheses, which fits well with DSL style.

beginner

Example: What does this Kotlin code do?

infix fun Int.times(str: String) = str.repeat(this)

This defines an infix function times that repeats a string a given number of times. For example, 2 times "Hi" returns "HiHi".

intermediate

What are the restrictions on parameters for infix functions?

Infix functions must have exactly one parameter. They cannot have varargs, default parameter values, or multiple parameters.

Which keyword is used to declare an infix function in Kotlin?

A fun

B inline

C infix

D override

The infix keyword marks a function as infix, allowing it to be called without dots or parentheses.

What is a requirement for the parameter of an infix function?

A It must be a vararg parameter

B It can have multiple parameters

C It must have default values

D It must be a single parameter without default values

Infix functions must have exactly one parameter and cannot have default values or varargs.

How would you call an infix function named connect on an object a with argument b?

A a.connect(b)

B a connect b

C connect(a, b)

D connect a b

Infix functions are called using infix notation: a connect b without dots or parentheses.

Why are infix functions popular in Kotlin DSLs?

A They make code look more natural and readable

B They improve performance

C They allow multiple parameters

D They require less memory

Infix functions improve readability and make DSL code look like natural language.

Which of these is NOT allowed in an infix function declaration?

A Multiple parameters

B Single parameter

C Extension function

D Member function

Infix functions cannot have multiple parameters; they must have exactly one.

Explain what an infix function is and how it helps in creating DSLs in Kotlin.

Think about how you write code that reads like a sentence.

Describe the rules and restrictions for declaring an infix function in Kotlin.

Focus on the function signature and keywords.

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