Why Returning functions from functions in Kotlin? - Purpose & Use Cases

The Big Idea

What if one function could hand you another function ready to solve your problem instantly?

The Scenario

Imagine you want to create different calculators for adding, subtracting, or multiplying numbers. Without returning functions, you'd have to write separate code for each calculator and call them manually every time.

The Problem

This manual way means repeating similar code again and again. It's slow to write, easy to make mistakes, and hard to change later if you want a new calculator type.

The Solution

By returning functions from functions, you can create a flexible calculator maker. One function can give you back the exact calculator you want, ready to use. This saves time, reduces errors, and makes your code neat and easy to update.

Before vs After

✗ Before

fun add(a: Int, b: Int) = a + b
fun subtract(a: Int, b: Int) = a - b
// Call add(5,3) or subtract(5,3) manually

✓ After

fun getCalculator(op: String): (Int, Int) -> Int = when(op) {
  "add" -> { a, b -> a + b }
  "subtract" -> { a, b -> a - b }
  else -> { _, _ -> 0 }
}
// val calc = getCalculator("add")
// calc(5,3)

What It Enables

This lets you build smart, reusable tools that create other tools on demand, making your programs powerful and flexible.

Real Life Example

Think of a coffee machine that can make espresso, latte, or cappuccino. Instead of building a new machine for each, you have one machine that returns the right coffee maker function based on your choice.

Key Takeaways

Manual repetition is slow and error-prone.

Returning functions creates flexible, reusable code.

It helps build smart tools that adapt to different needs.

Practice

1. What does it mean when a Kotlin function returns another function?

easy

A. The function can only return primitive data types like Int or String.

B. The function cannot have parameters if it returns another function.

C. The outer function produces a function as its result that can be called later.

D. The returned function must be called immediately inside the outer function.

Solution

1. Step 1: Understand function return types

   In Kotlin, a function can return any type, including another function type.

2. Step 2: Interpret returning a function

   Returning a function means the outer function gives back a function that can be stored or called later.

3. Final Answer:

   The outer function produces a function as its result that can be called later. -> Option C

4. Quick Check:

   Returning a function means output is a callable function [OK]

Hint: Returning function means output is a callable function [OK]

Common Mistakes:

• Thinking only primitive types can be returned
• Assuming returned function must be called immediately
• Believing returned functions cannot have parameters

2. Which of the following is the correct syntax to declare a Kotlin function that returns another function taking an Int and returning a String?

easy

A. fun outer(): (Int) -> String = { i -> i.toString }

B. fun outer(): (Int) -> String { return { i -> i.toString() } }

C. fun outer(): (Int) String { return { i -> i.toString() } }

D. fun outer(): Int -> String = return { i -> i.toString() } }

Solution

1. Step 1: Check function return type syntax

   In Kotlin, a function returning another function uses syntax like: fun name(): (ParamType) -> ReturnType

2. Step 2: Verify the return statement

   The function returns a lambda matching (Int) -> String, e.g., { i -> i.toString() }

3. Final Answer:

   fun outer(): (Int) -> String { return { i -> i.toString() } } -> Option B

4. Quick Check:

   Correct function type syntax = fun outer(): (Int) -> String { return { i -> i.toString() } } [OK]

Hint: Return type for function is (ParamType) -> ReturnType [OK]

Common Mistakes:

• Using arrow -> incorrectly in return type
• Mixing return type syntax with function body
• Omitting parentheses around function type

3. What is the output of this Kotlin code?

fun makeMultiplier(x: Int): (Int) -> Int {
    return { y -> x * y }
}

fun main() {
    val multiplyBy3 = makeMultiplier(3)
    println(multiplyBy3(5))
}

medium

A. 35

B. 8

C. Compilation error

D. 15

Solution

1. Step 1: Understand makeMultiplier function

   makeMultiplier returns a function that multiplies its input by x (captured from outer function).

2. Step 2: Trace main function calls

   multiplyBy3 is a function multiplying by 3; multiplyBy3(5) = 3 * 5 = 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   3 * 5 = 15 [OK]

Hint: Returned function uses captured outer variable [OK]

Common Mistakes:

• Confusing parameter names
• Forgetting the closure captures x
• Expecting immediate multiplication without calling returned function

4. Identify the error in this Kotlin code that tries to return a function:

fun outer(): (Int) -> Int {
    val inner = { x: Int -> x * 2 }
    return inner()
}

medium

A. Calling inner() instead of returning the function itself

B. Missing return type declaration for outer function

C. Incorrect lambda parameter syntax

D. Cannot return a lambda from a function

Solution

1. Step 1: Analyze return statement

   inner is a lambda function; inner() calls it and returns Int, not a function.

2. Step 2: Correct return value

   To return the function itself, return inner without parentheses.

3. Final Answer:

   Calling inner() instead of returning the function itself -> Option A

4. Quick Check:

   Return function, not function call [OK]

Hint: Return function name without () to return function itself [OK]

Common Mistakes:

• Returning function call instead of function
• Omitting function return type
• Misunderstanding lambda syntax

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

fun createCounter(start: Int): () -> Int {
    var count = start
    return {
        count += 1
        count
    }
}

fun main() {
    val counter = createCounter(10)
    println(counter())
    println(counter())
}

hard

A. 11
12

B. 10
11

C. 12
13

D. Compilation error

Solution

1. Step 1: Understand closure capturing variable

   The returned lambda captures 'count' variable and increments it each call.

2. Step 2: Trace calls to counter()

   First call: count = 10 + 1 = 11; prints 11. Second call: count = 11 + 1 = 12; prints 12.

3. Final Answer:

   11 12 -> Option A

4. Quick Check:

   Closure increments count on each call [OK]

Hint: Closures keep state between calls [OK]

Common Mistakes:

• Assuming count resets each call
• Confusing initial value with first output
• Expecting compilation error due to var usage