Returning functions from functions in Kotlin - Step-by-Step Execution

Concept Flow - Returning functions from functions

Call outerFunction

↓

outerFunction runs

↓

Create innerFunction

↓

Return innerFunction

↓

Call returned innerFunction

↓

innerFunction runs and returns result

The outer function creates and returns an inner function. Later, calling the returned function runs the inner function's code.

Execution Sample

Kotlin

fun outerFunction(): (Int) -> Int {
    return fun(x: Int): Int { return x * 2 }
}

val f = outerFunction()
println(f(5))

Defines a function that returns another function doubling its input, then calls it with 5.

Execution Table

Step	Action	Evaluation	Result
1	Call outerFunction()	Runs outerFunction body	Returns inner function (x) -> x * 2
2	Assign returned function to f	f now holds inner function	f = (x) -> x * 2
3	Call f(5)	Runs inner function with x=5	Returns 10
4	Print result	Output value	10

💡 Execution stops after printing the result of f(5)

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
f	undefined	function returned by outerFunction	function assigned	function called with 5	function remains assigned

Key Moments - 3 Insights

Why does outerFunction return a function instead of a value?

Because outerFunction's return type is a function type (Int) -> Int, it returns a function object, as shown in execution_table step 1.

How do we call the function returned by outerFunction?

We assign it to a variable (f) and then call f with an argument, as shown in execution_table steps 2 and 3.

What happens when we call f(5)?

The inner function runs with x=5 and returns 10, as shown in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of calling f(5)?

A Function object

B 10

C 5

D Error

💡 Hint

Check execution_table row 3 where f(5) is called and returns 10.

At which step is the inner function created and returned?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at execution_table step 1 where outerFunction returns the inner function.

If we change outerFunction to return a function that triples x, what would f(5) return?

A 15

B 10

C 5

D Error

💡 Hint

Think about the inner function's return value when multiplying x by 3 instead of 2.

Concept Snapshot

fun outerFunction(): (Int) -> Int {
  return fun(x: Int): Int { return x * 2 }
}

val f = outerFunction()  // f is a function
println(f(5))           // prints 10

- Functions can return other functions.
- Returned functions can be called later.

Full Transcript

This example shows how a Kotlin function can return another function. The outerFunction returns an inner function that doubles its input. When we call outerFunction, it returns this inner function, which we store in variable f. Calling f(5) runs the inner function with 5, returning 10. This lets us create functions dynamically and use them later.