What are pure functions in Kotlin?

In Kotlin, a pure function is a function that always returns the same output for the same input parameters and does not modify any external state or variables. They only use the input parameters passed to them to generate a result.

Pure functions have several advantages over non-pure functions:

  • They are easier to reason about, test, and debug.

  • They are less error-prone because they do not depend on external state.

  • They are more efficient since they can be memoized (i.e., caching the result of a function call for a specific input).

Pure vs. simple functions

The main difference between pure and simple functions is that pure functions always return the same output for the same given input, while simple functions might return different outputs for the same input, such as modifying a global variable or reading/writing to a file/database. These functions are not pure and can have unpredictable behavior.

Syntax

The syntax for defining a pure function in Kotlin is the same as a simple function.

fun functionName(parameter1: Type1, parameter2: Type2): ReturnType {
    // Function body
    // ...
    return result
}

Code example

Let’s take an example to understand the behavior of pure functions.

// Pure function 
fun sum(a: Int, b: Int): Int {
    return a + b
}
// Simple function 
var sum = 0
fun addToSum(num: Int) {
    sum += num
}
fun main() {
    val result = sum(3, 5)
    println(result) // Output: 8
    addToSum(2)
    addToSum(3)
    println(sum) // Output: 5
}

Explanation

Let’s discuss the above code in detail.

  • Lines 2–4: Define a pure function named sum that takes two integer parameters, a and b, adds them together, and returns the result.

  • Line 7: Declare a global variable named sum.

  • Lines 8–10: Define a simple function named addToSum that takes an integer parameter num and adds it to a variable named sum. Note that the sum is defined outside of the function and, therefore, can be modified by the function, making it impure.

  • Line 11: Define a main function that calls both sum and addToSum functions and prints the results to the console.

  • Lines 12–13: Call the sum function with the values 3 and 5 and assigns the result to a variable named result. Then, print the value of the result to the console.

  • Lines 15–17: Call the addToSum function twice with values 2 and 3, respectively, which modifies the value of the sum variable to be 2 + 3 = 5. Finally, print the value of sum to the console, which is 5.




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