Can you tell about the key principles underlying functional programming?

Key principles of functional programming (FP):

1. Pure functions — functions that always return the same result for the same inputs and have no side effects (do not modify external state).

2. Immutability — data does not change after creation. Instead, new copies are created with modifications.

3. Higher-order functions — functions that accept other functions as arguments or return functions.

4. Absence of side effects — executing a function does not affect the program's state outside its scope.

5. Recursion instead of loops — recursion is often used for repetitive operations.

6. Lazy evaluation — computations are deferred until necessary.

Example of a pure function in Kotlin:

fun sum(a: Int, b: Int): Int {
    return a + b
}

It does not change external state and always returns the same result for the same arguments.