9. functional programming is a programming
paradigm that treats computation as the evaluation of
mathematical functions and avoids changing-state
and mutable data.
FP란
17. pure function is a function that has the following properties:
Its return value depends only on its parameters and not on any internal
or external state (such as local static variables, non-local variables or
inputs from I/O devices).
Its evaluation has no side effect (such as mutation of local static
variables or non-local variables, or performing I/O operations).
Pure function