Скачать или смотреть 6. Pure Functions (Introduction to Functional Programming)

The next JIT nugget of background information we need to cover is the definition of a pure function.

A pure function is a function:

Whose only parameters from the outside world are the input parameters that are defined in its function signature.
- (I refer to the list of input parameters as a function’s front door.)
That does not modify those input parameters.
That does not modify other “hidden” variables that might be accessed through side doors, such as mutable variables in its class or any other global variables.
That does not communicate with the outside world. It has no I/O whatsoever: no file access, no internet access, no database access, no quantum entanglement, nothing.
That always returns the same output when its given the same input.
- For example, sum(2, 2) is always 4.
That returns output for all possible input values.
That does not throw exceptions.

Because of those last two attributes, FPers say that a function is total, meaning that it always returns a value.