18) Pure Functions

A pure function does not produce side effects. It communicates with the calling program only through parameters (which it does not modify) and a return value. Here is the doubleStufffunction from the previous section written as a pure function. To use the pure function version of double_stuff to modify things, you would assign the return value back to things.

Once again, codelens helps us to see the actual references and objects as they are passed and returned.

Which is Better?

Anything that can be done with modifiers can also be done with pure functions. In fact, some programming languages only allow pure functions. There is some evidence that programs that use pure functions are faster to develop and less error-prone than programs that use modifiers. Nevertheless, modifiers are convenient at times, and in some cases, functional programs are less efficient.

In general, we recommend that you write pure functions whenever it is reasonable to do so and resort to modifiers only if there is a compelling advantage. This approach might be called afunctional programming style.