Why Purity Matters

11 Jun, 2023

In the world of functional programming, the notion of purity often comes up. But what is it? Why should we as programmers care about such a concept? It turns out that pure functions enable us to write more straightforward and performant code.

What is a Pure Function?

A pure function:

• Always returns the same output for a given input
• Does not modify its surrounding environment
  • No mutating local variables, writing to files, reading from the console, etc.

In other words, a pure function is an isolated piece of code that only relies on its inputs. For example, consider a function is_even that returns whether an input integer is even or not. If you call is_even(5), it will always return false, no matter what. Notice that this isn't necessarily true for other functions, e.g. a function is_leap_year that tells you if the current year is a leap year. Sometimes calling is_leap_year() will return true, and other times false.

Reasonable Code

Another perspective is to think of function arguments as explicit dependencies, since a function's result depends on them. Then it becomes clear that is_even has no implicit dependencies, while is_leap_year implicitly depends on the current year. Implicit dependencies are dangerous, because they introduce additional state to our program, making it harder to reason about. If somebody makes changes to other parts of the codebase, your code might break!

To avoid this, we can make our dependencies explicit where possible. In the case of is_leap_year, we modify the function to take in a year, represented by an integer. This localizes all of the dependencies for our function; any bugs exist inside the function itself, and nowhere else, making them easier to reason about and debug.

Need for Speed

Since pure function calls don't modify their surrounding environments, they can't interfere with one another. Thus:

• They can be parallelized, since there are no race conditions (the threads cannot affect each other)
• They can be memoized, since repeated function calls will always yield the same result
  • Instead of recomputing values each time, we replace function calls with their results¹

These are (in my opinion) a few of the most important reasons to care about pure functions; more benefits can be found here.