A pure function does no funny business

Edit: I changed some wording to avoid potential bad connotations.

You don’t have to wander far into the functional programming world before you run into references to “purity”. Worse, you often hear it as an admonishment:

You should factor your code to use pure functions. Or, this function is complicated because it is impure. At the very edges there are even ways to write entire programs as “pure functions”. Those familiar with that practice have a tendency to lay blame for every fault at the feet of impurity.

Even the word is pretty polarizing. It suggests that your impure, dirty code is full of lurking issues and sits far away from some important ideal. But that’s a tragic point of view. Plenty of “impure” code is still simple, clear, and extremely valuable. The implied judgement is misguided at best.

But before we can talk about the tradeoffs of using pure functional style, we need to understand it. What does it mean for a function to be pure anyway?

Continue reading “A pure function does no funny business”