“Folds can be used to implement any function where you traverse a list once, element by element, and then return something based on that. Whenever you want to traverse a list to return something, chances are you want a fold. That’s why folds are, along with maps and filters, one of the most useful types of functions in functional programming.”
That’s a great definition because it includes (almost) everything, so I’ll add this:
- You traverse an entire list, once
- Element by element
- You apply your own custom algorithm to the elements as you traverse the list
- In the end, you return a single value based off of that traversal and the application of your algorithm
If you’re interested in the definition of a fold in computer programming, I hope that’s helpful. See my fold/reduce tutorial for many more details.