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I'm aware that fold-left produces left-leaning trees and fold-right produces right-leaning trees, but when I reach for a fold, I sometimes find myself getting bogged down in headache-inducing thought trying to determine which kind of fold is appropriate. I usually end up unwinding the entire problem and stepping through the implementation of the fold function as it applies to my problem. So what I want to know is: What are some rules of thumb for determining whether to fold left or fold right? How can I quickly decide which type of fold to use given the problem I'm facing? There is an example in Scala by Example (PDF) of using a fold to write a function called flatten which concatenates a list of element lists into a single list. In that case, a right fold is the proper choice (given the way the lists are concatenated), but I had to think about it a bit to arrive at that conclusion. Since folding is such a common action in (functional) programming, I'd like to be able to make these kinds of decisions quickly and confidently. So... any tips?

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