Any references that explain the "converts nesting to concatenation" idea? I find it fascinating, in particular because I write a lot of code that works on deeply nested data structures -- structs of values (which can be structs) or lists of values. The distinction between struct, list and value and the need to treat them differently in code is interesting and annoying, and goes beyond merely working with functors and applicables. I don't understand lenses at all, but I understand monads.