John Wiegley: Putting Lenses to Work

and you can learn enough lens to be productive in 1 hours with this talk without reading docs at all. You can skip the theory and go straight to usage: [VIDEO]

As with anything Haskell, one could simply reply with "here's the formal definition." But in easily digestible terms, it's a way to narrow your view over a collection to a subset that satisfies some property, and perhaps perform some operations on that subset. This video presentation (with a very humorous audience dynamic, might I add) serves as a great introduction by example: [VIDEO]

Good examples in "Putting lenses to work" by John Weigley: [VIDEO] Then for a great book that gives a great understanding see Chris Penner's amazing and fun "Optics by example: [LINK]

> Lenses let you do some amazing things, but I think most of their existence is just thanks to how truly horrible (nested) record field access and updates were. Only for the most basic uses. Architecting programs as folds or traversals can be powerful. One simple example of lenses i can think of being useful for more than record access is: > sumOf (folded . folded) [Just 1, Just 2] 3 More are in this presentation: [VIDEO]

Definitely agree on lenses being one of the more mind blowing things I've encountered and eventually grokked in programming. I would also recommend [VIDEO] as a good video on lenses in part because the latter portion has some pretty far-out examples of that deep well you mentioned.