I find the Lens operators in Haskell just the most hilarious API in all of computing, even knowing that there is some sense to it.

I very much enjoyed a Kmett talk where he described needing to parameterize some lens thing with an index, a monad it would operate under, the source focus, the target focus, and three different containing structures, with some pieces repeated... and then realizing the type arguments said `i m a s t a b u` and decided he had probably gone too far.

What's interesting is that Ed Kmett isn't even a dyed in the wool Haskeller. He has more than a foot in the C++ world.

He's quite brilliant, but I still haven't really gotten used to the full power of lenses, yet. I only used them in simple ways.

While I have found the majority of my interactions with Haskell (and ML family languages) deeply satisfying, I have never really used Lens in anger, having been put off each time I've tried to get into it. Although, I remain slightly simpathetic since - as you say - there is some theory underlying it all.

I may be mistaken, but I think I have seen a handful of "Lens-lite" type libraries for use in production since it's such a large library, which seems.. uh.. suboptimal.