4 HN users recommended this talk4 months2020–2024score 5
Years with an HN recommendation, 2007–2026
HN recommends it as the fastest way to learn lenses by example, with far-out examples later on.
John Wiegley's BayHac 2017 talk walks through Haskell lenses by example, showing many practical uses beyond simple record access. It is aimed at people who want to become productive with the lens library.
“you can learn enough lens to be productive in 1 hours with this talk without reading docs at all” (HN, 2020)
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: [this video]
Learn enough lens usage to be productive in an hour.
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: [this video]
Great introduction to lenses by example, with humorous audience.
Good examples in "Putting lenses to work" by John Weigley: [this 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: [this 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 [this 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.
Good lens video with far-out examples in the later portion.
Video list and Hacker News comment labels from HackerTube (prompted by @karpathy), released under ODC-By 1.0, built on the open-index/hacker-news archive. Hacker News data is provided by Y Combinator; comments remain the work of their authors. Video titles, channel names and thumbnails belong to their creators and are shown via YouTube. HackerTube Watch is an unofficial edition. Data export of 2026-10-31.
Plays via YouTube's privacy-enhanced embed
5 Hacker News comments
From the HN archive; up to 20 kept per video, favoring different commenters. Recommendations first.