To everyone who attended yesterday's NYC Clojure Meetup: thanks for listening to me, asking good questions and providing some pretty great answers as well.

Here are the slides. For more detail on nearly everything, see previous posts.

(Navigate using the compass arrows. Up/Down within a section; Left/Right betwen sections; ESC for overview.)

In two previous posts, I went on about lenses in Clojure. Pinholes comprised a small library of higher order functions to formalize and simplify the viewing and manipulation of complex nested structures. Tinholes did essentially the same thing, but with macros instead. In both cases, there's recursion going on, as we burrow through layers of nesting, but macros had the advantage of doing it all before compilation, giving core.typed a chance to check our work.

The macro post was inexplicably popular, catapulting me to levels of fame I never expected to achieve without consciously emulating an early De Niro …

In a previous post, I built up a framework for lens-like constructs in Clojure: essentially some fancified versions of assoc-in and get-in to allow for bidirectional transformations along the nesting path and some utilities to generate special-purpose getter/setter functions. The name, "pinhole," is supposed to suggest a more primitive, utilitarian mechanism for achieving focus.

While still ruing (sort of) other mistakes, I found myself worrying that a triumphal sentence near the end of the piece

What's more, thanks to the expressive power of dynamic Clojure,
and higher order functions, these lenses are not just simple to
use but …

Sseveral comments regarding the pinholes post, have forced me, against the deepest elements of my nature, to engage in thought. Since that might never happen again, I thought it meet to record the event.

I'm going to say "I" a lot, because this is mostly my opinions.

Bidirectional programming

As pointed out by Christian Schuhegger in a comment on the original post, lenses were originally introduced to computer science in the context of bidirectional programming, rather than as a tool for dealing with deeply nested immutable structures. He points to a good list of papers on the subject on the …

Lenses are a construct for getting, "setting" or "modifying" values within data structures, especially deeply nested data structures. The quotation marks have the usual meaning when they show up in funktionsprache:¹ not mutating anything per se, but instead producing an object, or reference thereto, that is identical except for the requested change.

In Scala, the need for lenses is pretty glaring, as illustrated in Eugene Yokota's great explanation of lenses in Scalaz, because of the centrality of case classes. In his example problem, a turtle is represented with three case classes:

  case class Point(x: Double, y: Double …