June 2012

Lenses are a great way to deal with functional references, but there are two common issues that arise from their use.

  1. There is a long-standing folklore position that lenses do not support polymorphic updates. This has actually caused a fair bit of embarrassment for the folks who'd like to incorporate lenses in any Haskell record system improvement.
  2. Access control. It'd be nice to have read-only or write-only properties -- "one-way" or "mirrored" lenses, as it were. Moreover, lenses are commonly viewed as an all or nothing proposition, in that it is hard to mix them with arbitrary user functions.
  3. Finally there is a bit of a cult around trying to generalize lenses by smashing a monad in the middle of them somewhere, it would be nice to be able to get into a list and work with each individual element in it without worrying about someone mucking up our lens laws, and perhaps avoid the whole generalized lens issue entirely.

We'll take a whack at each of these concerns in turn today.

No, I don't mean like this, but rather, If you spent any time trying to figure out xkcd's Umwelt April Fool comic this year, you may be interested in the Haskell source code. They used all sorts of information about you, the browser you were using, the resolution of your screen, to the geocoding of the network address you came from, etc. to serve up a custom web comic.

Today, davean posted to github the code for waldo, the engine he wrote to drive that comic.

Alas, he was not kind enough to actually supply the code for the umwelt comic strip itself, so you'll still be left wondering if the internet managed to find all of the Easter eggs. (Are they still Easter eggs when you release something a week before Easter?) You may find the list of links below useful if you want to get a feel for the different responses it gave people.

[ Article | xkcd's Forum | Hacker News | /r/haskell ]

[Update: Jun 10, 9:09pm] davean just posted a rather insightful post mortem of the development of waldo that talks a bit about why xkcd uses Haskell internally.