Comments

• By plaidfinch 2025-11-1514:50

  Ten years ago Dan Piponi's 2006 exploration of this same idea (http://blog.sigfpe.com/2006/11/from-l-theorem-to-spreadsheet...) inspired me to write the paper "Getting a Quick Fix on Comonads" (https://github.com/plaidfinch/GQFC), which ended up in Haskell Symposium 2015.

  It turns out there's a more powerful and arguably more interesting version of the Löb fixpoint in a setting where we have a ComonadApply, i.e. a structure that's not merely a functor, but also a comonad and comonad's version of Applicative. What emerges looks like spreadsheet evaluation in arbitrarily shaped spaces, with relative references provided by the comonadic structure.

  I've always found this stuff tremendously fun, and it's a delight that people keep stumbling across it who share my enthusiasm!

• By Twisol 2025-11-159:30

  > This shows how moeb is a generalization of fix.

  To be fair, loeb was already a generalization of fix. If `f` is the identity functor, then `loeb` has type `(a -> a) -> a`, and the `fmap` used in its definition resolves to `id`.

  It's a shame there aren't any other example applications of moeb. The author mentions using `traverse` and `foldMap`, but those are also based fundamentally on `fmap` in some sense, and I wouldn't be too surprised if they also ended up being literally `fmap` for some specific choice of functor.

• By gnabgib 2025-11-0823:17

  (2013) Popular in:

  2021 (153 points, 60 comments) https://news.ycombinator.com/item?id=34578411

  2018 (86 points, 10 comments) https://news.ycombinator.com/item?id=18159087