I’ve seen some great maps recently.

Monads are monoids

just how much category theory was needed to establish this fact?

Not so much after all!

Also by John Wiegley:

Program Reduction: A Win for Recursion Schemes

Contrarian Knuth

I also must confess to a strong bias against the fashion for reusable code. To me, “re-editable code” is much, much better than an untouchable black box or toolkit. I could go on and on about this. If you’re totally convinced that reusable code is wonderful, I probably won’t be able to sway you anyway, but you’ll never convince me that reusable code isn’t mostly a menace.

An implementation of AlphaGo Zero

A Typed, Algebraic Approach to Parsing

IMO parser combinators are way overused.

  • They get messy really quickly
  • They leave it to the user to define a parsing strategy
  • The error messages are horrible

I’m really looking forward to using something like this.


an elementary streaming prelude and general stream type.

Supposedly the last word in Haskell pipes / streaming libraries.

The Lady Astronaut of Mars

Dorothy lived in the midst of the great Kansas prairies, with Uncle Henry, who was a farmer, and Aunt Em, who was the farmer’s wife. She met me, she went on to say, when I was working next door to their farm under the shadow of the rocket gantry for the First Mars Expedition.

I have no memory of this.

She would have been a little girl and, oh lord, there were so many little kids hanging around outside the Fence watching us work. The little girls all wanted to talk to the Lady Astronaut. To me.

What You Needa Know about Yoneda

Guillaume Boisseau, Jeremy Gibbons

Abstract Profunctor optics are a neat and composable representation of bidirectional data accessors, including lenses, and their dual, prisms. The profunctor representation exploits higher-order functions and higher-kinded type constructor classes, but the relationship between this and the familiar representation in terms of “getter” and “setter” functions is not at all obvious. We derive the profunctor representation from the concrete representation, making the relationship clear. It turns out to be a fairly direct application of the Yoneda Lemma, arguably the most important result in category theory. We hope this derivation aids understanding of the profunctor representation. Conversely, it might also serve to provide some insight into the Yoneda Lemma.

Categorifying cardinal arithmetic

Emily Riehl.

Proves a x (b + c) = (a x b) + (a x c) via Yoneda.