Sjoerd Visscher

I'm currently working on the #Haskell library "proarrow", a category theory library, with the purpose of writing programs using string diagrams. My original goal was to generalize my "squares" package to support profunctors in any category instead of just Hask.

Released proarrow-0.4.0.0! Lots of small changes, and 2 new examples: a clocked digital circuit and a parallel build scheduler. The wiki explains what's going on: github.com/sjoerdvissch...

I haven't mentioned the embedded language for monoidal categories in proarrow yet! It is modular: only if you copy or discard a variable it requires a comonoid on the type, and similar for traces, duals, exponentials etc. Read more here: github.com/sjoerdvissch... #haskell #categorytheory

I haven't mentioned the embedded language for monoidal categories in proarrow yet! It is modular: only if you copy or discard a variable it requires a comonoid on the type, and similar for traces, duals, exponentials etc. Read more here: github.com/sjoerdvissch... #haskell #categorytheory

proarrow 0.3.0.0 is now released with this feature. #haskell #categorytheory

proarrow-0.1.0.0 is now live on hackage! hackage-content.haskell.org/package/proa... #haskell #categorytheory

If you have laws-as-code and you have code-as-diagrams, you get (you guessed it) laws-as-diagrams for free! github.com/sjoerdvissch...

If you have laws-as-code and you have code-as-diagrams, you get (you guessed it) laws-as-diagrams for free! github.com/sjoerdvissch...

playground.terrorjack.workers.dev weekend toy project: haskell playground based on ghc-10.0, with multi-file editor, ghci repl and haskell language server; transitive deps of hls like aeson are also included. note that this is a vibe coded poc, if you have allergies to this kind of thing

Apologies for the AI use, but I wanted to explore this in Oleg's visual style. Probably a lot of errors - I know spent too much time trying to get this even to this point. Script to generate it: gist.github.com/thomasmarsh/...

This is getting complicated! #haskell #categorytheory

That is, for any object A, we have an arrow α_A : F A -> G A, such that for all R∈Rel(A,B), F(R) ⊆ G(R(α,α)) If F is a functor, parametric arrows should be natural, for instance. This should give us an algebraic framework to talk about what we've been doing in PLT. 6/7

New optics just dropped! #haskell #category-theory

I’ve been working on proarrow for 3 years and only now I find out that all adjunctions between profunctors are trivial, as in they are always adjunctions between the two representables induced by a functor. At least it’s true for Hask enriched profunctors. Something to do with Cauchy completeness.

In today’s episode, we’re joined by Brent Yorgey, Associate Professor of CS at Hendrix College and the creator of the Diagrams vector graphics library and Swarm, a 2D programming and resource gathering game. We talk about teaching, DSLs, and competitive coding. haskell.foundation/podcast/85/

Dutch has this word "al", which frequently starts a sentence, and then it looks like "Al", indistinguishable from "AI" in most fonts, really annoying!

Cool observation: one can build a bicategory whose objects are free word monoids over alphabets and whose morphisms are Turing transducers. A Turing machine then appears as a specialization of a guarded trace between a Moore machine (tape) and a Mealy machine (controller).

Looking for non-trivial examples of virtual double categories arising in TCS or logic, ideally many-object examples where the virtuality has a concrete computational/logical meaning.

Using AI to write your code is like hiring a consultant to write your code, but continuously hiring a different one. No matter how good you set it up, documenting everything etc., over time the result is terrible.

after two years of development, I finally demoed Jermaine for the first time - a language with fully symmetric data and codata, and native call-cc.

I never thought the proarrow library would grow this big. I expected to hit a major roadblock at some point but that just didn't happen. sjoerdvisscher.github.io/proarrow/ #haskell

I want to explain what's going on here. Because it's quite interesting. The type of monads over the category of presheaves is given by class PMonad f where return :: p i -> f i (>>=) :: f p i -> (forall j. p j -> f q j) -> f q i (just follow the textbooks) 1/11

Pre-registration for this year's ICFP programming contest is open! You may register your team at your leisure at icfpcontest2026.com/signup The contest begins on July 24th - just under two weeks to go!

Are you also annoyed that the Bluesky Haskell feed is full of hurricane warnings for the town of Haskell? I just found this alternative feed: bsky.app/profile/did:...

I spelled out a complete type inference algorithm for one of the trickiest corners of Nix to typecheck: the biased record concatenation operator ("//") The post even includes a reference implementation in ≈400 lines of Haskell code haskellforall.com/2026/07/mech...

Haskell koan: Type-checked non-empty strings. ~ Mike Ledger. exploring-better-ways.bellroy.com/haskell-koan... #Haskell #FunctionalProgramming

‘De wereld voedt ons, en dan vooral ons vee’

For some Friday afternoon fun, allow me to hijack your productivity with this game: Escape the Moon! It started as a side project to kick the tyres on #SpacetimeDB, but snowballed into a whole cards-and-crafting web game. Hope you enjoy. Let me know if you do. 🙂 escapethemoon.vercel.app

There is a European-owned place called Aisle that is doing world class security bug finding. Beating Mythos even. But no one in Europe likes talking up our own talent. Been trying to get them mentioned in Dutch news, but all we can do apparently is promote US tech. aisle.com/blog/aisle-d...

Is de hittekracht-schaal nu al weer toe aan een update? www.hittekracht.net/verwachting @knmi.nl @helgavanleur.bsky.social

Before we solve data workflows in Haskell we have to teach people Haskell. sabela.datahaskell.com/c/1ea40000

Haskell ecosystem activities report: March–May 2026 www.well-typed.com/blog/2026/06... #Haskell

Mark your calendars! 🗓️ This year's iteration of the annual ICFP programming contest will take place from ** July 24 to July 27 ** Stay tuned for more info...! icfpcontest2026.com

Latest and maybe last (?) blogpost on my Haskell for @elmlang developers series: IO! 🚀 (This was cooked at @ZuriHac btw ☺️) flaviocorpa.com/haskell-for-...

The ICFP 2026 Programming Contest will run from July 24th to July 27th. We'll get icfpcontest2026.com updated with more information on the contest in the coming weeks :)

Fact: When abroad, James Bond is known as +44 07.

The past years have seen horrendous attacks towards trans people. We have a duty to protect the most vulnerable members of our communities. There is no Haskell without the trans people who have shaped it through the years. This June is a month of celebrations, but the struggle continues. ✊🏳️‍⚧️λ

Serokell’s Work on GHC: Dependent Types, Part 5 serokell.io/blog/serokel... #Haskell #DependentTyples