Come to sunny Glasgow this summer for this year's edition of the Scottish Programming Languages and Verification summer school.

https://spli.scot/splv/2026-glasgow/

Absolutely unbeatable accommodation rates available too!

@counting_is_hard Is their some public document that details the redundancies you're talking about?

Also, best of luck to you (and congratulations on your LICS paper)!

@ncf Yes, the "non-discreteness" enters via the colimits you consider. And then the hope/trick is that these interact nicely with the kind of construction you wish to consider.

@jdw @dwarn @maxsnew

@jdw @ncf @dwarn See also the proof of Lemma 3.28 in https://doi.org/10.1017/jsl.2024.76 and the references to Harting and Taxerås Flaten.

@ncf @jdw @maxsnew (1/3) Let me try a telegraphic summary here, as a teaser for a longer text I might write in the future. Follow-up questions are very much welcome! :-)

In three sentences: Baby Barr (also known as "Friedman's trick", "A-translation", "nontrivial exit continuation", "harnessing the power of and eventually escaping the continuation monad") and full Barr are two very useful constructivization techniques. Another is to rewrite things using pointfree/formal topology. A very nice reference is the survey of Thierry Coquand linked at the bottom of https://rt.quasicoherent.io/.

"Baby Barr": Every (Grothendieck or elementary) topos ℰ can be covered by a Boolean topos. "Covered" means that there is a surjective geometric morphism from the covering topos to the original topos. "Boolean" means that the covering topos validates LEM. The covering topos can be explicitly written down, it is Sh_¬*¬*(ℰ/Ω), the topos of double negation* sheaves over ℰ/Ω (which is obtained from ℰ by adjoining a "generic truth value"). The proof that this topos has the required properties is constructive.

"Full Barr": Every Grothendieck topos can be covered by a topos in which LEM and AC hold. The covering topos can be explicitly written down, and the proof that it validates LEM is constructive, but the proof that it also validates the axiom of choice requires Zorn's lemma in the metatheory.

To appreciate the utility of these abstract results, and in the end to express them in non-topos theoretic language, one needs to know two things:

@jaycech3n Interesting meeting! I wasn't aware of these

@VojtechStep Please do come and say hi!

The 37th European Summer School in Logic, Language and Information (ESSLLI), taking place from August 3rd to August 14th, 2026 at the Faculty of Civil Engineering, Czech Technical University, Prague, Czechia.
https://2026.esslli.eu/

= Overview =
The European Summer School in Logic, Language and Information (ESSLLI) is a yearly recurring event, organized under the auspices of the Association for Logic, Language and Information (FoLLI), and has been running since 1989. The ESSLLI Summer School provides an interdisciplinary setting in which courses and workshops are offered in logic, linguistics and computer science, also from wider scientific, historical, and philosophical perspectives.

ESSLLI attracts around 400 participants from Europe, the Middle East, Asia and Africa, as well as from North America and Latin America. ESSLLI has become the main meeting place for young researchers and students in logic, linguistics and computer science to discuss current research and to share knowledge. The event is unique in its interdisciplinary set-up, with no equivalents in Europe.

Early-bird registration deadline: 15 June
https://2026.esslli.eu/registration/registration.html

= Programme =
ESSLLI offers an exciting two-week programme, consisting of:

Courses in three areas:
- Language and Computation, Logic and Computation, and Logic and Language
Workshops in logic, linguistics and computer science
Student session
Evening lectures
Social activities

https://2026.esslli.eu/courses-workshops-accepted/week-1-and-2-schedule.html

= Accommodation =
A number of two-person student rooms in a dormitory of Charles University have been reserved. This is a practical and affordable student option with a very modest level of comfort.
https://2026.esslli.eu/location/accommodation-general.html

#logic #computerscience

RE: https://mathstodon.xyz/@de_Jong_Tom/116435680670519184

The early-bird registration deadline for the European Summer School in Logic, Language and Information (ESSLLI) has been extended to 15 June.
https://2026.esslli.eu/registration/registration.html

[More details in the reply to this post.]

@antoinechambertloir Such a beautiful tune, a true favourite of mine.

@pounce Right, the workarounds are not satisfactory.

@amy

Our new preprint with @marco_abbadini (UCLouvain) and @RodrigoNAlmeida (University of Amsterdam) is out!

"A topos for étale-finite Heyting algebras"

https://arxiv.org/abs/2606.03861

It concerns Pitts' problem: can every Heyting algebra be realised as the lattice of truth values of an elementary topos?

In other words, for every Heyting algebra \( H \), is there an elementary topos \(\mathcal{E}\) such that

\(\text{Sub}_\mathcal{E}(1) \cong H\)?

Here is a thread.

1/15

Does anybody know how to typeset pullback/pushout corners in typst/fletcher?
The code provided at https://github.com/Jollywatt/typst-fletcher/issues/50#issuecomment-2851846670 does not quite work for me and even when it does, it requires experimenting with degrees which is bad.

[Update] I added my own workaround as a reply to the GitHub issue.

#typst #typesetting #categorytheory

@mevenlennonbertrand I do in the sense that mathematical work is about more than correctness. Having the community evaluate its significance is important.

@mevenlennonbertrand They do (and this is sensible to me), but I would consider it a bug if signatures appeared (even temporarily) while their associated comments are pending moderation.

@gadmm @dif Thanks, that's good! (Attaching a screenshot as "proof" that it wasn't showing earlier.)

@dif I just noticed that my comment wasn't listed. I don't know why. Maybe it was the result of a technical issue? I guess I should try to contact the organizers.

[Update] The comment is listed now.

@yforster @dif Yes, at least some are, see the attached screenshot (I would link instead if I could).

@dif [Update] The comment is now showing. I don't know why it wasn't earlier.

I signed it, although I'm less than happy that my accompanying comment [*] was removed.

[*] While the recommendations may be underwhelming and prove insufficient, I consider the declaration's identification and description of values and threats to be urgent and valuable.