Zulip Chat Archive
Stream: general
Topic: Universe of pi type
Sebastian Reichelt (Feb 08 2022 at 22:22):
I have a basic question about universes in Lean: Would any known inconsistency arise if the type of (x : α) → β x
with α : Sort u
and β : α → Sort v
was simply Sort v
instead of Sort (imax u v)
?
(I guess the answer must be "yes" because otherwise imax
wouldn't need to exist, but I'd like to be sure because it seems to be a blocker for something I would like to do.)
Yaël Dillies (Feb 08 2022 at 22:24):
Do you mean Sort (max u v)
?
Sebastian Reichelt (Feb 08 2022 at 22:30):
No, max
would be much "worse" than imax
of course; there wouldn't be any universally quantified propositions then as far as I understand. You can interpret my question as asking if "universally quantified data" can be treated in the same way as universally quantified propositions, or equivalently if the "size" of a function could just be the "size" of its output. I hope there is not some obvious type-theoretic reason why it cannot work.
Reid Barton (Feb 08 2022 at 22:31):
It can't be because e.g. Type -> bool
has to be bigger than Type
, so at least in the same universe as Type
.
Reid Barton (Feb 08 2022 at 22:33):
You need LEM to cook up this kind of example, I think without it a system like this might be consistent, but it would be anticlassical.
Reid Barton (Feb 08 2022 at 22:33):
(the key phrase is "impredicative universe")
Sebastian Reichelt (Feb 08 2022 at 22:40):
Thanks, I'll check that out.
Just curious, is the Type -> bool
example problematic only in the presence of choice
, via propDecidable
or something? (Assuming LEM as you say.)
Reid Barton (Feb 08 2022 at 22:48):
I mean there are definitely systems with an impredicative universe out there, but they are unsuitable for mathematics in the strong sense of proving classically false statements, so I don't know that much about exactly what you can and can't put into such a theory.
Reid Barton (Feb 08 2022 at 22:49):
But computer scientists often don't care about classical mathematics (because they cannot compute LEM anyways), so they think it is reasonable. System F is one of these systems, I understand.
Sebastian Reichelt (Feb 08 2022 at 23:03):
Ah, now I understand, you are arguing that α → Bool
is set-theoretically larger than α
, right? I missed that, oops. :-)
Trebor Huang (Feb 09 2022 at 08:37):
if you do that, there cannot be more than two universes (in which case we get the CoC). Otherwise your system contains a subsystem named System , where you can formulate Girards paradox.
Trebor Huang (Feb 09 2022 at 08:38):
(I haven't really checked all the axioms of because I simply can't remember them, so correct me if I'm wrong.)
Mario Carneiro (Feb 09 2022 at 09:16):
@Trebor Huang See docs#girard
Mario Carneiro (Feb 09 2022 at 09:19):
Oh shoot, since it's been moved to the archive I can't find it in the docs anymore. @Rob Lewis What are your thoughts on this? Can we add the mathlib archive to docgen?
Mario Carneiro (Feb 09 2022 at 09:19):
You can find it at an old version of the mathlib docs: https://cs.brown.edu/courses/cs1951x/docs/logic/girard.html
Yaël Dillies (Feb 09 2022 at 09:21):
Exactly the same happened to Bhavik and I yesterday :sweat_smile:
Yaël Dillies (Feb 09 2022 at 09:21):
And, yes, documentation for the archive would be great.
Eric Wieser (Feb 09 2022 at 12:50):
https://github.com/leanprover-community/doc-gen/issues/147 tracks that task
Eric Wieser (Feb 07 2023 at 14:46):
Mario Carneiro said:
Trebor Huang See docs#girard
https://leanprover-community.github.io/mathlib_docs_demo/girard.html
Mario Carneiro (Feb 07 2023 at 16:02):
Oh, you fixed the archive links? :tada:
Eric Wieser (Feb 07 2023 at 16:03):
Yes, see https://github.com/leanprover-community/doc-gen/pull/176 and #general > docs archive.
Last updated: Dec 20 2023 at 11:08 UTC