Questions tagged [universe]

In mathematics, and particularly in set theory, category theory, type theory, and the foundations of mathematics, a universe is a collection that contains all the entities one wishes to consider in a given situation. (from Wikipedia)

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Heterogeneous lists, large indices

Recently I had cause to define a type of heterogeneous lists in Lean, and wrote ...
Brendan Murphy's user avatar
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Universe polymorphism and Coq standard library

When developing in Coq with the Universe Polymorphism flag on, the standard library introduces unwelcome universe constraints because it is universe monomorphic. Is there an alternative standard ...
Jon's user avatar
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increasing the universe level of a type in Lean by force

Given some term A : Sort u in Lean, is there a way to artificially increase the universe level and promote A to a term ...
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Feferman's universes for proof assistants?

I asked this question on MathOverflow a few months ago, but received no answers. There are some mathematicians who are comfortable with ZFC but uneasy with large cardinals. For them, it may be ...
Timothy Chow's user avatar
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Abstracting over large types in type theory

In dependent type theory usually the typing context lets you abstract over arbitrary elements of some (dependent) types. Now if one wants to abstract over an arbitrary (but not quite) type, they can ...
Russoul's user avatar
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Naive computation of propositional resizing

As far as I know, it is not known whether cubical type theories with propositional resizing admit computational interpretations. But here is an obvious attempt: Turn off universe checking locally, and ...
Trebor's user avatar
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Stacks versus universes

This is a vague question, and I apologize in advance for it. I do not need a definite answer, I am happy to just get some general ideas. An elementary topos can model higher order logic over a ...
Nico's user avatar
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Universe inconsistency as an effect

The Internet tells me there is some work on languages that permit general recursion but carry information about possible divergence in the type system. For instance, the simply-typed language Koka ...
Mike Shulman's user avatar
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4 votes
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Universe polymorphism and modules in Coq

The following code (without universe polymorphism) is accepted by Coq (8.16.0) : ...
Dave's user avatar
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Descriptions of heterogenous datatypes

When attempting to describe the datatype as appearing in my previous question, using indexed descriptions in style of The Gentle Art of Levitation to describe this datatype (using Agda for examples): <...
Ilk's user avatar
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Is it possible to prove (-> (= (Either Trivial Trivial) (left sole) (right sole)) Absurd) in Pie?

In this question, I am talking about the language Pie described in the book The Little Typer. One can derive that $0=1$ is contradictory: ...
CrabMan's user avatar
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Unexpected eta expansion in constant definition

Start from this example in section 2.1.14: Polymorphic Universes in Coq's reference maunual (slightly modified): ...
Indprinciple's user avatar
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Is coercion to a higher universe injective?

A type that is a member of a universe can be coerced into a higher universe. Is that coercion injective? That is, if two elements of U1 are equal after being coerced to U2, does that imply they are ...
Kyle Stemen's user avatar
7 votes
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Cardinality of Type in a given universe

I'm trying to determine the cardinality of Type u in Lean 3. So far I've been able to prove two inequalities: ...
Matt Diamond's user avatar
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Why not have `Prop : Set` in Coq?

My understanding of Coq is that Prop : Type_1, Set : Type_1, and then Type_1 : Type_2, ...
Emma Hudson's user avatar
13 votes
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Unintentionally proven false theorem with type-in-type outside logic and foundations?

We are all familiar with Russell's paradox, and it is known that Per Martin-Löf proved that type-in-type is normalizing and consistent (which is false), by accidentally using an assumption in his meta-...
user21820's user avatar
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Parameterized Datatypes in a Universe à la Tarski?

I'm wondering, is there a way to make a Universe à la Tarski that models all of the types in an open type theory, where there can be user defined parameterized inductive types? For context, I'm trying ...
Joey Eremondi's user avatar
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How much duplication does universe polymorphism actually save us?

From my rough impression, in (formalizing) everyday mathematics we almost never use universe polymorphism in a way that stretches the proof-theoretic strength. It merely saves us some duplication. My ...
Trebor's user avatar
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11 votes
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What is a universe?

What is a universe? More specifically, Is a type system without a term-type distinction, are universes just ordinary types? If (1) is true, can we the system-builders freely choose which types are &...
Greg Nisbet's user avatar
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Explicit vs implicit universes in lean

I've seen in mathlib several cases where the universes are explicit, that is Type u instead of Type*. Is there any advantage in ...
mcd's user avatar
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Is there any universe polymorphic version of univalence?

People would say in univalent type theory, anything you defined for types should respect equivalence since univalence told you equivalence equivalent to equality. But that's not correct. Only ...
KANG Rongji's user avatar