nLab hierarchy of universes

 Idea

In dependent type theory, having a type of all types results in various paradoxes, such as Russell's paradox and Girard's paradox. There are two ways to resolve this issue. One way is to simply add a universe of small types and accept that not all types are small. On the other hand, one can use a hierarchy of universes or universe hierarchy and postulate that every type is an element of universes in this universe hierarchy: Given an universe level $\kappa$, each type universe $U_\kappa$ consists of only the $U_\kappa$-small types, and $U_\kappa$ itself, while not an element of $U_\kappa$, is an element of the successor universe $U_{\kappa^+}$, where $\kappa^+$ is the successor universe level. Most commonly used for the universe levels are the natural numbers, but other options are available as well, such as the integers or the ordinal numbers.

There are a few advantages of using a hierarchy of universes where every type is an element of the hierarchy, instead of a single separate type judgment, in the formulation of dependent type theory:

1. Having a hierarchy of universes where every type in the type theory is an element of the hierarchy of universes avoids having to use long annotations everywhere. For example, if there is a single separate type judgment where not all types are elements of types, then given the type family $x:A \vdash B(x)$, if it is not small relative to the hierarchy of universes, then the heterogeneous identity type, elements $a:A$, $b:A$, $p:\mathrm{Id}_A(a, b)$, $y:B(a)$, and $z:B(b)$ is represented by $\mathrm{hId}_{x:A.B(x)}(a, b, p, y, z)$, where the subscript $x:A.B(x)$ is a long annotation used to represent the type family $x:A \vdash B(x)$. With a hierarchy of universes where every type in the type theory is an element of the hierarchy of universes, the type family $x:A \vdash B(x):U$ is represented by the function $B:A \to U$, and the same heterogeneous identity type is then represented by $\mathrm{hId}_{B}(a, b, p, y, z)$, which is more concise when written out, and in addition, the subscript $B$ is now represents that the type depends upon the universe, rather than merely being an annotation, and can be written as the dependent type $\mathrm{hId}(B, a, b, p, y, z)$.

2. In the split context formalization of spatial type theory and cohesive type theory with an additional judgment for crisp terms of types, with a hierarchy of universes, one can define crisp types by simply postulating the type to crisply be an element of a type universe. However, without type universes, one has to add to the theory a separate judgment for crisp types, and all the requisite inference rules, structural rules, and congruence rules for crisp type judgments.

In addition, in dependent type theory with a hierarchy of universes,

• the congruence rules for substitution implies the congruence rules for every type in the type theory

• one can define a symbol $B$ as a type $A:U$ using the propositional equality $B =_U A$ or judgmental equality of terms $A \equiv B:U$.

However, with a single separate judgment for types, where not all types are elements of the hierarchy of universes, not all types can be compared using judgmental equality of terms, so neither of these are possible. There are two solutions to this, both of which are unwieldy:

• One can add judgmental equality of types, and the associated structural rules for strict judgmental equality of types. However, the congruence rules for substitution no longer implies the congruence rules for every type former in the type theory, because not all types are terms of universes. Thus, the congruence rules for every type in the type theory have to be added separately.

• Alternatively, one could add strict transport across judgmental equality of terms as strict equivalences of types, whether using the strict equivalence type or using the natural deduction inference rules of strict negative copies. This results in a simpler formal theory, since one doesn’t need any of the structural and congruence rules for judgmental equality of types.

• If one doesn’t even have judgmental equality of terms, one would need to use transport across identity types or path types. However, one needs to define weak equivalence of types and prove the congruence rules for weak equivalences of types, before weak equivalence of types could be used for definitions. Weak equivalences are significantly more complex to define, since the symbol $B \simeq A$ usually representing the weak equivalence type hasn’t been formally defined in the theory yet, and any definition of weak equivalence type or isEquiv for functions is itself a very complex expression when only using dependent function types, function types, dependent pair types, pair types, and identity types in the expression. Alternatively, one can add natural deduction inference rules for $B$ as a positive copy or weak negative copy of $A$, from which one can prove that the function in the introduction rule carries the structure of a weak equivalence of types. The proofs of the various congruence rules of type formers are also very complex; see dependent product type § typal congruence rules § using weak equivalences of types for a proof of the associated congruence rule for the formation rule of dependent product types and section 11.1.6 of Rijke22 for a proof of the associated congruence rule for the formation rule of dependent sum types.

However, this all comes at the cost of having to formalize the theory of universe levels of the hierarchy of universes before formalizing the type theory.

Definition

There are two main ways to define a hierarchy of universes:

• The first way is done in dependent type theories with only a single separate type judgment, by defining a type of universe levels $\mathrm{Level}$ inside the type theory and then defining a family of type universes indexed by $\mathrm{Level}$. Not all types are elements of universes in the universe hierarchy.

• The second way is done in dependent type theories with either no separate type judgment à la Russell, or a separate type judgment for every type universe in the type theory à la Coquand, where every type in the theory is an element of the hierarchy of universes. Instead of having an internal type of universe levels, one has either a separate judgment $\kappa \mathrm{level}$ which is in an untyped first-order theory or a meta-theoretic sort $\mathrm{Level}$ which is in a typed first-order theory or a dependent type theory, with an operation which takes level $\kappa$ to the successor level $\kappa^+$. Then for type theories à la Russell one has for each level $\kappa$ a universe $U_\kappa:U_{\kappa^+}$, and for type theories à la Coquand one has for each level $\kappa$ a type judgment $A \; \mathrm{type}_\kappa$ and a universe $U_\kappa \; \mathrm{type}_{\kappa^+}$.

Cumulativity

Types of a universe $U_\kappa$ in an hierarchy of universes are also types of the successor universe $U_{\kappa^+}$; i.e. $U_\kappa$ is a subtype of $U_{\kappa^+}$. There are two different ways of representing this, reminiscent of the distinction between Russell universes and Tarski universes.

• Given a universe level $\kappa$, the universe $U_\kappa$ is cumulative if given an element $A:U_\kappa$, one can derive that $A:U_{\kappa^+}$. This is similar to Russell universes in that elements of a cumulative universe $U_\kappa$ are literally elements of $U_{\kappa^+}$, similar to how elements of a Russell universe are literally types.

• Given a universe level $\kappa$, the universe $U_\kappa$ is non-cumulative if given a $U_\kappa$-small type $A:U_\kappa$, one can derive that $\mathrm{Lift}_\kappa(A):U_{\kappa^+}$. This is similar to Tarski universes in that elements of a cumulative universe $U_\kappa$ are codes for elements of $U_{\kappa^+}$, represented by the family of elements $A:U_\kappa \vdash \mathrm{Lift}_\kappa(A):U_{\kappa^+}$, similar to how elements of a Tarski universe are codes for types, represented by the family of types $A:U_\kappa \vdash \mathrm{Lift}_\kappa(A) \; \mathrm{type}$.

Examples

Some examples of type theories with a hierarchy of universes are as follows:

• Coq uses a hierarchy of Russell universes. For practical purposes, it also has cumulativity, although there is some question (perhaps mainly semantic) of whether this is true internally or whether it uses casts that are simply hidden from the user.

• Agda uses a hierarchy of non-cumulative Russell universes.

• UFP13 (first edition) uses a hierarchy of cumulative Russell universes.

• Rijke 22 uses a hierarchy of non-cumulative Tarski universes.

 Related concepts

• type universe

• coercion

• reflection principle

References

The hierarchy of universes is discussed in section 1.3 of:

• Homotopy Type Theory: Univalent Foundations of Mathematics, The Univalent Foundations Project, Institute for Advanced Study, 2013. (web, pdf)

and in section 6.2 of:

• Egbert Rijke, Introduction to Homotopy Type Theory, Cambridge Studies in Advanced Mathematics, Cambridge University Press (arXiv:2212.11082)