nLab simplicial type

Idea

In dependent type theory, given a bounded poset $I$, simplicial types are those for which $I$ behaves as a bounded total order. This is important in triangulated type theory to identify the objects which behave as simplicial objects relative to the distributive lattice interval $\mathbb{I}$, in order to define things such as $(\infty,1)$-categories, which are the $\mathbb{I}$-simplicial Rezk types in triangulated type theory.

 Definition

In dependent type theory, let $I$ be a bounded poset, such as a distributive lattice. A type $A$ is $I$-simplicial if for all elements $i:I$ and $j:I$ the function

$$\lambda x.\lambda t.x:A \to ((i \leq j \vee j \leq i) \to A)$$

which takes elements $x:A$ to constant functions $\lambda t.x:(i \leq j \vee j \leq i) \to A$ is an equivalence of types

$$\mathrm{isSimp}_I(A) \coloneqq \mathrm{isEquiv}(\lambda x.\lambda t.x)$$

Theorem

Suppose that $I$ is a total order. Then every type $A$ is $I$-simplicial.

Proof

In a total order, the type $i \leq j \vee j \leq i$ is a contractible type for all $i:I$ and $j:I$, and given any contractible type $B$, the function $\lambda x.\lambda t.x:A \to (B \to A)$ is an equivalence of types. Thus, $A$ is $I$-simplicial for all types $A$.

In particular, this means that in simplicial type theory with bounded total order $\mathbb{I}$, every type is $\mathbb{I}$-simplicial.

Related concepts

• skeletal type

• triangulated type theory

• simplicial type theory

 References

• Daniel Gratzer, Jonathan Weinberger, Ulrik Buchholtz, Directed univalence in simplicial homotopy type theory (arXiv:2407.09146)

• Jonathan Sterling, Lingyuan Ye, Domains and Classifying Topoi (arXiv:2505.13096)