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\newtheorem{prop}{Proposition} \newtheorem{cor}{Corollary} \newtheorem*{utheorem}{Theorem} \newtheorem*{ulemma}{Lemma} \newtheorem*{uprop}{Proposition} \newtheorem*{ucor}{Corollary} \theoremstyle{definition} \newtheorem{defn}{Definition} \newtheorem{example}{Example} \newtheorem*{udefn}{Definition} \newtheorem*{uexample}{Example} \theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{note}{Note} \newtheorem*{uremark}{Remark} \newtheorem*{unote}{Note} %------------------------------------------------------------------- \begin{document} %------------------------------------------------------------------- \section*{categorical semantics} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{category_theory}{}\paragraph*{{Category theory}}\label{category_theory} [[!include category theory - contents]] \hypertarget{type_theory}{}\paragraph*{{Type theory}}\label{type_theory} [[!include type theory - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{of_dependent_type_theory}{Of dependent type theory}\dotfill \pageref*{of_dependent_type_theory} \linebreak \noindent\hyperlink{ContextsAndTypeJudgements}{Contexts and type judgements}\dotfill \pageref*{ContextsAndTypeJudgements} \linebreak \noindent\hyperlink{terms}{Terms}\dotfill \pageref*{terms} \linebreak \noindent\hyperlink{variables}{Variables}\dotfill \pageref*{variables} \linebreak \noindent\hyperlink{substitution}{Substitution}\dotfill \pageref*{substitution} \linebreak \noindent\hyperlink{of_homotopy_type_theory}{Of homotopy type theory}\dotfill \pageref*{of_homotopy_type_theory} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{related_pages}{Related pages}\dotfill \pageref*{related_pages} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} One may interpret mathematical \emph{[[logic]]} as being a formal language for talking about the collection of [[monomorphisms]] into a given [[object]] of a given [[category]]: the [[poset of subobjects]] of that object. More generally, one may interpret [[type theory]] and notably [[dependent type theory]] as being a formal language for talking about [[slice categories]], consisting of all morphisms into a given object. Conversely, starting with a given theory of logic or a given type theory, we say that it has a \emph{categorical semantics} if there is a category such that the given theory is that of its slice categories, if it is the \emph{[[internal logic]]} of that category. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} For the general idea, for the moment see at \emph{[[type theory]]} the section \emph{\href{type+theory#CategoricalSemantics}{An introduction for category theorists}} and see at \emph{[[relation between type theory and category theory]]}. \hypertarget{of_dependent_type_theory}{}\subsubsection*{{Of dependent type theory}}\label{of_dependent_type_theory} We discuss how to interpret judgements of [[dependent type theory]] in a given [[category]] $\mathcal{C}$ with [[finite limits]]. For more see \emph{[[categorical model of dependent types]]}. Write $cod : \mathcal{C}^I \to \mathcal{C}$ for its [[codomain fibration]], and write \begin{displaymath} \chi : \mathcal{C}^{op} \to Cat \end{displaymath} for the corresponding [[Grothendieck construction|classifying functor]], the self-[[indexed category|indexing]] \begin{displaymath} \chi : \Gamma \mapsto \mathcal{C}_{/\Gamma} \end{displaymath} that sends an [[object]] of $\mathcal{C}$ to the [[slice category]] over it, and sends a morphism $f : \Gamma \to \Gamma'$ to the [[pullback]]/[[base change]] functor \begin{displaymath} f^\ast : \mathcal{C}_{/\Gamma'} \to \mathcal{C}_{/\Gamma} \,. \end{displaymath} We give now rules for choices ``$[x y z]$'' that associate with every string ``$x y z$'' of symbols in [[type theory]] objects and morphisms in $\mathcal{C}$. A collection of such choices following these rules is \emph{an interpretation} / a choice of \emph{categorical semantics} of the type theory in the category $\mathcal{C}$. \hypertarget{ContextsAndTypeJudgements}{}\paragraph*{{Contexts and type judgements}}\label{ContextsAndTypeJudgements} \begin{enumerate}% \item The empty [[context]] $()$ in [[type theory]] is interpreted as the [[terminal object]] of $\mathcal{C}$ \begin{displaymath} [ () ] := * \,. \end{displaymath} \item If $\Gamma$ is a context which has already been given an interpretation $[\Gamma] \in Obj(\mathcal{C})$, then a judgement of the form \begin{displaymath} \Gamma \vdash A : Type \end{displaymath} is interpreted as an object in the slice over $\Gamma$ \begin{displaymath} [\Gamma \vdash A : Type] \in Obj(\mathcal{C}_{/\Gamma}) \,, \end{displaymath} hence as a choice of morphism \begin{displaymath} \itexarray{ [(\Gamma, x : A)] \\ \downarrow^{\mathrlap{[\Gamma \vdash A : Type]}} \\ [\Gamma] } \end{displaymath} in $\mathcal{C}$. \item If a judgement of the form $\Gamma \vdash A : Type$ has already found an interpretation, as above, then an extended [[context]] of the form $(\Gamma, x : A)$ is interpreted as the domain object $[(\Gamma, x : A)]$ of the above choice of morphism. \end{enumerate} \hypertarget{terms}{}\paragraph*{{Terms}}\label{terms} Assume for a context $\Gamma$ and a judgement $\Gamma \vdash A : Type$ we have already chosen an interpretation $[\Gamma, x : A] \stackrel{[\Gamma \vdash A : Type]}{\to} [\Gamma]$ as above. A judgement of the form $\Gamma \vdash a : A$ (a [[term]] of [[type]] $A$) is to be interpreted as a [[section]] of this morphism, equivalently as a morphism in $\mathcal{C}_{/\Gamma}$ \begin{displaymath} [\Gamma \vdash : a : A] : * \to [\Gamma, x : A] \end{displaymath} from the [[terminal object]] to $[\Gamma \vdash A : Type]$, which in $\mathcal{C}$ is a [[commuting diagram|commuting triangle]] \begin{displaymath} \itexarray{ [\Gamma] &&\stackrel{[(\Gamma \vdash a : A)]}{\to}&& [\Gamma, x : A] \\ & {}_\mathllap{[\Gamma]}\searrow && \swarrow_{\mathrlap{[\Gamma \vdash A : Type]}} \\ && [\Gamma] } \,. \end{displaymath} \hypertarget{variables}{}\paragraph*{{Variables}}\label{variables} For a term $\Gamma \vdash a : A$ the context $\Gamma$ is the collection of [[free variables]] in $a$. (\ldots{}) \hypertarget{substitution}{}\paragraph*{{Substitution}}\label{substitution} Assume that interpretations for judgements \begin{displaymath} \Gamma , x : A \vdash B(x) : Type \end{displaymath} and \begin{displaymath} \Gamma \vdash a : A \end{displaymath} have been given as above. Then the substitution judgement \begin{displaymath} \Gamma \vdash B[a/x] : Type \end{displaymath} is to be interpreted as follows. The interpretation of the first two terms corresponds to a diagram in $\mathcal{C}$ of the form \begin{displaymath} \itexarray{ &&&& [(\Gamma, x : A, y : B(x))] \\ &&&& \downarrow^{\mathrlap{[\Gamma, x : A \vdash B(x) : Type]}} \\ [\Gamma] &&\stackrel{[\Gamma \vdash a : A]}{\to}&& [(\Gamma, x : A)] \\ & {}_{id}\searrow && \swarrow_{\mathrlap{[\Gamma \vdash A : Type]}} \\ && [\Gamma] } \end{displaymath} The interpretation of the substitution statement is then [[generalized the|the]] [[pullback]] \begin{displaymath} [\Gamma \vdash B[a/x] : Type] := [\Gamma \vdash a : A]^* [\Gamma, x : A \vdash B(x) : Type] \,, \end{displaymath} hence the morphism in $\mathcal{C}$ that universally completes the above diagram as \begin{displaymath} \itexarray{ [(\Gamma, y : B[x/a])] &&\to&& [(\Gamma, x : A, y : B(x))] \\ {}^{\mathllap{[\Gamma \vdash B[a/x] : Type] }}\downarrow &&&& \downarrow^{\mathrlap{[\Gamma, x : A \vdash B(x) : Type]}} \\ [\Gamma] &&\stackrel{[\Gamma \vdash a : A]}{\to}&& [(\Gamma, x : A)] \\ & {}_{id}\searrow && \swarrow_{\mathrlap{[\Gamma \vdash A : Type]}} \\ && [\Gamma] } \end{displaymath} \hypertarget{of_homotopy_type_theory}{}\subsubsection*{{Of homotopy type theory}}\label{of_homotopy_type_theory} See \emph{[[categorical semantics of homotopy type theory]]}. \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \begin{itemize}% \item [[models in presheaf toposes]] \end{itemize} \hypertarget{related_pages}{}\subsection*{{Related pages}}\label{related_pages} \begin{itemize}% \item [[relation between type theory and category theory]] \item [[categorical model of dependent types]] \item [[Awodey's conjecture]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} A standard textbook reference for categorical semantics of logic is section D1.2 of \begin{itemize}% \item [[Peter Johnstone]], \emph{[[Sketches of an Elephant]]} \end{itemize} The categorical semantics of [[dependent type theory]] in [[locally cartesian closed categories]] is essentially due to \begin{itemize}% \item [[R. A. G. Seely]], \emph{Locally cartesian closed categories and type theory}, Math. Proc. Camb. Phil. Soc. (1984) 95 (\href{http://www.math.mcgill.ca/rags/LCCC/LCCC.pdf}{pdf}) \end{itemize} For more references on this see at \emph{[[relation between category theory and type theory]]}. Lecture notes on this include for instance. \begin{itemize}% \item [[Martin Hofmann]], \emph{Syntax and semantics of dependent types}, Semantics and Logics of Computation (P. Dybjer and A. M. Pitts, eds.), Publications of the Newton Institute, Cambridge University Press, Cambridge, (1997) pp. 79-130. (\href{http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.36.8985}{web}, ) \item Roy Crole, \emph{Categories for types} \end{itemize} See also section B.3 of \begin{itemize}% \item [[Michael Warren]], \emph{Homotopy Theoretic Aspects of Constructive Type Theory} (\href{http://www.andrew.cmu.edu/user/awodey/students/warren.pdf}{pdf}) \end{itemize} A comprehensive definition of semantics of [[homotopy type theory]] in [[type-theoretic model categories]] is in section 2 of \begin{itemize}% \item [[Michael Shulman]], \emph{Univalence for inverse diagrams and homotopy canonicity}, Mathematical Structures in Computer Science, Volume 25, Issue 5 ( \emph{From type theory and homotopy theory to Univalent Foundations of Mathematics} ) June 2015 (\href{https://arxiv.org/abs/1203.3253}{arXiv:1203.3253}, \href{https://doi.org/10.1017/S0960129514000565}{doi:/10.1017/S0960129514000565}) \end{itemize} \end{document}