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Point-free semantics of dependent type theories

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The developments we did in our stay at the University of Canterbury, kindly hosted by Douglas Bridges and Hannes Diener.

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Point-free semantics of dependent type theories

  1. 1. Point-free semantics of dependent type theories M Benini, R Bonacina Università degli Studi dell’Insubria University of Canterbury, December 4th, 2017
  2. 2. Why Also known as constructive type theory, or Martin-Löf type theory, dependent type theory has recently seen a huge raise of interest because it is the basis for homotopy type theory. Semantics for dependent type theories are known: they are variations on the semantics of typed λ-calculi. Usually, they are complex: they are either based on the advanced theory of orders (specialised domains), or on category theory; in the case of categorical models, they use non-elementary constructions (fibrations, higher-order cells, . . . ); homotopy type theory has an intended semantics based on ∞-groupoids; the only categorical semantics (Seely) which does not use those advanced constructions, contains a problem (i.e., it does not work). Locally Cartesian closed categories are not enough to properly model dependent type theory (Hoffman, Dybjer). ( 2 of 17 )
  3. 3. Why Why are complex, higher-order models needed? Are they, really? These were the initial questions that have been addressed in Roberta’s master thesis. The answer was that there is no need for such complex constructions: a categorical model, using no higher-order constructions, suffices to provide a sound and complete explanation to dependent type theory. However, the result was not completely satisfactory because the notion of inductive theory was sketched but not precisely defined in all the details; some passages in the soundness and completeness proofs were reasonable but not formal; in the overall, there was the feeling that the result had to be polished to reach its maximal generality. ( 3 of 17 )
  4. 4. What So, the visit here, at the University of Canterbury had the purpose to precisely define the syntactic notion of inductive theory, and pave the way toward its extension to higher-inductive types, as defined in homotopy type theory; polish the semantics and fix all the passages in the soundness and completeness proofs. In short, we did it! In the following, I am going to give a glance to the semantics. ( 4 of 17 )
  5. 5. Point-free semantics Consider the following inductive types: the 0 type, which is characterised as the type having no terms; the N− , which is characterised by the rule having the inductive step n : N− → succ(n) : N− but not the basic step 0 : N− . Usually, these two types are considered equivalent because they have no terms belonging to them: the minimal fix point of their constructing rules is the same. However, they can be distinguished: if x : 0 and y : N− stand for some objects in the types 0 and N− , it is evident that no object other than x is forced to be in 0, while succ(y), succ(succ(y)), ... are all in N− . We want a semantics in which the meaning of a type depends on the context in which it is defined, so to be able to distinguish 0 from N− . ( 5 of 17 )
  6. 6. Point-free semantics The semantics is then point-free: types and their terms do not identify entities (“points”) in some universe. They explain how judgements are kept together by the logical inferences. And they do so by taking local values, which change under different assumptions. So N− is not equivalent to the 0 type: they are the same thing in the empty context, while they differ in a context in which we assume both types contain at least one term. ( 6 of 17 )
  7. 7. The big picture The semantics is based on category theory. An inductive theory, i.e., a series of inductive types defined in the standard dependent type theory, has models in the class of ML-categories. These categories allow to interpret dependent type theory and inductive types in a natural way. A category which makes valid all the judgements of an inductive theory is a model, and it has holds that every inductive theory has a model (Soundness); every judgement which is valid in any model of an inductive theory is derivable (Completeness); for every inductive theory there is a model which is contained in every other model (Classification). ( 7 of 17 )
  8. 8. The big picture Mctx Γ MΓ ∆ M∆ • M• Mctx is a partial order with minimum • in which all paths are finite; each MΓ is a preorder with Γ as its minimum such that each pair of elements has a least upper bound; Mctx and all the MΓ form the ML-category (for Martin-Löf). ( 8 of 17 )
  9. 9. Inside pyramids Each MΓ has the following structure: Γ π π π π a ∈ a ∈ b ∈ b ∈A ∈ B ∈Ui ∈ Ui+1
  10. 10. Inside pyramids Each MΓ has the following structure: Γ π π π π a ∈ a ∈ b ∈ b ∈A ∈ B ∈Ui ∈ Ui+1 context proper terms proper types universes ( 9 of 17 )
  11. 11. Interpretation x : A,y : B,z : C ctx = x : A y : B z : C •ctx x : Actx x : A,y : Bctx x : A,y : B,z : C ctx in Mctx Γ a : A = Γ a A π ∈ in MΓ Γ a ≡ b : A = Γ a b A π ∈ π ∈ i in MΓ ( 10 of 17 )
  12. 12. Variables Variables have more than one role in dependent type theory: they are hypotheses in the context; they are terms in the language; they are the only entities which may be substituted. The first role is captured by interpreting contexts as objects in Mctx and their way to be written down as path of irreducible arrows in the same category. The second role requires that there is an object in MΓ deputed to interpret x. The third role imposes a deeper structure on the ‘pyramids’ over Mctx. ( 11 of 17 )
  13. 13. Variables Γ x:A A Ui π ∈ A Ui π ∈ x π ∈ an irreducible arrow in Mctx induces a new term x in the pyramid over the codomain;
  14. 14. Variables Γ x:A A Ui π ∈ A Ui π ∈ x π ∈ a a an irreducible arrow in Mctx induces a new term x in the pyramid over the codomain; a term a of type A in Γ is so also in the extended context;
  15. 15. Variables Γ x:A A Ui π ∈ A Ui π ∈ x π ∈ a a ∼= ∼= an irreducible arrow in Mctx induces a new term x in the pyramid over the codomain; a term a of type A in Γ is so also in the extended context; substituting a in x, i.e., making them isomorphic and closing for type generation, forces the pyramids to be equivalent. ( 12 of 17 )
  16. 16. Inductive types An inductive type is the minimal collection of terms closed under the interpretation of its introduction rules. Semantically, this means an inductive type is the colimit of the diagram composed by the terms-in-context which are the result of the closure of the transformation associated to the introduction rules. For example, the dependent sum has the following introduction rule: Γ b : B[a/x] Γ a : A Γ,x : A B : Ui Γ A : Ui Σ−I Γ (a,b) : Σx : A.B The associated semantic transformation θ maps each pair of objects α and β in MΓ such that there are ∈: α → A Γ and ∈: β → B[a/x] Γ in an object θ(α,β) of MΓ. ( 13 of 17 )
  17. 17. Inductive types In general, the guiding principle is that the formation rule is used to identify the space S of terms which is transformed to construct the terms in the new inductive type. So, θ: S → MΓ. The space S forms a category, and θ becomes a functor. As such, θ has to be free, in one of the way to interpret this adjective in category theory, to ensure that the construction is inductive. The idea is that θ must be associated with T, the inductive theory as a whole, not to each specific type. Then, MΓ is the minimal fix point of the θ transformation in a category having enough terms to interpret the context Γ in which every variable is a distinct term. ( 14 of 17 )
  18. 18. Inductive types Difficult to express, but mathematically straightforward, this idea of inductive theory allows to capture at once recursive types, mutual recursive types, and even more esoteric beasts. The formal framework had also a pleasant consequence: namely, all the canonical types are inductive. So, the dependent sum Σ, the dependent product Π, the coproduct +, the empty type 0, the unit type 1, and equality in a type =A can be treated in the very same way as natural numbers. In fact, the non-inductive part of dependent type theory is reduced to the structural rules (context formation, variables, judgemental equality) and the rules about universes, which are structural in a sense, because they are used to distinguish types. ( 15 of 17 )
  19. 19. The state of the art We are in the process of writing down all of this. In this moment, we have the definition of ML-category, a few of its properties, and the definition of interpretation written down with all the details. Inductive types are completely developed: however, only the syntactic side has been polished. The semantics of them needs to be written and checked once again. We have the definition of syntactic category, the classification, soundness and completeness theorem to polish: they have been developed in all the details, but not yet written down in the proper mathematical style. We have a rough sketch of how everything should work on higher-inductive types, but this is far from being a result, even with a very optimistic view. . . not yet, at least! ( 16 of 17 )
  20. 20. The end Questions? ( 17 of 17 )

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