146 research outputs found
Constructivist and Structuralist Foundations: Bishop's and Lawvere's Theories of Sets
Bishop's informal set theory is briefly discussed and compared to Lawvere's
Elementary Theory of the Category of Sets (ETCS). We then present a
constructive and predicative version of ETCS, whose standard model is based on
the constructive type theory of Martin-L\"of. The theory, CETCS, provides a
structuralist foundation for constructive mathematics in the style of Bishop.Comment: 28 page
Constructing categories and setoids of setoids in type theory
In this paper we consider the problem of building rich categories of setoids,
in standard intensional Martin-L\"of type theory (MLTT), and in particular how
to handle the problem of equality on objects in this context. Any
(proof-irrelevant) family F of setoids over a setoid A gives rise to a category
C(A, F) of setoids with objects A. We may regard the family F as a setoid of
setoids, and a crucial issue in this article is to construct rich or large
enough such families. Depending on closure conditions of F, the category C(A,
F) has corresponding categorical constructions. We exemplify this with finite
limits. A very large family F may be obtained from Aczel's model construction
of CZF in type theory. It is proved that the category so obtained is isomorphic
to the internal category of sets in this model. Set theory can thus establish
(categorical) properties of C(A, F) which may be used in type theory. We also
show that Aczel's model construction may be extended to include the elements of
any setoid as atoms or urelements. As a byproduct we obtain a natural extension
of CZF, adding atoms. This extension, CZFU, is validated by the extended model.
The main theorems of the paper have been checked in the proof assistant Coq
which is based on MLTT. A possible application of this development is to
integrate set-theoretic and type-theoretic reasoning in proof assistants.Comment: 14 page
Internalising modified realisability in constructive type theory
A modified realisability interpretation of infinitary logic is formalised and
proved sound in constructive type theory (CTT). The logic considered subsumes
first order logic. The interpretation makes it possible to extract programs
with simplified types and to incorporate and reason about them in CTT.Comment: 7 page
Coequalisers of formal topology
We give a predicative construction of quotients of formal topologies. Along with earlier results on the match up between of continuous functions on real numbers (in the sense of Bishop\u27s constructive mathematics) and approximable mappings on the formal space of reals, we argue that formal topology gives an adequate foundation for constructive algebraic topology, also in the predicative sense. Predicativity is of essence when formalising the subject in logical frameworks based on Martin-Löf type theories
Categories with families and first-order logic with dependent sorts
First-order logic with dependent sorts, such as Makkai's first-order logic
with dependent sorts (FOLDS), or Aczel's and Belo's dependently typed
(intuitionistic) first-order logic (DFOL), may be regarded as logic enriched
dependent type theories. Categories with families (cwfs) is an established
semantical structure for dependent type theories, such as Martin-L\"of type
theory. We introduce in this article a notion of hyperdoctrine over a cwf, and
show how FOLDS and DFOL fit in this semantical framework. A soundness and
completeness theorem is proved for DFOL. The semantics is functorial in the
sense of Lawvere, and uses a dependent version of the Lindenbaum-Tarski algebra
for a DFOL theory. Agreement with standard first-order semantics is
established. Applications of DFOL to constructive mathematics and categorical
foundations are given. A key feature is a local propositions-as-types
principle.Comment: 83 page
On Equality of Objects in Categories in Constructive Type Theory
In this note we remark on the problem of equality of objects in categories
formalized in Martin-L\"of's constructive type theory. A standard notion of
category in this system is E-category, where no such equality is specified. The
main observation here is that there is no general extension of E-categories to
categories with equality on objects, unless the principle Uniqueness of
Identity Proofs (UIP) holds. We also introduce the notion of an H-category, a
variant of category with equality on objects, which makes it easy to compare to
the notion of univalent category proposed for Univalent Type Theory by Ahrens,
Kapulkin and Shulman.Comment: 7 page
Computable decision making on the reals and other spaces via partiality and nondeterminism
Though many safety-critical software systems use floating point to represent
real-world input and output, programmers usually have idealized versions in
mind that compute with real numbers. Significant deviations from the ideal can
cause errors and jeopardize safety. Some programming systems implement exact
real arithmetic, which resolves this matter but complicates others, such as
decision making. In these systems, it is impossible to compute (total and
deterministic) discrete decisions based on connected spaces such as
. We present programming-language semantics based on constructive
topology with variants allowing nondeterminism and/or partiality. Either
nondeterminism or partiality suffices to allow computable decision making on
connected spaces such as . We then introduce pattern matching on
spaces, a language construct for creating programs on spaces, generalizing
pattern matching in functional programming, where patterns need not represent
decidable predicates and also may overlap or be inexhaustive, giving rise to
nondeterminism or partiality, respectively. Nondeterminism and/or partiality
also yield formal logics for constructing approximate decision procedures. We
implemented these constructs in the Marshall language for exact real
arithmetic.Comment: This is an extended version of a paper due to appear in the
proceedings of the ACM/IEEE Symposium on Logic in Computer Science (LICS) in
July 201
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