propositional resizing




Type theory

natural deduction metalanguage, practical foundations

  1. type formation rule
  2. term introduction rule
  3. term elimination rule
  4. computation rule

type theory (dependent, intensional, observational type theory, homotopy type theory)

syntax object language

computational trinitarianism =
propositions as types +programs as proofs +relation type theory/category theory

logiccategory theorytype theory
trueterminal object/(-2)-truncated objecth-level 0-type/unit type
falseinitial objectempty type
proposition(-1)-truncated objecth-proposition, mere proposition
proofgeneralized elementprogram
cut rulecomposition of classifying morphisms / pullback of display mapssubstitution
cut elimination for implicationcounit for hom-tensor adjunctionbeta reduction
introduction rule for implicationunit for hom-tensor adjunctioneta conversion
logical conjunctionproductproduct type
disjunctioncoproduct ((-1)-truncation of)sum type (bracket type of)
implicationinternal homfunction type
negationinternal hom into initial objectfunction type into empty type
universal quantificationdependent productdependent product type
existential quantificationdependent sum ((-1)-truncation of)dependent sum type (bracket type of)
equivalencepath space objectidentity type/path type
equivalence classquotientquotient type
inductioncolimitinductive type, W-type, M-type
higher inductionhigher colimithigher inductive type
-0-truncated higher colimitquotient inductive type
coinductionlimitcoinductive type
completely presented setdiscrete object/0-truncated objecth-level 2-type/preset/h-set
setinternal 0-groupoidBishop set/setoid
universeobject classifiertype of types
modalityclosure operator, (idempotent) monadmodal type theory, monad (in computer science)
linear logic(symmetric, closed) monoidal categorylinear type theory/quantum computation
proof netstring diagramquantum circuit
(absence of) contraction rule(absence of) diagonalno-cloning theorem
synthetic mathematicsdomain specific embedded programming language

homotopy levels




In homotopy type theory, given a universe, 𝒰\mathcal{U}, we may define a type of mere propositions within that universe, A:𝒰isProp(A)\sum_{A: \mathcal{U}} isProp(A). For universes, 𝒰 i\mathcal{U}_i and 𝒰 i+1\mathcal{U}_{i+1}, we have that A:𝒰 iA: \mathcal{U}_i entails A:𝒰 i+1A:\mathcal{U}_{i+1}. We thus have a natural map

Prop 𝒰 iProp 𝒰 i+1. Prop_{\mathcal{U}_i} \to Prop_{\mathcal{U}_{i+1}}.

The axiom of proposition resizing states that this map is an equivalence. Any mere proposition in the larger universe may be resized to an equivalent mere proposition in the smaller universe.

The axiom is a form of impredicativity for mere propositions, and by avoiding its use, the type theory is said to remain predicative.


Created on September 24, 2018 at 06:45:28. See the history of this page for a list of all contributions to it.