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
equivalence classquotientquotient type
inductioncolimitinductive type, W-type, M-type
higher inductionhigher colimithigher inductive 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, (idemponent) 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 formal logic, a metalanguage is a language (formal or informal) in which the symbols and rules for manipulating another (formal) language – the object language – are themselves formulated. That is, the metalanguage is the language used when talking about the object language.

For instance the symbol ϕ\phi may denote a proposition in a deductive system, but the statement “ϕ\phi can be proven” is not itself a proposition in the deductive system, but a statement in the metalanguage, often denoted by a sequent of the form

ϕtrue \vdash \phi \, true

and then called a judgement instead (in type theory one might also omit the “truetrue”, see at propositions-as-types for details on this). Or, more generally, if the truth of ϕ\phi depends on the truth of some other proposition ψ\psi then

ψtrueϕtrue \psi \, true \vdash \phi \, true

is the hypothetical judgement in the metalanguage that there is a proof of ϕ\phi in the context in which ψ\psi is assumed.

In contrast, the implication expression (ψϕ)(\psi \to \phi) may denote another proposition in the object language, a conditional statement. A deduction theorem connects the two, in that it says that if the judgement

ψtrueϕtrue \psi \, true \vdash \phi \, true

holds in the metalanguage, then the judgement

(ψϕ)true \vdash (\psi \to \phi) \, true

may be deduced; the converse is the rule of modus ponens. (Actually, both the deduction theorem and modus ponens are slightly more general, being relativized to an arbitrary context, but we needn't get into that here.)



Detailed discussion of the difference between judgements in the metalanguage and propositions in the object language is in the foundational lectures

  • Per Martin-Löf, On the meaning of logical constants and the justifications of the logical laws, leture series in Siena (1983) (web)

Last revised on February 24, 2014 at 12:32:06. See the history of this page for a list of all contributions to it.