dependent type

**natural deduction** metalanguage, practical foundations

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

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

logic | category theory | type theory |
---|---|---|

true | terminal object/(-2)-truncated object | h-level 0-type/unit 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

</table>

In type theory, a *dependent type* or *type in context* is a family or bundle of types which vary over the elements (terms) of some other type. It can be regarded as a formalization of the notion of “indexed family,” providing a structural account of families (in contrast to the material approach which requires sets to be able to contain other sets as elements).

Type theory with the notion of dependent types is called *dependent type theory*.

In the categorical semantics of type theory, a dependent type

$x:A \; \vdash \;B(x):Type$

is represented by a particular morphism $p\colon B\to A$, the intended meaning being that each type $B(x)$ is the fiber of $p$ over $x\in A$. The morphism in a category that may represent dependent types in this way are sometimes called display morphisms (especially when not every morphism is a display morphism).

When the theory of a category is phrased in dependent type theory then there is one type “$obj$” of objects and a type $hom$ of morphisms, which is dependent on two terms of type $obj$, so that for any $x,y:obj$ there is a type $hom(x,y)$ of arrows from $x$ to $y$. This dependency is usually written as $x,y:obj \vdash hom(x,y):Type$. In some theories, it makes sense to say that the type of “$hom$” itself is $obj, obj\to Type$ (usually understood as $obj \to (obj \to Type)$ or $(\obj \times \obj) \to Type$), i.e. a function from pairs of elements of $A$ to the universe? $Type$ of types.

- Blog post: In praise of dependent types

In Coq:

- Yves Bertot,
*Introduction to dependent types in Coq*(pdf)

Last revised on October 25, 2017 at 14:54:42. See the history of this page for a list of all contributions to it.