nLab
intensional type theory

Idea

Intensional type theory is the flavor of type theory in which identity types are not necessarily propositions (that is, (-1)-truncated). Martin-Löf’s original definition of identity types, and the equivalent formulation as an inductive type, are by default intensional; one has to impose extra axioms or rules in order to get extensional type theory (in which identity types are propositions).

In particular, homotopy type theory is intensional, because identity types represent path objects.

Note that some type theorists use “intensional type theory” to refer to type theory which fails to satisfy function extensionality. This is in general an orthogonal requirement to how we are using the term here.

Properties

Decidability

Only the intensional but not the extensional Martin-Löf type theory is decidable. (Martin-Löf, Hofmann).

Examples

• CiC?

Related concepts

• homotopy type theory
• extensional type theory, observational type theory
• function extensionality

References

• Per Martin-Löf, An intuitionistic theory of types: predicative part, Logic Colloquium ‘73 (Amsterdam) (H. E. Rose and J. C. Shepherdson, eds.), North-Holland, 1975, pp. 73-118.

• Martin Hofmann, Extensional concepts in intensional type theory, Ph.D. thesis, University of Edinburgh, (1995) (web)