Intuitionistic Type Theory - Extensional Versus Intensional

Extensional Versus Intensional

A fundamental distinction is extensional vs intensional Type Theory. In extensional Type Theory definitional (i.e., computational) equality is not distinguished from propositional equality, which requires proof. As a consequence type checking becomes undecidable in extensional type theory. This is because relying on computational equality means that the equality depends on computations that could be Turing complete in general and thus the equality itself is undecidable due to the halting problem. Some type theories enforce the restriction that all computations be decidable so that definitional equality may be used.

In contrast in intensional Type Theory type checking is decidable, but the representation of standard mathematical concepts is somewhat complex, since extensional reasoning requires using setoids or similar constructions. It is a subject of current discussion whether this tradeoff is unavoidable and whether the lack of extensional principles in intensional Type Theory is a feature or a bug.