canonical form


Type theory

natural deduction metalanguage, practical foundations

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


syntax object language


logiccategory theorytype theory

falseinitial objectempty type


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


implicationinternal homfunction type

negationinternal hom into initial objectfunction type into empty type

universal quantificationdependent productdependent product type


equivalencepath space objectidentity type

equivalence classquotientquotient type

inductioncolimitinductive type, W-type, M-type

higher inductionhigher colimithigher inductive type


setinternal 0-groupoidBishop set/setoid

universeobject classifiertype of types



proof netstring diagramquantum circuit


synthetic mathematicsdomain specific embedded programming language


homotopy levels


Constructivism, Realizability, Computability



Systems of formal logic, such as type theory, try to transform expressions into a canonical form which then serves as the end result of the given computation or deduction. A formal system is said to enjoy canonicity if every expression reduces to canonical form.

More precisely, in type theory, a term belonging to some type is said to be of canonical form if it is explicitly built up using the constructors of that type. A canonical form is in particular a normal form (one not admitting any reductions), but the converse need not hold.

For example, the terms of type \mathbb{N} (a natural numbers type) of canonical form are the numerals

S(S(S((0)))). S(S(S(\cdots (0)\cdots ))).

A type theory is said to enjoy canonicity if every term computes to a canonical form. This is held to be an important meta-theoretic property of type theory, especially considered as a programming language or as a computational foundation for mathematics.

Canonicity vs axioms

Adding axioms to type theory, such as the principle of excluded middle or the usual version of the univalence axiom, can destroy canonicity. The axioms result in “stuck terms” which are not of canonical form, yet neither can they be “computed” any further.

For instance, if we assume the law of excluded middle, then we can build a term case(LEM(Goldbach),0,1):case(LEM(Goldbach),0,1) \colon \mathbb{N} which is 00 or 11 according as the Goldbach conjecture is true or false. Clearly this term doesn’t “compute”, but neither is it of canonical form (a numeral).

Similarly, using the univalence axiom, we can obtain a term p:(2=2)p : (\mathbf{2}=\mathbf{2}) corresponding to the automorphism of the type 2\mathbf{2} which switches 0 20_\mathbf{2} and 1 21_\mathbf{2}. Then the term transport(p,0 2)transport(p,0_\mathbf{2}) also has type 2\mathbf{2}, but doesn’t “compute” because the computer gets “stuck” on the univalence term.

It is conjectured that univalence, unlike excluded middle, can be given a “computational” interpretation while preserving canonicity. Some partial progress towards this can be found here.


Discussion of canonicity in homotopy type theory with univalence is discussed in

Last revised on June 11, 2018 at 16:53:33. See the history of this page for a list of all contributions to it.