In mathematical logic, System U and System U are pure type systems, i.e. special forms of a typed lambda calculus with an arbitrary number of sorts, axioms and rules (or dependencies between the sorts). System U was proved inconsistent by Jean-Yves Girard in 1972[1] (and the question of consistency of System U was formulated). This result led to the realization that Martin-Löf's original 1971 type theory was inconsistent as it allowed the same "Type in Type" behaviour that Girard's paradox exploits.

Formal definition

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System U is defined[2]: 352  as a pure type system with

  • three sorts  ;
  • two axioms  ; and
  • five rules  .

System U is defined the same with the exception of the   rule.

The sorts   and   are conventionally called “Type” and “Kind”, respectively; the sort   doesn't have a specific name. The two axioms describe the containment of types in kinds ( ) and kinds in   ( ). Intuitively, the sorts describe a hierarchy in the nature of the terms.

  1. All values have a type, such as a base type (e.g.   is read as “b is a boolean”) or a (dependent) function type (e.g.   is read as “f is a function from natural numbers to booleans”).
  2.   is the sort of all such types (  is read as “t is a type”). From   we can build more terms, such as   which is the kind of unary type-level operators (e.g.   is read as “List is a function from types to types”, that is, a polymorphic type). The rules restrict how we can form new kinds.
  3.   is the sort of all such kinds (  is read as “k is a kind”). Similarly we can build related terms, according to what the rules allow.
  4.   is the sort of all such terms.

The rules govern the dependencies between the sorts:   says that values may depend on values (functions),   allows values to depend on types (polymorphism),   allows types to depend on types (type operators), and so on.

Girard's paradox

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The definitions of System U and U allow the assignment of polymorphic kinds to generic constructors in analogy to polymorphic types of terms in classical polymorphic lambda calculi, such as System F. An example of such a generic constructor might be[2]: 353  (where k denotes a kind variable)

 .

This mechanism is sufficient to construct a term with the type   (equivalent to the type  ), which implies that every type is inhabited. By the Curry–Howard correspondence, this is equivalent to all logical propositions being provable, which makes the system inconsistent.

Girard's paradox is the type-theoretic analogue of Russell's paradox in set theory.

References

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  1. ^ Girard, Jean-Yves (1972). "Interprétation fonctionnelle et Élimination des coupures de l'arithmétique d'ordre supérieur" (PDF).
  2. ^ a b Sørensen, Morten Heine; Urzyczyn, Paweł (2006). "Pure type systems and the lambda cube". Lectures on the Curry–Howard isomorphism. Elsevier. doi:10.1016/S0049-237X(06)80015-7. ISBN 0-444-52077-5.

Further reading

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