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Many-sorted logic can reflect formally our intention not to handle the universe as a homogeneous collection of objects, but to partition it in a way that is similar to types in typeful programming. Both functional and assertive "parts of speech" in the language of the logic reflect this typeful partitioning of the universe, even on the syntax level: substitution and argument passing can be done only accordingly, respecting the "sorts".

There are more ways to formalize the intention mentioned above; a many-sorted logic is any package of information which fulfills it. In most cases, the following are given:

The domain of discourse of any structure of that signature is then fragmented into disjoint subsets, one for every sort.

Contents

ExampleEdit

When reasoning about biological organisms, it is useful to distinguish two sorts:   and  . While a function   makes sense, a similar function   usually does not. Many-sorted logic allows one to have terms like  , but to discard terms like   as syntactically ill-formed.

AlgebraizationEdit

The algebraization of many-sorted logic is explained in an article by Caleiro and Gonçalves,[1] which generalizes abstract algebraic logic to the many-sorted case, but can also be used as introductory material.

Order-sorted logicEdit

While many-sorted logic requires two distinct sorts to have disjoint universe sets, order-sorted logic allows one sort   to be declared a subsort of another sort  , usually by writing   or similar syntax. In the above example, it is desirable to declare

 ,
 ,
 ,
 ,
 ,
 ,

and so on.

Wherever a term of some sort   is required, a term of any subsort of   may be supplied instead. For example, assuming a function declaration  , and a constant declaration  , the term   is perfectly valid and has the sort  . In order to supply the information that the mother of a dog is a dog in turn, another declaration   may be issued; this is called function overloading, similar to overloading in programming languages.

Order-sorted logic can be translated into unsorted logic, using a unary predicate   for each sort  , and an axiom   for each subsort declaration  . The reverse approach was successful in automated theorem proving: in 1985, Christoph Walther could solve a then benchmark problem by translating it into order-sorted logic, thereby boiling it down an order of magnitude, as many unary predicates turned into sorts.[2]

In order to incorporate order-sorted logic into a clause-based automated theorem prover, a corresponding order-sorted unification algorithm is necessary, which requires for any two declared sorts   their intersection   to be declared, too: if   and   are variables of sort   and  , respectively, the equation   has the solution  , where  .

Smolka generalized order-sorted logic to allow for parametric polymorphism. [3][4] In his framework, subsort declarations are propagated to complex type expressions. As a programming example, a parametric sort   may be declared (with   being a type parameter as in a C++ template), and from a subsort declaration   the relation   is automatically inferred, meaning that each list of integers is also a list of floats.

Schmidt-Schauß generalized order-sorted logic to allow for term declarations.[5] As an example, assuming subsort declarations   and  , a term declaration like   allows to declare a property of integer addition that could not be expressed by ordinary overloading.

See alsoEdit

ReferencesEdit

  1. ^ Carlos Caleiro, Ricardo Gonçalves (2006). "On the algebraization of many-sorted logics". Proc. 18th int. conf. on Recent trends in algebraic development techniques (WADT) (PDF). Springer. pp. 21–36. ISBN 978-3-540-71997-7.
  2. ^ Walther, Christoph (1985). "A Mechanical Solution of Schubert's Steamroller by Many-Sorted Resolution" (PDF). Artif. Intell. 26 (2): 217–224. doi:10.1016/0004-3702(85)90029-3.
  3. ^ Smolka, Gert (Nov 1988). "Logic Programming with Polymorphically Order-Sorted Types". Int. Workshop Algebraic and Logic Programming. LNCS. 343. Springer. pp. 53–70.
  4. ^ Smolka, Gert (May 1989), Logic Programming over Polymorphically Order-Sorted Types, Univ. Kaiserslautern, Germany
  5. ^ Schmidt-Schauß, Manfred (Apr 1988). Computational Aspects of an Order-Sorted Logic with Term Declarations. LNAI. 395. Springer.

Early papers on many-sorted logic include:

External linksEdit