Many-sorted logic explained

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

• a set of sorts, S
• an appropriate generalization of the notion of signature to be able to handle the additional information that comes with the sorts.

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

Example

When reasoning about biological organisms, it is useful to distinguish two sorts:

plant and animal . While a function mother \colon animal \to animal makes sense, a similar function mother \colon plant \to plant usually does not. Many-sorted logic allows one to have terms like mother ( lassie ) , but to discard terms like mother ( my\ _{favorite\} _{oak} ) as syntactically ill-formed.

Algebraization

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 logic

While many-sorted logic requires two distinct sorts to have disjoint universe sets, order-sorted logic allows one sort

s _{1} to be declared a subsort of another sort s _{2} , usually by writing s _{1} \subseteq s _{2} or similar syntax. In the above biology example, it is desirable to declare dog \subseteq carnivore ,dog \subseteq mammal ,carnivore \subseteq animal ,mammal \subseteq animal , animal \subseteq organism , plant \subseteq organism , and so on; cf. picture.

Wherever a term of some sort

s is required, a term of any subsort of s may be supplied instead (Liskov substitution principle). For example, assuming a function declaration mother : animal \longrightarrow animal , and a constant declaration lassie : dog , the term mother ( lassie ) is perfectly valid and has the sort animal . In order to supply the information that the mother of a dog is a dog in turn, another declaration mother : dog \longrightarrow dog 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

p _{i(x)} for each sort s _{i} , and an axiom \forall x (p _{i(x)} → p _{j(x))} for each subsort declaration s _{i} \subseteq s _{j} . 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

s _{1,} s _{2} their intersection s _{1} \cap s _{2} to be declared, too: if x _{1} and x _{2} are variables of sort s _{1} and s _{2} , respectively, the equation x _{1} \stackrel{?}{ = }x _{2} has the solution \{ x _{1} = x, x _{2} = x\} , where x: s _{1} \cap s _{2} .

Smolka generalized order-sorted logic to allow for parametric polymorphism.^[3] In his framework, subsort declarations are propagated to complex type expressions.As a programming example, a parametric sort

list (X) may be declared (with X being a type parameter as in a C++ template), and from a subsort declaration int \subseteq float the relation list ( int ) \subseteq list ( float ) 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.^[4] As an example, assuming subsort declarations

even \subseteq int and odd \subseteq int , a term declaration like \forall i: int . (i + i): even allows to declare a property of integer addition that could not be expressed by ordinary overloading.

See also

• Categorical logic
• First-order_logic#Many-sorted_logic

References

Early papers on many-sorted logic include:

• Wang, Hao. Logic of many-sorted theories. Journal of Symbolic Logic. 1952. 17. 2. 105–116. 10.2307/2266241. 2266241., collected in the author's Computation, Logic, Philosophy. A Collection of Essays, Beijing: Science Press; Dordrecht: Kluwer Academic, 1990.
• Gilmore, P.C.. An addition to "Logic of many-sorted theories". Compositio Mathematica. 1958. 13. 277–281.
• A. Oberschelp. Untersuchungen zur mehrsortigen Quantorenlogik. Mathematische Annalen. 1962. 145. 4. 297–333. 10.1007/bf01396685. 123363080. 2013-09-11. https://web.archive.org/web/20150220005037/http://gdz.sub.uni-goettingen.de/index.php?id=11&L=4&PPN=GDZPPN002289989&L=1. 2015-02-20. dead.
• F. Jeffry Pelletier. Sortal Quantification and Restricted Quantification. Philosophical Studies. 1972. 23. 6. 400–404. 10.1007/bf00355532. 170303654.

External links

• "Many-sorted Logic", the first chapter in Lecture Notes on Decision Procedures by Calogero G. Zarba

Notes and References

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