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This is a book about isomorphisms 0/ types, arecent difficult research topic in type theory that turned out to be able to have valuable practical applications both for programming language design and far more human centered information retrieval in software libraries. By means of a deep study of the syntax of the now widely known typed A-ca1culus, it is possible to identify some simple equations between types that on one hand allow to improve the design of the ML language, and on the other hand provide the basis for building radically new information retrieval systems for functional software libraries. We present in this book both the theoretical aspects of these researches and a fully functional implementation of some of their applications in such a way to provide interesting material both for the theoretician looking for proofs and for the practitioner interested in implementation details. In order to make it possible for these different types of readers to use this book effectively, some special signs are used to designate material that is particularly technical or applied or that represents a digression. When the symbol appears at the beginning of a section or a subsection, it warns that the material contained in such section is particularly technical with respect to the general level of the chapter or section where it is located. This material is generally reserved to theoreticians and does not need to be read by the casual reader.
1 Introduction.- 1.1 What is a type?.- 1.2 Types in mathematical logic.- 1.3 Types for programming.- 1.3.1 Imperative languages.- 1.3.2 Limits of static type-checking.- 1.3.3 Functional languages.- 1.3.4 The lambda calculus.- 1.4 Exploring typed ?-calculi.- 1.4.1 Church-style types.- 1.4.2 Curry-style types.- 1.4.3 Explicit polymorphic types.- 1.4.4 Implicit polymorphic types.- 1.5 The typed ? -calculi used in this work.- 1.5.1 The calculus ?2????.- 1.5.2 General notations for terms and substitutions.- 1.6 The Curry-Howard Isomorphism.- 1.7 Using types to classify and retrieve software.- 1.7.1 Object-oriented languages.- 1.7.2 Functional languages.- 1.8 When are two types equal?.- 1.8.1 Isomorphic types.- 1.8.2 Isomorphisms in category theory.- 1.8.3 Digression: Tarski’s High School Algebra Problem.- 1.8.4 Isomorphisms in logic.- 1.9 Isomorphisms and the lambda calculus.- 1.9.1 Isomorphisms and invertibility.- 1.9.2 The theories of isomorphisms for typed ?-calculi.- 1.9.3 Soundness.- 2 Confluence Results.- 2.1 Introduction.- 2.2 Extensionality.- 2.2.1 Survey.- 2.3 Overview.- 2.3.1 Weakly confluent reduction.- 2.3.2 Investigating strong normalization.- 2.3.3 A general criterion for confluence.- 2.4 Confluence.- 2.5 Weak normalization.- 2.6 Decidability and conservative extension results.- 2.7 Other related works.- 3 Strong normalization results.- 3.1 Normalization without ?2 on gentop n.f.’s.- 3.1.1 Reducibility with parameters.- 3.2 Normalization without ?top and SPtop.- 4 First-Order Isomorphic Types.- 4.1 Rewriting types.- 4.2 From ?1???? to the classical ?1??.- 4.3 Using finite hereditary permutations.- 4.4 The complete theories of ?1??? and ?1???.- 5 Second-Order Isomorphic Types.- 5.1 Towards completeness.- 5.1.1 Outline of the section.- 5.1.2 Reduction to a subclass of types.- 5.1.3 Reduction to a subclass of terms.- 5.2 Characterizing canonical terms.- 5.2.1 Outline of the section.- 5.2.2 Projection of invertibility over coordinates.- 5.2.3 Reduction of coordinates to ?2??.- 5.2.4 Syntactic characterization of canonical bijections.- 5.3 Completeness for isomorphisms.- 5.3.1 Uniform isomorphisms.- 5.4 Decidability of the equational theory.- 5.5 The complete theories of ?2??? and ?2???.- 5.6 Conclusions.- A Properties of n-tuples.- B Technical lemmas.- C Miscellanea.- 6 Isomorphisms for ML.- 6.1 Introduction.- 6.2 Isomorphisms of types in ML-style languages.- 6.2.1 A formal setting for valid isomorphisms in ML-like languages.- 6.3 Completeness and conservativity results.- 6.3.1 Completeness.- 6.3.2 Relating Th2xT and ThML.- 6.4 Deciding ML isomorphism.- 6.4.1 An improved decision procedure.- 6.4.2 Equality as unification with variable renamings.- 6.4.3 Dynamic programming.- 6.4.4 Experimental results.- 6.5 Adding isomorphisms to the ML type-checker.- 6.5.1 Type-inference with just (Split).- 6.5.2 What is special in (Split).- 6.5.3 Choosing the right isomorphisms.- 6.5.4 Right isomorphisms in impure context.- 6.6 Conclusion.- 6.7 Some technical Lemmas.- 6.8 Completeness.- 6.9 Conservativity.- 7 Related Works, Future Perspectives.- 7.1 Equational matching of types.- 7.1.1 Decomposing the matching problem.- 7.2 Using equational unification.- 7.3 Extending the paradigm.- 7.3.1 Searching through type classes.- 7.3.2 Searching with more powerful specifications.- 7.3.3 Recursive terms and types.- 7.3.4 Other applications of type isomorphisms.- 7.4 Future work and perspectives.- 7.4.1 Design of type systems for functional languages.- 7.4.2 Object retrieval in object-oriented libraries.- 7.4.3 Dynamic composition of software components.- 7.4.4 Representation optimization.- Citation Index.