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Isaac Newton Institute for Mathematical Sciences

Semantics of Computation

1 July - 31 December 1995

Organisers: S Abramsky (Imperial College, London), G Kahn (INRIA, Sophia-Antipolis), J C Mitchell (Stanford), A M Pitts (Cambridge)

Semantics of Computation Seminar

Friday 8 December, 2:15 pm

Classes = Objects + Data Abstraction

John Mitchell (Stanford and Newton Inst.)

This talk presents a type-theoretic foundation for object systems that include "interface types" and "implementation types", in the process accounting for access controls such as C++ private, protected and public levels of visibility. The approach begins with a basic object calculus that provides a notion of object, method lookup, and object extension (an object-based form of inheritance). In this calculus, the type of an object gives an interface, as a set of methods (public member functions) and their types, but does not imply any implementation properties such as the presence or layout of any hidden internal data.

We extend the core object calculus with a higher-order form of data abstraction mechanism that allows us to declare supertypes of an abstract type and a list of methods guaranteed not to be present. This results in a flexible framework for studying and improving practical programming languages where the type of an object gives certain implementation guarantees, such as would be needed to statically determine the offset of a function in a method lookup table or safely implement binary operations without exposing the internal representation of objects.

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