Title: NOOP: A Domain-Theoretic Model of Nominally-Typed OOP,
Abstract: The majority of industrial-strength object-oriented (OO) software is written
using nominally-typed OO programming languages. Extant domain-theoretic models
of OOP developed to analyze OO type systems miss, however, a crucial feature of
these mainstream OO languages: nominality. This paper presents the construction
of NOOP as the first domain-theoretic model of OOP that includes full
class/type names information found in nominally-typed OOP. Inclusion of nominal
information in objects of NOOP and asserting that type inheritance in
statically-typed OO programming languages is an inherently nominal notion allow
readily proving that type inheritance and subtyping are completely identified
in these languages. This conclusion is in full agreement with intuitions of
developers and language designers of these OO languages, and contrary to the
belief that "inheritance is not subtyping," which came from assuming
non-nominal (a.k.a., structural) models of OOP.
To motivate the construction of NOOP, this paper briefly presents the
benefits of nominal-typing to mainstream OO developers and OO language
designers, as compared to structural-typing. After presenting NOOP, the paper
further briefly compares NOOP to the most widely known domain-theoretic models
of OOP. Leveraging the development of NOOP, the comparisons presented in this
paper provide clear, brief and precise technical and mathematical accounts for
the relation between nominal and structural OO type systems. NOOP, thus,
provides a firmer semantic foundation for analyzing and progressing
nominally-typed OO programming languages.