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Summary: Combining Structural Subtyping and External Dispatch
Donna Malayeri Jonathan Aldrich
Carnegie Mellon University
{donna+, aldrich+}@cs.cmu.edu
Abstract
Nominal subtyping (or user-defined subtyping) and structural sub-
typing each have their own strengths and weaknesses. Nominal
subtyping allows programmers to explicitly express design intent,
and, when types are associated with run time tags, enables run-
time "type" tests (e.g., downcasts) and external/multimethod dis-
patch. On the other hand, structural subtyping is flexible and com-
positional, allowing unanticipated reuse. To date, nearly all object-
oriented languages fully support only one subtyping paradigm or
the other.
In this paper, we describe a core calculus for a language that
combines the key aspects of nominal and structural subtyping in a
unified framework. Our goal is to combine the flexibility of struc-
tural subtyping while still allowing static typechecking of external
methods. We prove type safety for this language and illustrate its
practical utility through examples that are not easily expressed in
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