Intensional polymorphism, the ability to dispatch to different routines based on types
at run time, enables a variety of advanced implementation techniques for polymorphic
languages, including tag-free garbage collection, unboxed function arguments, polymorphic
marshalling, and flattened data structures. To date, languages that support intensional
polymorphism have required a type-passing (as opposed to type-erasure) interpretation
where types are constructed and passed to polymorphic functions at run time.
Unfortunately, type-passing suffers from a number of drawbacks; it requires duplication of
constructs at the term and type levels, it prevents abstraction, and it severely
complicates polymorphic closure
We present a type-theoretic framework that supports intensional polymorphism, but avoids many of the disadvantages of type passing. In our approach, run-time type information is represented by ordinary terms. This avoids the duplication problem, allows us to recover abstraction, and avoids complications with closure conversion. In addition, our type system provides another improvement in expressiveness; it allows unknown types to be refined in place thereby avoiding certain beta-expansions required by other frameworks.
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