We describe a denotational semantics for a sequential functional language with nondeterministic choice over a countably infinite set (the natural numbers), and prove that it is fully abstract with respect to may-and-must testing.

Our model is based on biordered sets similar to Berry's bidomains, and stable, monotone functions. However, (as in prior models of unbounded non-determinism) these functions may not be continuous. Working in a biordered setting allows us to exploit the different properties of both extensional and stable orders to construct a Cartesian closed category of sequential, discontinuous functions, with least and greatest fixpoints having strong enough properties to prove computational adequacy.

We establish full abstraction of the semantics by showing that it contains a simple, first-order ``universal type-object'' within which all types may be embedded using functions defined by (countable) ordinal induction.

  • Extended abstract (.pdf)
  • Draft (.pdf) of an extension to recursive types with countable nondeterminism.
  • Slides (.ps) from the conference presentation.
  • Bibtex entry:

    author = "J. Laird",
    title = "Bidomains and Full abstraction for countable non-determinism",
    booktitle = "Proceedings of FoSSaCS'06",
    publisher = "Springer",
    series = "LNCS",
    number = 3921,
    pages = "352--366",
    year = 2006}