Semi-continuous sized types and termination

  • Authors:
  • Andreas Abel

  • Affiliations:
  • Institut für Informatik, Ludwig-Maximilians-Universität München, München, Germany

  • Venue:
  • CSL'06 Proceedings of the 20th international conference on Computer Science Logic
  • Year:
  • 2006

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Abstract

A type-based approach to termination uses sized types: an ordinal bound for the size of a data structure is stored in its type. A recursive function over a sized type is accepted if it is visible in the type system that recursive calls occur just at a smaller size. This approach is only sound if the type of the recursive function is admissible, i.e., depends on the size index in a certain way. To explore the space of admissible functions in the presence of higher-kinded data types and impredicative polymorphism, a semantics is developed where sized types are interpreted as functions from ordinals into sets of strongly normalizing terms. It is shown that upper semi-continuity of such functions is a sufficient semantical criterion for admissibility. To provide a syntactical criterion, a calculus for semi-continuous function is developed.