(Co-) iteration for higher-order nested datatypes

  • Authors:
  • Andreas Abel;Ralph Matthes

  • Affiliations:
  • Department of Computer Science, University of Munich;PPS, CNRS, Université Paris VII

  • Venue:
  • TYPES'02 Proceedings of the 2002 international conference on Types for proofs and programs
  • Year:
  • 2002

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Abstract

The problem of defining iteration for higher-order nested datatypes of arbitrary (finite)rank is solved within the framework of System Fω of higher-order parametric polymorphism. The proposed solution heavily relies on a general notion of monotonicity as opposed to a syntactic criterion on the shape of the type constructors such as positivity or even being polynomial. Its use is demonstrated for some rank-2 heterogeneous/nested datatypes such as powerlists and de Bruijn terms with explicit substitutions. An important feature is the availability of an iterative definition of the mapping operation (the functoriality)for those rank-1 type transformers (i. e., functions from types to types)arising as least fixed-points of monotone rank-2 type transformers. Strong normalization is shown by an embedding into Fω. The results dualize to greatest fixed-points, hence to coinductive constructors with coiteration.