THEORETICAL PEARLS: A bargain for intersection types: a simple strong normalization proof

  • Authors:
  • Peter Møller Neergaard

  • Affiliations:
  • Michtom School of Computer Science, Brandeis University, Waltham, MA 02454, USA (e-mail: turtle@achilles.linearity.org)

  • Venue:
  • Journal of Functional Programming
  • Year:
  • 2005

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Abstract

This pearl gives a discount proof of the folklore theorem that every strongly $\beta$-normalizing $\lambda$-term is typable with an intersection type. (We consider typings that do not use the empty intersection $\omega$ which can type any term.) The proof uses the perpetual reduction strategy which finds a longest path. This is a simplification over existing proofs that consider any longest reduction path. The choice of reduction strategy avoids the need for weakening or strengthening of type derivations. The proof becomes a bargain because it works for more intersection type systems, while being simpler than existing proofs.