Life without the Terminal Type

  • Authors:
  • Lutz Schröder

  • Affiliations:
  • -

  • Venue:
  • CSL '01 Proceedings of the 15th International Workshop on Computer Science Logic
  • Year:
  • 2001

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Abstract

We introduce a method of extending arbitrary categories by a terminal object and apply this method in various type theoretic settings. In particular, we show that categories that are cartesian closed except for the lack of a terminal object have a universal full extension to a cartesian closed category, and we characterize categories for which the latter category is a topos. Both the basic construction and its correctness proof are extremely simple. This is quite surprising in view of the fact that the corresponding results for the simply typed λ-calculus with surjective pairing, in particular concerning the decision problem for equality of terms in the presence of a terminal type, are comparatively involved.