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On the building of affine retractions

Published online by Cambridge University Press:  01 August 2008

ALEKSY SCHUBERT*
Affiliation:
Institute of Informatics, University of Warsaw, ul. Banacha 2, 02-097 Warsaw, Poland Email: alx@mimuw.edu.pl
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Abstract

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A simple type σ is retractable to a simple type τ if there are two terms Cσ→τ and Dτ→σ such that D ○ C λx.x. The retractability of types is affine if the terms C and D are affine, that is, when every bound variable occurs in them at most once in the scope of its declaration. This paper presents a system that derives affine retractability for simple types. It also studies the complexity of constructing these affine retractions. The problem of affine retractability is NP-complete even for the class of types over a single type atom and having limited functional order. In addition, a polynomial algorithm for types of orders less than three is presented.