| Summary: | Some type-based approaches to termination use 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 semantic criterion for
admissibility. To provide a syntactical criterion, a calculus for
semi-continuous functions is developed.
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