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Recurrence extraction and denotational semantics with recursive definitions

With one exception, our previous work on recurrence extraction and denotational semantics has focused on a source language that supports inductive types and structural recursion. The exception handles general recursion via an initial translation into call-by-push-value. In this note we give an extraction function from a language with general recursive function definitions and recursive types directly to a PCF-like recurrence language. We prove the main soundness result (that the syntactic recurrences in fact bound the operational cost) without the use of a logical relation, thereby significantly simplifying the proof compared to our previous work (at the cost of placing more demands on the models of the recurrence language). We then define two models of the recurrence language, one for analyzing merge sort, and another for analyzing quick sort, as case studies to understand model definitions for justifying the extracted recurrences.