Space-Efficient Manifest Contracts

Abstract

The standard algorithm for higher-order contract checking can lead to unbounded space consumption and can destroy tail recursion, altering a program's asymptotic space complexity. While space efficiency for gradual types-contracts mediating untyped and typed code-is well studied, sound space efficiency for manifest contracts-contracts that check stronger properties than simple types, e.g., "is a natural" instead of "is an integer"-remains an open problem.We show how to achieve sound space efficiency for manifest contracts with strong predicate contracts. The essential trick is breaking the contract checking down into coercions: structured, blame-annotated lists of checks. By carefully preventing duplicate coercions from appearing, we can restore space efficiency while keeping the same observable behavior.

Complete monitors for gradual types

“…Our third contribution is a semantics that satisfies the same type soundness property as Typed Racket but checks the same first-order properties as Transient. This proof-of-concept semantics is dubbed Amnesic; it is inspired by forgetful and heedful contracts [Greenberg 2015].…”

Does blame shifting work?

“…Findler and Felleisen [2002] use delayed checks to lift contracts to the world of higher order functions. This work has since led to a significant body of research on the design of higher order contract systems [Disney et al 2011;Feltey et al 2018;Findler and Blume 2006;Findler et al 2007;Greenberg 2015;Greenberg et al 2010;Heidegger et al 2012;Hinze et al 2006;Jia et al 2016;Keil and Thiemann 2015;Moore et al 2016Moore et al , 2014Scholliers et al 2015;Strickland and Felleisen 2009a,b;Swords et al 2015;Takikawa et al 2012;Waye et al 2017] and their semantics [Blume and McAllester 2006;Degen et al 2008Degen et al , 2009Degen et al , 2010Degen et al , 2012Findler et al 2004].…”

A spectrum of type soundness and performance

“…Herman et al [2010] recognize the problem of space-inefficiency in the higher-order embedding and propose a theoretical solution. Other theoretical solutions address the issue for gradual typing [Herman et al 2010;Siek et al 2009;Siek and Wadler 2010], and more generally for higher-order contracts [Greenberg 2015].…”

“…Co-natural enforces all non-base types with monitors [Degen et al 2012;Findler et al 2007]. Forgetful limits each value to at most one monitor [Greenberg 2015]. Full definitions and proofs are in the supplement [Greenman and Felleisen 2018].…”