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arXiv · 2510.13725

A Complementary Approach to Incorrectness Typing

Celia Mengyue Li·Sophie Pull·Steven Ramsay
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Abstract

We introduce a new two-sided type system for verifying the correctness and incorrectness of functional programs with atoms and pattern matching. A key idea in the work is that types should range over sets of normal forms, rather than sets of values, and this allows us to define a complement operator on types that acts as a negation on typing formulas. We show that the complement allows us to derive a wide range of refutation principles within the system, including the type-theoretic analogue of co-implication, and we use them to certify that a number of Erlang-like programs go wrong. An expressive axiomatisation of the complement operator via subtyping is shown decidable, and the type system as a whole is shown to be not only sound, but also complete for normal forms.

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Celia Mengyue Li, Sophie Pull, Steven Ramsay. 2026-05-08. A Complementary Approach to Incorrectness Typing. https://doi.org/10.1145/3776724

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