Abstract
Functional programs with side effects represented by monads are amenable to equational reasoning. This approach to program verification has been experimented several times using proof assistants based on dependent type theory. These experiments have been performed independently but reveal similar technicalities such as how to build a hierarchy of interfaces and how to deal with non-structural recursion. As an effort towards the construction of a reusable framework for monadic equational reasoning, we report on several practical improvements of Monae, a Coq library for monadic equational reasoning. First, we reimplement the hierarchy of effects of Monae using a generic tool to build hierarchies of mathematical structures. This refactoring allows for easy extensions with new monad constructs. Second, we discuss a well-known but recurring technical difficulty due to the shallow embedding of monadic programs. Concretely, it often happens that the return type of monadic functions is not informative enough to complete formal proofs, in particular termination proofs. We explain library support to facilitate this kind of proof using standard Coq tools. Third, we augment Monae with an improved theory about nondeterministic permutations so that our technical contributions allow for a complete formalization of quicksort derivations by Mu and Chiang.
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Notes
- 1.
Hierarchy-Builder version 1.1.0 (2021-03-30).
- 2.
The existence of this intermediate interface is further justified by its use in the definition of the interface of the backtrackable state monad [14].
- 3.
The
command can be configured by the user so that Coq provides proofs automatically. - 4.
Since Coq already has a
tactic, we call the function
. - 5.
There is another axiom sorted-cat3, but its proof is easy using lemmas from MathComp.
- 6.
The type Z_eqType is the type of integers equipped with decidable equality. This is a slight generalization of the original definition that is using natural numbers.
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Acknowledgements
The authors would like to thank Cyril Cohen and Enrico Tassi for their assistance with Hierarchy-Builder, Kazuhiko Sakaguchi and Takafumi Saikawa for their comments, the members of the Programming Research Group of the Department of Mathematical and Computing Science at the Tokyo Institute of Technology for their input, and the anonymous reviewers for many comments that improved this paper. The second author acknowledges the support of the JSPS KAKENHI grants 18H03204 and 22H00520.
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Saito, A., Affeldt, R. (2022). Towards a Practical Library for Monadic Equational Reasoning in Coq. In: Komendantskaya, E. (eds) Mathematics of Program Construction. MPC 2022. Lecture Notes in Computer Science, vol 13544. Springer, Cham. https://doi.org/10.1007/978-3-031-16912-0_6
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