CakeML: a verified implementation of ML

  • Authors:

  • Ramana Kumar;Magnus O. Myreen;Michael Norrish;Scott Owens

  • Affiliations:

  • University of Cambridge, Cambridge, United Kingdom;University of Cambridge, Cambridge, United Kingdom;NICTA, Canberra, Australia;University of Kent, Kent, United Kingdom

  • Venue:
  • Proceedings of the 41st ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages
  • Year:
  • 2014

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Abstract

We have developed and mechanically verified an ML system called CakeML, which supports a substantial subset of Standard ML. CakeML is implemented as an interactive read-eval-print loop (REPL) in x86-64 machine code. Our correctness theorem ensures that this REPL implementation prints only those results permitted by the semantics of CakeML. Our verification effort touches on a breadth of topics including lexing, parsing, type checking, incremental and dynamic compilation, garbage collection, arbitrary-precision arithmetic, and compiler bootstrapping. Our contributions are twofold. The first is simply in building a system that is end-to-end verified, demonstrating that each piece of such a verification effort can in practice be composed with the others, and ensuring that none of the pieces rely on any over-simplifying assumptions. The second is developing novel approaches to some of the more challenging aspects of the verification. In particular, our formally verified compiler can bootstrap itself: we apply the verified compiler to itself to produce a verified machine-code implementation of the compiler. Additionally, our compiler proof handles diverging input programs with a lightweight approach based on logical timeout exceptions. The entire development was carried out in the HOL4 theorem prover.