We present F*, a new language that works both as a proof assistant as well as a general-purpose, verification-oriented, effectful programming language.
In support of these complementary roles, F* is a dependently typed, higher-order, call-by-value language with primitive effects including state, exceptions, divergence and IO. Although primitive, programmers choose the granularity at which to specify effects by equipping each effect with a monadic, predicate transformer semantics. F* uses this to efficiently compute weakest preconditions and discharges the resulting proof obligations using a combination of SMT solving and manual proofs. Isolated from the effects, the core of F* is a language of pure functions used to write specifications and proof terms—its consistency is maintained by a semantic termination check based on a well-founded order.
We evaluate our design on more than 55,000 lines of F* we have authored in the last year, focusing on three main case studies. Showcasing its use as a general-purpose programming language, F* is programmed (but not verified) in F*, and bootstraps in both OCaml and F#. Our experience confirms F*’s pay-as-you-go cost model: writing idiomatic ML-like code with no finer specifications imposes no user burden. As a verification-oriented language, our most significant evaluation of F* is in verifying several key modules in an implementation of the TLS-1.2 protocol standard. For the modules we considered, we are able to prove more properties, with fewer annotations using F* than in a prior verified implementation of TLS-1.2. Finally, as a proof assistant, we discuss our use of F* in mechanizing the metatheory of a range of lambda calculi, starting from the simply typed lambda calculus to F-omega and even micro-F*, a sizeable fragment of F* itself—these proofs make essential use of F*’s flexible combination of SMT automation and constructive proofs, enabling a tactic-free style of programming and proving at a relatively large scale.
Wed 20 Jan
|16:30 - 16:55|
Nikhil SwamyMicrosoft Research, Cătălin HriţcuINRIA Paris, Chantal KellerMSR-INRIA, Aseem RastogiUniversity of Maryland, College Park, Antoine Delignat-LavaudINRIA, Simon ForestENS, Karthikeyan BhargavanINRIA, Cédric FournetMicrosoft Research, Pierre-Yves StrubIMDEA Software Institute, Markulf KohlweissMicrosoft Research, Jean-Karim ZinzindohouéINRIA, Santiago Zanella-BéguelinMicrosoft ResearchPre-print Media Attached
|16:55 - 17:20|
Johannes BorgströmUppsala University, Andrew D. GordonMicrosoft Research and University of Edinburgh, Long OuyangStanford University, Claudio RussoMicrosoft Research, Adam ŚcibiorUniversity of Cambridge, Marcin SzymczakUniversity of EdinburghMedia Attached
|17:20 - 17:45|
Bjørn Bugge GrathwohlDIKU, University of Copenhagen, Fritz HengleinDIKU, Denmark, Ulrik Terp RasmussenDIKU, University of Copenhagen, Kristoffer Aalund SøholmJobindex, Denmark, Sebastian Paaske TørholmJobindex, DenmarkMedia Attached