Dependent Types and Multi-Monadic Effects in F*

Dependent Types and Multi-Monadic Effects in F*

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Nikhil Swamy1    Cătălin Hriţcu2    Chantal Keller1,3    Aseem Rastogi4
Antoine Delignat-Lavaud2,5    Simon Forest2,5    Karthikeyan Bhargavan2    Cédric Fournet1,3
Pierre-Yves Strub6    Markulf Kohlweiss1    Jean-Karim Zinzindohoue2,5    Santiago Zanella-Béguelin1

1Microsoft Research    2Inria    3MSR-Inria    4UMD    5ENS Paris    6IMDEA Software Institute

Symposium on Principles of Programming Languages, POPL 2016

Abstract

We present a new, completely redesigned, version of F*, a 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 System 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.