Section: New Results

The OCaml language and system

OCaml

Participants : Damien Doligez, Alain Frisch [Lexifi SAS] , Jacques Garrigue [University of Nagoya] , Sébastien Hinderer, Fabrice Le Fessant, Xavier Leroy, Luc Maranget, Gabriel Scherer, Mark Shinwell [Jane Street] , Leo White [Jane Street] , Jeremy Yallop [OCaml Labs, Cambridge University] .

This year, we released versions 4.03.0 and 4.04.0 of the OCaml system. These are major releases that introduce a large number of new features. The most important features are:

• A new optimization subsystem called flambda, which does inlining and specialization of functions as well as static allocation of some data structures, etc.

• ephemerons: a generalization of weak pointers that is better suited for memoization of mutually-recursive functions.

• A fine-grained memory profiler to help programmers understand the allocation behavior of their programs.

• unboxed types: a user-controlled optimized representation for some simple data types.

Infrastructure for OCaml

Participant : Sébastien Hinderer.

Sébastien Hinderer worked on improving the test infrastructure of the OCaml compiler. These tests aim at verifying that the compiler works as expected. Currently, they are driven by a set of Makefiles which are hard to maintain and extend and make it difficult to add new tests. Sébastien developed the ocamltest driver, which parses test descriptions written in a domain-specific language and runs the appropriate tests.

Sébastien Hinderer also worked on merging the Makefiles used for building the compiler under Unix and Windows. The existence of separate sets of Makefiles, which is the result of a long development history, makes it especially hard to maintain and extend the compiler's build system. Sébastien worked on eliminating this redundancy, so that a single build system can be used on every platform. This is a prerequisite for using the GNU autoconf tools and for building easy-to-use cross-compilers for OCaml. A cross-compiler is required, for instance, to build iOS apps using OCaml.

Continuous integration of OCaml packages

Participant : Fabrice Le Fessant.

OPAM is a repository of OCaml source packages. It is now advertised as the official way of installing the OCaml distribution. To maintain a high level of quality for the thousands of source packages distributed in the repository, it is crucial to provide feedback to the developers on the impact of their modifications to the repository, in real-time, despite the high churn and the cascading costs of package recompilations.

We have designed and prototyped a simple modular architecture for a service that monitors the OPAM repository, and triggers recompilation of packages that are impacted by the latest modifications to the repository, for all major and minor OCaml versions since 3.12.1. Previous attempts to design such a system have failed to scale, although they targeted cloud systems of thousands of virtual machines. On the contrary, the new prototype has been deployed on a single quadcore server, and has been able to follow the OPAM repository for eight months, providing feedback in almost real-time. To achieve such a result, it uses many optimizations and caching techniques, to make recompilations as incremental as possible [37].

Global analyses of OCaml programs

Participants : Thomas Blanc [ENSTA-ParisTech & OCamlPro] , Pierre Chambart [OCamlPro] , Vincent Laviron [OCamlPro] , Fabrice Le Fessant, Michel Mauny.

Exception handling in OCaml can be used for managing and reporting errors, as well as to express complex control flow constructs. As such, exceptions can be the source of errors, when, for instance, a function that may raise an exception is called in a context where this exception cannot be handled. In such situations, the program may fail unexpectedly, and the source of the error can be difficult to identify.

This work aims at performing global static analyses of OCaml programs using abstract interpretation techniques, with a particular focus on the detection of uncaught exceptions. Starting from one of the OCaml intermediate languages, we produce a hypergraph that represents the program to be analyzed. Each node of this hypergraph is a program state and each edge is an operation. Operations that may or may not raise an exception (such as function calls) have one or two successors. A fixpoint iteration is then performed on the graph, where function application edges are dynamically replaced by the corresponding subgraphs. In essence, environment information is propagated through the graph, adding at each node a superset of all possible values of each variable, until no additional information can be found. A description of the framework was presented at the 2015 OCaml workshop. We expect concrete results as well as Thomas Blanc's thesis manuscript during 2017.

Type-checking the OCaml intermediate languages

Participants : Pierrick Couderc [ENSTA-ParisTech & OCamlPro] , Grégoire Henry [OCamlPro] , Fabrice Le fessant, Michel Mauny.

This work aims at propagating type information through the intermediate languages used by the OCaml compiler. We started by the design and implementation of a consistency checker of the type-annotated abstract syntax trees (TASTs) produced by the OCaml compiler. It appears that, when presented as inference rules, the different cases of this TAST checker can be read as the rules of the OCaml type system. Proving the correctness of (part of) the checker would prove the soundness of the corresponding part of the OCaml type system. A preliminary report on this work has been presented at the 17th Symposium on Trends in Functional Programming (TFP 2016).

Optimizing OCaml for satisfiability problems

Participants : Sylvain Conchon [LRI, Univ. Paris Sud] , Albin Coquereau [ENSTA-ParisTech] , Fabrice Le fessant, Michel Mauny.

This work aims at improving the performance of the Alt-Ergo SMT solver, implemented in OCaml. For safety reasons, the implementation of Alt-Ergo uses as much as possible a functional programming style and persistent data structures, which are sometimes less efficient that the imperative style and mutable data structures. We would like to first obtain a better understanding of the OCaml memory and cache behavior, so as to understand where efficiency could be gained, and then design dedicated data structures (for instance, semi-persistent data structures) and compare their efficiency to the current ones. This work is still at a preliminary stage: we have selected benchmarks and profiled their execution in order to discover sources of inefficiency.

Type compatibility checking for dynamically loaded OCaml data

Participants : Florent Balestrieri [ENSTA-ParisTech] , Michel Mauny.

The SecurOCaml project (FUI 18) aims at enhancing the OCaml language and environment in order to make it more suitable for building secure applications, following recommendations published by the French ANSSI in 2013. Michel Mauny and Florent Balistrieri (ENSTA-ParisTech) represent ENSTA-Paristech in this project for the two-year period 2016-2017.

The goal of this first year was to design and produce an effective OCaml implementation that checks whether a memory graph – typically the result obtained by un-marshalling some data – is compatible with a given OCaml type, following the algorithm designed by Henry et al. in 2012. As the algorithm needs a runtime representation of OCaml types, Florent Balestrieri implemented a library for generic programming in OCaml [21]. He also implemented a type-checker which, when given a type and a memory graph, checks whether the former could be the type of the latter. The algorithm handles sharing and polymorphism, but currently supports neither functional values nor existential types.

Pattern matching

Participants : Luc Maranget, Gabriel Scherer [Northeastern University, Boston] , Thomas Réfis [Jane Street LLC] .

A new pattern matching diagnostic message, which should help OCaml programmers to detect rare but vicious programming errors, was integrated in the yearly release of the OCaml compiler, and was presented at the OCaml Users and Developers Workshop [39].

Error diagnosis in Menhir parsers

Participant : François Pottier.

In 2015, François Pottier proposed a reachability algorithm for LR automata, which he implemented in the Menhir parser generator. He applied this approach to the C grammar in the front-end of the CompCert compiler, therefore allowing CompCert to produce better syntax error messages. This work has been presented at the conferences JFLA 2016 [31] and CC 2016 [26].