Section: New Results

The OCaml language and system

The OCaml system

Participants : Damien Doligez, Alain Frisch [Lexifi SAS] , Jacques Garrigue [University of Nagoya] , Fabrice Le Fessant, Xavier Leroy, Gabriel Scherer.

This year, we released version 4.01.0 of the OCaml system. This is a major release that fixes about 140 bugs and introduces 44 new features suggested by users. Damien Doligez acted as release manager for this version.

The major innovations in OCaml 4.01 are:

• The overloading of variant constructors and record field labels, resolved using typing information. Before this, programmers had to use globally unique field labels across all record types. The new typechecking algorithm enables programmers to use more natural names for fields in their data structures. The algorithm is carefully engineered to preserve principality of inferred types.

• New warnings give the programmer the option of applying very strict checking of problematic constructs in the source code.

Other features of this release include:

• Suggestion of possible typos in case of “unbound identifier” error.

• New infix application operators in the standard library.

• Options to reduce the verbosity (and enhance the readability) of error messages.

• Many internal improvements, especially in compiler performance.

In parallel, we designed and experimented with several new features that are candidate for inclusion in the next major release of OCaml in 2014:

• Module aliases: a more efficient way of typechecking and compiling module declarations of the form module M = ModuleName , providing a lighter, more practical alternative to packed modules and reducing the need for name spaces.

• Extension points and preprocessing by rewriting abstract syntax trees: this approach provides an alternative to Camlp4 for macro processing and automatic code generation.

• A native code generator for the new ARM 64 bit instruction set (also known as AArch64).

• Several ongoing experiments to improve the performance of OCaml-compiled code: more aggressive function inlining and constant propagation; more unboxing of numbers; and a pass of common subexpression elimination.

Run-time types for the OCaml language

Participants : Grégoire Henry, Jacques Garrigue [University of Nagoya] , Fabrice Le Fessant.

With the addition of GADTs to OCaml in version 4.00, it is now possible to provide a clean implementation of run-time types in the language, thus allowing the definition of polytypic function, a.k.a. generic function defined by case analysis on the structure of its argument's type. However, when integrating this mechanism into the language, its interaction with other parts of the type-system proved delicate, the main difficulty being the semantic of abstract types.

In collaboration with Jacques Garrigue during a 3 month stay in Japan, Grégoire Henry worked on different semantics for the runtime representation of abstract types. They tried to design a mechanism that preserves abstraction by default, and still allows to propagate type information when requested by the programmer.

Multi-runtime OCaml

Participants : Luca Saiu, Fabrice Le Fessant.

Multicore architectures are now broadly available, and developers expect their programs to be able to benefit from them. In OCaml, there is no portable way to use such architectures, as only one OCaml thread can run at any time.

As part of the ANR project “BWare”, Luca Saiu and Fabrice Le Fessant developed a multi-runtime version of OCaml that takes advantage of multicore architectures. In this version, a program can start several runtimes that can run on different cores. As a consequence, OCaml threads running on different runtimes can run concurrently. This implementation required a lot of rewriting of the OCaml runtime system (written in C), to make all global variables context-dependent and all functions reentrant. The compiler was also modified to generate reentrant code and context-dependent variables. The sources of the prototype were released in September 2013, to be tested by users.

Luca Saiu then developed a library based on skeletons to facilitate the development of parallel applications that take advantage of the multi-runtime architecture.

Evaluation strategies and standardization

Participants : Thibaut Balabonski, Flávio de Moura [Universidade de Brasília] .

During the past years, Thibaut Balabonski studied evaluation strategies, laziness and optimality for functional programming languages, in particular in relation to pattern matching. These investigations continued this year, with two highlights:

• Publication in the ICFP conference [14] of a theoretical result relating fully lazy evaluation (as can be found in some Haskell compilers) to optimal reduction in the weak $\lambda$-calculus.

• Collaboration with Flávio de Moura (Universidade de Brasília) on so-called “standard” evaluation strategies for a calculus with rich pattern matching mechanisms (the Pure Pattern Calculus of Jay and Kesner  [42] ). The challenge here lies in that the calculus does not satisfies the usual stability properties. As a consequences, standard strategies are not unique anymore, and new approaches are needed. A paper is in preparation.