A modular foreign function interface

Abstract

Foreign function interfaces (FFIs) between high-level languages and system libraries typically intertwine the actions of describing the interface of a system library and selecting a binding strategy for linking to it. This tight coupling makes it difficult for programmers to switch between different binding strategies, and discourages the development of new approaches to binding, since more exotic approaches are unlikely to attract sufficient users to justify the cost of development.We present Cmeleon, a replacement for the standard OCaml FFI that exposes typed constructors that correspond to the operations of the type algebra of C, and binding strategies that interpret this type structure as separate program stages. Cmeleon parameterises external calls across binding strategies, isolating interface descriptions from choices relating to call construction (code generation vs dynamic call frames), concurrency style (blocking, cooperatively or preemptively threaded), and separation (in-process, address space or a network connection).This flexibility enables significant code reuse of bindings in many different contexts, from rapid interactive development in a REPL to production deployments with generated code and privilege separation. Cmeleon has been used for the past two years to bind to a broad variety of real-world OCaml libraries, and entirely supplants the need for the low-level C FFI for the vast majority of applications.

A practical unification of multi-stage programming and macros

“…Modular Macros [44] o ered a compile-time variant of BER MetaOCaml by introducing a new keyword to enable macro expansion. In their work they demonstrate that an existing staged library needs intrusive changes to sprinkle the code with the aforementioned keywords.…”

“…Monnier et al [25] rst expressed inlining as staged computation but MacroML o ered a user-level perspective by reusing the same mechanisms of quotes and splices; where splices can appear at the top-level (not nested in a quote). Modular Macros [44] prototyped a compile-time variant of MetaOCaml which also comprises part of our inspiration. While the same line of work inspired many metaprogramming libraries and language features, to our knowledge builtin support for both run-time MSP and generative macros has not been implemented previously in a unifying manner.…”

Lambda: the ultimate sublanguage (experience report)

“…Generalized algebraic data types (GADTs), whose usefulness once appeared to be restricted to length-indexed vectors and well-typed evaluators, are now commonplace in functional programming, appearing in applications from parsers [Pottier and Régis-Gianas 2006] to foreign function interfaces [Yallop et al 2018]. However, novices still find GADTs challenging to understand.…”

CAMLroot: revisiting the OCaml FFI

“…Ctypes (Yallop et al, 2018) proposes an alternative way to bind libraries. Rather than writing C code, a specification of the library is described in OCaml code.…”