Real World Haskell

Chapter 17. Interfacing with C: The FFI

Programming languages do not exist in perfect isolation. They inhabit an ecosystem of tools and libraries, built up over decades, and often written in a range of programming languages. Good engineering practice suggests we reuse that effort. The Haskell Foreign Function Interface (the FFI) is the means by which Haskell code can use, and be used by, code written in other languages. In this chapter, we’ll look at how the FFI works and how to produce a Haskell binding to a C library, including how to use an FFI preprocessor to automate much of the work. The challenge: take PCRE, the standard Perl-compatible regular expression library, and make it usable from Haskell in an efficient and functional way. Throughout, we’ll seek to abstract out manual effort required by the C implementation, delegating that work to Haskell to make the interface more robust, yielding a clean, high-level binding. We assume only some basic familiarity with regular expressions.

Binding one language to another is a nontrivial task. The binding language needs to understand the calling conventions, type system, data structures, memory allocation mechanisms, and linking strategy of the target language, just to get things working. The task is to carefully align the semantics of both languages so that both can understand the data that passes between them.

For Haskell, this technology stack is specified by FFI to the Haskell report. The FFI report describes how to correctly bind ...