Cxx foreign function interface
[To do: link here from other pages, add an FFI category?]
This page outlines the challenges involved in building a foreign function interface (FFI) from Haskell to C++ libraries, beyond the usual difficulties of FFI, and then describes some (possible) solutions to the difficulties presented by C++.
Understanding the FFI in general, and with respect to the C programming language in particular, is vital background for anyone wishing to build a Haskell interface to C++. The following pages provide background information:
Difficulties of Interfacing to C++
Foreign function interfaces (FFIs) from Haskell to C are well developed, but writing an FFI to a C++ library remains a difficult undertaking. In general, the Haskell FFI apparatus specifies how to call a function (not a method) using the "ccall" calling mechanism used for C programs. The FFI specification (Definition#Addenda_to_the_report) mentions, but does not require, an alternative calling mechanism for C++; no Haskell compiler has ever implemented this. Extra complications arise in interfacing to C++ libraries:
- A C++ program allows overloading, using the same function name with different argument lists. But at the binary level, each different version of the function must have its own name. The C++ compiler "mangles" the function names to differentiate them.
- A C++ program may have classes with constructors, destructors, and methods. These are, obviously, not quite the same as functions, and have no obvious counterpart in Haskell.
- Inheritance (maybe)
- Template functions, template classes, etc.
Building Interfaces to C++
- IO inside describes how to interface to a C++ library using the "export C" declaration (in C++) which makes it look like a C library. This seems to be the standard technique for bridging the gap between Haskell and C++.
- CPlusPlus from Haskell describes "the hard way" of interfacing to C++, using the mangled C++ names directly without using "export C". It also suggests the possibility of a more automated approach using gcc-xml.
- Hacanon automatically generates an FFI to C++ using Template Haskell, but its author has placed it "on hold" and sees some inherent flaws in using Template Haskell. (See David Himmelstrup, 2006, "About Lemmih", accessed 20 June 2009)
- Hacanon-light is a reduced form of Hacanon, but with the same problems involving Template Haskell. (See ibid.)
- ZeroTH (zeroth) fixes the problems of Hacanon and Hacanon-light. ZeroTH by itself only removes Template Haskell dependencies from Haskell programs; so, you could use either (Hacanon + ZeroTH) or (Hacanon-light + ZeroTH), but not ZeroTH by itself, to generate C++ bindings.
- The HQK project reports having developed a small Haskell module, qoo, for accessing object-oriented libraries conveniently.
- SWIG (Simplified Wrapper and Interface Generator) can be used to connect C and C++ with many languages, such as Python, Tcl, Java, several versions of Lisp, and OCAML), but alas, not Haskell. Or at least not directly. But with MissingPy, which provides access to the Python interpreter, it might be possible to do this indirectly: first, make the C++ library accessible to Python (already done, in many cases), then, let your Haskell program use the Python interpreter to make calls into the C++ library. Maybe this is also possible with some other scripting languages.