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Cxx foreign function interface

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== Foreign Function Interfaces to C++ Libraries ==
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This page outlines the challenges involved in building a foreign function interface (FFI) from Haskell to C++ libraries, references some general background about the FFI, and then describes some solutions to the difficulties of C++.
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== Difficulties of Interfacing to C++ ==
   
 
Foreign function interfaces (FFIs) from Haskell to C are well developed,
 
Foreign function interfaces (FFIs) from Haskell to C are well developed,
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In general, the Haskell FFI apparatus specifies how to call a function
 
In general, the Haskell FFI apparatus specifies how to call a function
 
(not a method) using the "ccall" calling mechanism used for C programs.
 
(not a method) using the "ccall" calling mechanism used for C programs.
The FFI specification (** need link) 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:
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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 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.
 
# 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.
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# Inheritance (maybe)
 
# Template functions, template classes, etc.
 
# Template functions, template classes, etc.
   
Here are some approaches to building interfaces to C++:
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== Background Information ==
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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:
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* [[FFI_Introduction]]
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* [[Applications_and_libraries/Interfacing_other_languages]]
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== 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 briding the gap between Haskell and 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 briding 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.
 
* [[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 it is unmaintained and its author seems dissatisfied with it.
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* [http://darcs.haskell.org/~lemmih/hacanon/ 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, [http://darcs.haskell.org/~lemmih/aboutMe.html "About Lemmih"], accessed 20 June 2009)
** Need link
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** [http://hackage.haskell.org/package/hacanon-light Hacanon-light] is a reduced form of Hacanon, but with the same problems involving Template Haskell. (See ibid.)
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** [http://darcs.haskell.org/%7Elemmih/zerothHead Zeroth] fixes the problems of Hacanon and Hacanon-light, but is "on hold". (See ibid.)
 
* The [[HQK]] project reports having developed a small Haskell module, qoo, for accessing object-oriented libraries conveniently.
 
* The [[HQK]] project reports having developed a small Haskell module, qoo, for accessing object-oriented libraries conveniently.

Revision as of 16:20, 20 June 2009

This page outlines the challenges involved in building a foreign function interface (FFI) from Haskell to C++ libraries, references some general background about the FFI, and then describes some solutions to the difficulties of C++.

1 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:

  1. 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.
  2. 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.
  3. Inheritance (maybe)
  4. Template functions, template classes, etc.

2 Background Information

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:

3 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 briding 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 fixes the problems of Hacanon and Hacanon-light, but is "on hold". (See ibid.)
  • The HQK project reports having developed a small Haskell module, qoo, for accessing object-oriented libraries conveniently.