SWIG, Re: Language Bindings for Erlang
Sun May 28 06:18:38 CEST 2006
On Saturday 27 May 2006 20:25, Gerd Flaig wrote:
> Romain Lenglet <> writes:
> >> An Erlang add-on to SWIG would be nice...
> >> http://www.swig.org/
> >> Preferably, one should be able to choose creating a
> >> port-program, linked-in driver, C-node interface.
> >> It is up for grabs; anyone ?
> > I have been working on it for some time.
> would you please tell why you abandoned that approach and chose to
> roll your own instead?
Sorry, my sentence was ambiguous. I never considered SWIG.
I meant that I have been working on my own solution that would allow "to
choose creating a port-program, linked-in driver, C-node interface." But not
based on SWIG.
I believe that the SWIG approach is OK when languages are similar, and have
similar data models, etc. But Erlang and C are so different, and the port
driver model so... restrictive, that I believe that an approach that takes a
C function signature and automagically generates an Erlang binding can work
only for a limited category of signatures.
Or a lot must be done for a binding in additional hand-written wrappers, on
the Erlang side and on the C side, to make the bound function signatures fit
the limitations of the binding generator. See the previous email from Serge
Aleynikov about the necessity to use C wrappers to overcome limitations of
The problem is that a complete binding specification becomes scattered accross
several files / wrappers, and maintenance becomes difficult. It is better to
specify everything about a binding in one place, and to have consistency
ensured automatically by tools (compilers...).
EDTK goes one step further than a SWIG approach, e.g. by allowing to specify
when and how to map C data into integer identifiers, but since EDTK is also
purely declarative and it offers little control without entering into the
implementation details of the port driver, it is also not general enough.
The point about Dryverl is that it provides extensive *programmatic control*
over the implementation of a binding, which is necessary to handle all kinds
Dryverl has different objectives than SWIG and EDTK. Both aim at simplicity.
Dryverl instead aims at flexibility and generality.
However, if anybody would write a SWIG-like binding generator to handle a
limited category of simple cases, and which would generate Dryverl
specifications (or even only skeletons of specifications), that would be very
> I wonder what options there are for C++. The obvious approach seems to
> be to map classes to processes and method calls to sending a message
> (and receiving one if the method has a type different from void). But
> this only feels right for large objects.
You can make the "this" pointer explicit, and pass it as the first argument in
normal function calls. On the Erlang side of the binding it would be a
numeric identifier. On the C side of the binding, it would be mapped into an
object pointer used as the target of C++ method calls.
The value map mechanism in Dryverl can be used to implement this easily.
Anyway, of course, this would require that you make all of your methods
virtual. I don't see how to handle non-virtual methods (e.g. non-virtual
destructors) in a clean way (and automatically!).
More information about the erlang-questions