I've written a ffi wrapper for the free image library, which is here:

http://cfis.github.com/free-image-ruby/

The problem is that I've coded against the latest version of
FreeImage, and most linux distributions have older versions
installed.  Thus when I try to load the gem it fails since
attach_function throws an error because the older versions don't have
all the functions.

This isn't what I want - the older versions work perfectly fine, they
just don't include as much functionality.  So instead of blowing up, I
would prefer if FFI throw a warning message but continue loading the
library.

If a user does call one of these unimplemented functions, then FFI
should throw an error and allows the wrapping library to customize
that message to say something useful (sorry, to use this function you
need to upgrade to a newer version of freeimage).

I can think of lots of ways of doing this, here is one possibility:

* If a function is not implemented, instead of throwing an error, wrap
it with a NotImplemented function (to go along with Function and
Variadic).

* When a NotImplemented function is called, throw an error.  This
could:
    - Just hard-code an error, not allow customization
    - Call a method on library, not_implemented, that developers could
override
    - Call a proc which would be passed to attach_function which would
be called

I lean towards the 2nd choice, but don't feel very strongly about it.

Does this sound like an idea that could be accepted to FFI?  If so,
I'll code up a patch for review.

Thanks - Charlie