lqml/cpp-lib

Build

$ cd cpp
$ qmake lib.pro
$ make
$ cd ..

Run

$ lqml ~/slime/lqml-start-swank.lisp
$ emacs run.lisp

After M-x slime-connect and loading run.lisp, you can see that, despite the argument and return type simply being defined as QVariant, you may also pass lists, because a QVariant can also be of type QVariantList, so this is a perfect fit for (nested) Lisp lists.

So, we pass a nested Lisp list, and it gets converted and shown on Qt side with the respective types. Then the QVariantList is returned to Lisp, where it is automatically converted back to a nested Lisp list.

Really convenient!

From the second function -- which calls back to Lisp -- we can see that it suffices to simply pass some intuitive, primitive C++ values to ecl_fun, which will be converted automatically (using QVariant) to the appropriate Lisp values.

Conclusion: by only allowing QVariant arguments for calls between Lisp and C++/Qt, we simplify things to a point where it becomes trivial, especially considering nested lists on both sides.