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IMHO, you want to do it from scratch if you want it to really work with Erlang, but that means a ton of work. ex11 exists, but I don't know that it was ever very complete, and it was built in the old era of X11 where clients didn't render fonts, etc, so there'd be a lot of work to complete it. The most active fork has moved towards using it for embedded systems.

Personally, I used ex11 because the wxWidgets interfaces are hard for me to grasp and then accept; it was easier for me to understand X11 as a distributed communication protocol that has a side effect of occasionally outputting graphics; but my application only outputs a bitmap once in a while, so really I could have made anything work. ex11 doesn't help you with mobile though, because Android and iOS don't run X11.

There's also gtkNode, but I don't know how well that works, because it didn't seem any less confusing than wxWidgets. gtk has some ability to run on mobile, I think.