I notice that included in the source of the OpaWhiteBoard example, there is a .ml file here: https://github.com/hhugo/OpaWhiteBoard/blob/master/src/opacairo/cairo.ml
This appears to be OCaml with some directives ("##"). I'm guessing this means it's possible to write extensions to Opa in OCaml, correct?
Yes you can with OPA-plugin-builder.
The book is still a work in progress (http://opalang.org/resources/book/index.html) and we will complete the plugin builder part as soon as we can.
But you can guess how it works looking at the Makefile: https://github.com/hhugo/OpaWhiteBoard/blob/master/Makefile
And the cairo.ml plugin: https://github.com/hhugo/OpaWhiteBoard/blob/master/src/opacairo/cairo.ml
The '##' directive are here in particularly to register functions and their types in OPA.
More precisely, you can get some info and example here : http://opalang.org/resources/book/index.html#hello_binding
