I notice that OpaWhiteBoard is included in the example source, there is a .ml file here: https://github.com/hhugo/OpaWhiteBoard/blob/master/src/opacairo/cairo.ml
OCaml seems to have some directives ("##"). I suppose this means that you can write extensions for Opa in OCaml, right?
Yes you can with the OPA-plugin-builder.
The book is still under development (http://opalang.org/resources/book/index.html), and as soon as we can complete the work with the plugin creator.
, , Makefile: https://github.com/hhugo/OpaWhiteBoard/blob/master/Makefile
cairo.ml: https://github.com/hhugo/OpaWhiteBoard/blob/master/src/opacairo/cairo.ml
'##' , , OPA.
, : http://opalang.org/resources/book/index.html#hello_binding