The Fo compiler parses and type-checks Fo source code and then generates and outputs Go code. It generates a unique concrete type for each usage of a generic type. You can look at the examples directory of the repo to see what this output looks like: https://github.com/albrow/fo/tree/master/examples/box.
Generic interfaces are pretty fundamental IMO. It's something I plan to add to the language before the v1 release.