Currently, nannou::app and, in general, the app::Builder API expect an argument of type fn, but it would be nice to be able to pass a closure instead.

My use case is that I'm writing a CLI wrapper to speed up my workflow, and I need to close over the environment in model, update, and view, but I can't because they have to be of type fn.

I think this wouldn't be a breaking change because you can pass a function in a place where a closure is expected, but not the other way around. However, there may be some nuance I am not aware of.