[WJW]

The "app-as-function" is not at all contrived though, it has some very concrete applications that make use of the composability advantages that functions have. The [rack-test](https://github.com/rack/rack-test) gem (and similar languages) work exactly like this: they run the "app" part without the "web server" part to enable much easier testing. Rack middlewares similarly treat the enclosed app as just a function taking some specified input and returning some output value.