A function available in the entire app
Be careful, this approach might be considered as a bad design for angular apps, since it requires programmers to remember both where functions are placed in the scope tree, and to be aware of scope inheritance. In many cases it would be preferred to inject a service (Angular practice - using scope inheritance vs injection.
This example only show how scope inheritance could be used for our needs, and the how you could take advantage of it, and not the best practices of designing an entire app.
In some cases, we could take advantage of scope inheritance, and set a function as a property of the rootScope. This way - all of the scopes in the app (except for isolated scopes) will inherit this function, and it could be called from anywhere in the app.