Chapter 24 Scoping

https://rstudio.com/resources/rstudioconf-2018/make-shiny-fast-by-doing-as-little-work-as-possible/

https://shiny.rstudio.com/articles/scoping.html

Elliott, Conal, and Paul Hudak. 1997. “Functional Reactive Animation.” In International Conference on Functional Programming. http://conal.net/papers/icfp97/.