In the previous section, we introduced the functional programming facilities in R. You learned that functions are just another type of object we can pass around. When we create a new function, say fun, the environment we create will be associated with the function. This environment is called the enclosing environment of the function, which can be accessed via environment(fun). Each time we call the function, a new executing environment that contains the unevaluated arguments (promises) will be created to host the execution of the function, which enables lazy evaluation. The parent of the executing environment is the enclosing environment of the function, which enables lexical scoping.
Functional programming allows us to write code in higher level of abstraction. Metaprogramming goes even further. It allows us to tweak the language itself and make certain language constructs easier to use in a certain scenario. Some popular R packages use metaprogramming in their functions...