Lexical scope and forced configuration

The idea

An R function carries its enclosing environment, so a returned function can keep access to local configuration after its factory finishes. Name lookup follows lexical scope rather than the environment of whichever caller happens to invoke the function later. Separate factory calls can create separate environments, which supports small configurable transformations without global variables.

Function arguments are promises evaluated when needed. A factory that captures an argument without evaluating it may observe a changed referenced variable when the returned function eventually forces that promise. Use force inside the factory when configuration should be fixed at creation time. Document whether state is a snapshot or a live dependency, and avoid superassignment when ordinary return values can express the update more clearly.

make_adder <- function(offset) {
  force(offset)
  function(x) x + offset
}
n <- 3
add <- make_adder(n)
n <- 9
cat(add(2), "\n", sep="")

5

Forcing offset captures its evaluated value before n is reassigned. The returned function uses the factory’s environment.