17.5 Customizing evaluations with functions

• Can also bind names to functions in supplied environment
• Allows overriding function behaviour
• This is how dplyr generates SQL for working with databases

For example…

string_math <- function(x) {
  e <- env(
    caller_env(),
    `+` = function(x, y) paste(x, y),
    `*` = function(x, y) strrep(x, y)
  )

  eval(enexpr(x), e)
}

cohort <- 9
string_math("Hello" + "cohort" + cohort)
#> [1] "Hello cohort 9"
string_math(("dslc" + "is" + "awesome---") * cohort)
#> [1] "dslc is awesome---dslc is awesome---dslc is awesome---dslc is awesome---dslc is awesome---dslc is awesome---dslc is awesome---dslc is awesome---dslc is awesome---"