Why are strings and other constants enquosed in the empty environment?

Function arguments are defused into quosures that keep track of the environment of the defused expression.

quo(1 + 1)
#> <quosure>
#> expr: ^1 + 1
#> env:  global

You might have noticed that when constants are supplied, the quosure tracks the empty environment instead of the current environmnent.

quos("foo", 1, NULL)
#> <list_of<quosure>>
#>
#> [[1]]
#> <quosure>
#> expr: ^"foo"
#> env:  empty
#>
#> [[2]]
#> <quosure>
#> expr: ^1
#> env:  empty
#>
#> [[3]]
#> <quosure>
#> expr: ^NULL
#> env:  empty

The reason for this has to do with compilation of R code which makes it impossible to consistently capture environments of constants from function arguments. Argument defusing relies on the promise mechanism of R for lazy evaluation of arguments. When functions are compiled and R notices that an argument is constant, it avoids creating a promise since they slow down function evaluation. Instead, the function is directly supplied a naked constant instead of constant wrapped in a promise.

Concrete case of promise unwrapping by compilation

We can observe this optimisation by calling into the C-level findVar() function to capture promises.

# Return the object bound to `arg` without triggering evaluation of
# promises
f <- function(arg) {
  rlang:::find_var(current_env(), sym("arg"))
}

# Call `f()` with a symbol or with a constant
g <- function(symbolic) {
  if (symbolic) {
    f(letters)
  } else {
    f("foo")
  }
}

# Make sure these small functions are compiled
f <- compiler::cmpfun(f)
g <- compiler::cmpfun(g)

When f() is called with a symbolic argument, we get the promise object created by R.

g(symbolic = TRUE)
#> <promise: 0x7ffd79bac130>

However, supplying a constant to "f" returns the constant directly.

g(symbolic = FALSE)
#> [1] "foo"

Without a promise, there is no way to figure out the original environment of an argument.

Do we need environments for constants?

Data-masking APIs in the tidyverse are intentionally designed so that they don't need an environment for constants.

• Data-masking APIs should be able to interpret constants. These can arise from normal argument passing as we have seen, or by injection with !!. There should be no difference between dplyr::mutate(mtcars, var = cyl) and dplyr::mutate(mtcars, var = !!mtcars$cyl).

• Data-masking is an evaluation idiom, not an introspective one. The behaviour of data-masking function should not depend on the calling environment when a constant (or a symbol evaluating to a given value) is supplied.