R: non-standard evaluation with nested functions - r

Question

I have a function f1 on a dataframe which calls another function f2 and then does stuff with f2's output. f2 works interactively on its own but how do I get it to run when called by f1?
f1 <- function(x, y) {
z <- f2(x, y)
# do stuff with z
w <- z
return(w)
}
f2 uses subset() to de-select certain columns:
f2 <- function(x, y) {
y <- substitute(y)
subset(x, select = -eval(y))
}
As you can see, f2 works interactively. I don't care about that, but I do want it to work when called by f1.
# This works fine interactively (but I don't care about that)
f2(mtcars,mpg)
# This is what I want to work
f1(mtcars,mpg)
Error in -eval(y) : invalid argument to unary operator
I would prefer not to change f1 or its arguments. How do I re-write f2 so that it works within f1?
Here is a similar question with solutions that I'm having trouble applying to my context: R: passing expression to an inner function

Answer 1

The easiest would be to use rlang and tidyverse functions for compatibility with quasiquotation :
library(dplyr)
library(rlang)
f1 <- function(x, y) {
z <- f2(x, -!!enquo(y))
# do stuff with z
w <- z
return(w)
}
f2 <- function(x, y) {
select(x, !!enquo(y))
}
f1(mtcars,mpg)
# cyl disp hp drat wt qsec vs am gear carb
# Mazda RX4 6 160.0 110 3.90 2.620 16.46 0 1 4 4
# Mazda RX4 Wag 6 160.0 110 3.90 2.875 17.02 0 1 4 4
# Datsun 710 4 108.0 93 3.85 2.320 18.61 1 1 4 1
# ...
This works in base R :
f1 <- function(x, y) {
z <- f2(x, substitute(y))
# do stuff with z
w <- z
return(w)
}
f2 <- function(x, y) {
eval(substitute(subset(x, select = -Y), list(Y = y)))
}
f1(mtcars,mpg)
# cyl disp hp drat wt qsec vs am gear carb
# Mazda RX4 6 160.0 110 3.90 2.620 16.46 0 1 4 4
# Mazda RX4 Wag 6 160.0 110 3.90 2.875 17.02 0 1 4 4
# Datsun 710 4 108.0 93 3.85 2.320 18.61 1 1 4 1
# ...

Answer 2

With the newer version of rlang 0.4.0, we can use the {{...}} (curly-curly) that makes it easier to do the evaluation
library(rlang)
f1 <- function(x, y) {
z <- f2(x, -{{y}})
# do stuff with z
w <- z
return(w)
}
f2 <- function(x, y) {
select(x, {{y}})
}
f1(mtcars,mpg) %>%
head
# cyl disp hp drat wt qsec vs am gear carb
#Mazda RX4 6 160 110 3.90 2.620 16.46 0 1 4 4
#Mazda RX4 Wag 6 160 110 3.90 2.875 17.02 0 1 4 4
#Datsun 710 4 108 93 3.85 2.320 18.61 1 1 4 1
#Hornet 4 Drive 6 258 110 3.08 3.215 19.44 1 0 3 1
#Hornet Sportabout 8 360 175 3.15 3.440 17.02 0 0 3 2
#Valiant 6 225 105 2.76 3.460 20.22 1 0 3 1