Using glue-like constructs on RHS in R/Tidyeval - r

Question

I've spent hours trying to make glue on the RHS of a formula work and out of clues. Here is a simple reprex.
meta <- function(x, var, suffix){
x<- x %>% mutate("{{var}}_{suffix}":= 5)
x<- x %>% mutate("{{var}}_{suffix}_new":= {{var}} - "{{var}}_{suffix}")
}
x<- meta(mtcars, mpg, suf)
#Should be equivalent to
x<- mtcars %>% mutate(mpg_suf:= 5)
x<- x%>% mutate(mpg_suf_new:= mpg - mpg_suf)
#N: Tried https://stackoverflow.com/questions/70427403/how-to-correctly-glue-together-prefix-suffix-in-a-function-call-rhs but none of the methods in it worked, unfortunately
Meta function gives me "Error in local_error_context(dots = dots, .index = i, mask = mask) :
promise already under evaluation: recursive default argument reference or earlier problems? "
Went over all hits for the searchwords for it on SO but nothing worked at the moment.
Would really appreciate any insights. Thank you!

Answer 1

Here is a working version:
meta <- function(x, var, suffix){
new_name <- rlang::englue("{{ var }}_{{ suffix }}")
x %>%
mutate("{new_name}" := 5) %>%
mutate("{new_name}_new" := {{ var }} - .data[[new_name]])
}
names(meta(mtcars, mpg, suf))
#> [1] "mpg" "cyl" "disp" "hp"
#> [5] "drat" "wt" "qsec" "vs"
#> [9] "am" "gear" "carb" "mpg_suf"
#> [13] "mpg_suf_new"
To understand what is going on:
Learn about the difference between "{{ var }}" and "{var}" in tidyeval glue strings: https://rlang.r-lib.org/reference/glue-operators.html
Learn about englue() to create glue strings outside of the LHS of :=: https://rlang.r-lib.org/reference/englue.html. This part is not necessary but I thought it was nicer to create and reuse a variable.
Tricky part, you create a new column with a constructed name and then want to use the new column that this name refers to. You'll have to subset it with .data, see: https://rlang.r-lib.org/reference/dot-data.html
See also the general topic: https://rlang.r-lib.org/reference/topic-data-mask-programming.html

Answer 2

I think it's best if we define the pieces we need first, then we can use them as needed on the LHS or the RHS of the calculation. I will add that it doesn't make much sense to me to pass the suffix argument as a bare name. I think it would be a clearer choice to make it string only.
library(dplyr)
meta <- function(x, var, suffix) {
var <- rlang::as_name(enquo(var))
suffix <- rlang::as_name(enquo(suffix)) # Remove this to make "suffix" string only.
new_var <- glue::glue("{var}_{suffix}")
x %>%
mutate("{new_var}" := 5,
"{new_var}_new" := !!sym(var) - !!sym(new_var))
}
mtcars %>%
head() %>%
meta(mpg, suf)
mpg cyl disp hp drat wt qsec vs am gear carb mpg_suf mpg_suf_new
Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4 5 16.0
Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4 5 16.0
Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1 5 17.8
Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1 5 16.4
Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2 5 13.7
Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1 5 13.1