How can I create colum names in a custom function, pass it to dplyr's select and use purrr's pmap to recycle the custom function?

The following code is supposed to create a list, df_test_purrr, with two dataframes. The two dataframes should contain the columns vs, vs_txt and am, am_txt, respectively.

As a first step, the function fn_test takes an argument for a dataframe, dta, and an argument for a column, col. Using the function directly with the argument dta = df_mtcars, col = expr(vs) creates the dataframe df_test as expected.

# Libaries
library(dplyr)
library(purrr)

# Data
df_mtcars <- mtcars %>%
  dplyr::mutate(vs_txt = dplyr::case_when(vs == 1 ~ "ABC", T ~ "XYZ"),
                am_txt = dplyr::case_when(am == 1 ~ "TTT", T ~ "BBB"))

# Function for one column
fn_test <- function(dta, col) {
  {{dta}} %>% 
    dplyr::select( {{col}}, as.name( paste0( {{col}}, "_txt") ) )
}

# Result for one column
df_test <- fn_test( dta = df_mtcars, col = expr(vs) )

# Purrr
fn_test_purrr <- function() {
  purrr::pmap(list(
    list(df_mtcars, df_mtcars),
    list(expr(vs), expr(am))
  ), fn_test )
}

df_test_purrr <- fn_test_purrr()

Defining and calling the function fn_test_purrr gives the error:

Error in `instrument_base_errors()`:
! object 'vs' not found
Caused by error:
! object 'vs' not found
Run `rlang::last_error()` to see where the error occurred.

How can I make the function work?

Please note that the function fn_test_purrr has more than two arguments in my real-world application such that I must use purrr's pmap (I guess).

Could you please try the below code

# Function for one column
fn_test <- function(dta, col) {
  col2 <- paste0(col,'_txt')
  col2 <- col2[col2!='c_txt']
  dta %>% 
    dplyr::select( {{col}}, col2) 
}

# Result for one column
df_test <- fn_test( dta = df_mtcars, col = expr(c(vs,am)) )

#output

                    vs am vs_txt am_txt
Mazda RX4            0  1    XYZ    TTT
Mazda RX4 Wag        0  1    XYZ    TTT
Datsun 710           1  1    ABC    TTT
Hornet 4 Drive       1  0    ABC    BBB
Hornet Sportabout    0  0    XYZ    BBB
Valiant              1  0    ABC    BBB
Duster 360           0  0    XYZ    BBB
Merc 240D            1  0    ABC    BBB
Merc 230             1  0    ABC    BBB
Merc 280             1  0    ABC    BBB
Merc 280C            1  0    ABC    BBB
Merc 450SE           0  0    XYZ    BBB
Merc 450SL           0  0    XYZ    BBB
Merc 450SLC          0  0    XYZ    BBB
Cadillac Fleetwood   0  0    XYZ    BBB
Lincoln Continental  0  0    XYZ    BBB
Chrysler Imperial    0  0    XYZ    BBB
Fiat 128             1  1    ABC    TTT
Honda Civic          1  1    ABC    TTT
Toyota Corolla       1  1    ABC    TTT
Toyota Corona        1  0    ABC    BBB
Dodge Challenger     0  0    XYZ    BBB
AMC Javelin          0  0    XYZ    BBB
Camaro Z28           0  0    XYZ    BBB
Pontiac Firebird     0  0    XYZ    BBB
Fiat X1-9            1  1    ABC    TTT
Porsche 914-2        0  1    XYZ    TTT
Lotus Europa         1  1    ABC    TTT
Ford Pantera L       0  1    XYZ    TTT
Ferrari Dino         0  1    XYZ    TTT
Maserati Bora        0  1    XYZ    TTT
Volvo 142E           1  1    ABC    TTT

Please check updated code which returns the list with 2 dataframes

df_mtcars <- mtcars %>%
  dplyr::mutate(vs_txt = dplyr::case_when(vs == 1 ~ "ABC", T ~ "XYZ"),
                am_txt = dplyr::case_when(am == 1 ~ "TTT", T ~ "BBB"))

fn_test <- function(dta, col) {
  col2 <- paste0(col,'_txt')
  col2 <- col2[col2!='c_txt']

  if (length(col)==1){
    len=1
    col2 <- paste0(col,'_txt')
  } else if(length(col)>1){
    len <- length(col)-1  
  }
  
  my.list <- list()
  
  for(i in 1:len){
  colx <- paste0({{col}})[i+1]
  colx2 <- col2[i]
  dat <- dta %>% 
    dplyr::select( {{colx}}, colx2) 
    
  my.list[[i]] <- dat
  }
  return(my.list)
}

# Result for one column
mylist <- fn_test( dta = df_mtcars, col = expr(c(vs,am)) )

df1 <- as.data.frame(mylist[1])
df2 <- as.data.frame(mylist[2])

For those who are interested, I came up with a different approach that also works for my purpose:

# Libaries
library(dplyr)
library(purrr)

# Data
df_mtcars <- mtcars %>%
  dplyr::mutate(vs_txt = dplyr::case_when(vs == 1 ~ "ABC", T ~ "XYZ"),
                am_txt = dplyr::case_when(am == 1 ~ "TTT", T ~ "BBB"))


# Function for one column
fn_test <- function(dta, var) {
  dta %>% 
    dplyr::select( {{var}} )
}

# Function call
test <- fn_test(dta = df_mtcars, var = c("vs", "vs_txt"))

# Purrr
fn_test_purrr <- function() {
  purrr::pmap(
    list(
      list(df_mtcars, df_mtcars),
      list(c("vs", "vs_txt"), c("am", "am_txt"))
    ), fn_test
  )
}

# Function call
test_purrr <- fn_test_purrr()

# Results
test_purrr[[1]]
test_purrr[[2]]