Using dplyr's enquo to access Spark table columns via sparklyr - r

Question

I would like to be ebale to use dplyr's enquo within lapply call while jumping through Spark table columns.
lapply(tbl_vars(sprkTbl),
function(col_nme) {
print(col_nme)
# Enquoe column name
quo_col_nme <- enquo(col_nme)
print(quo_col_nme)
sprkTbl %>%
select(!!quo_col_nme) %>%
# do stuff
collect -> dta_res
}) -> l_res
However, when I try to run this code I keep on getting error:
Error in (function (x, strict = TRUE) : the argument has already
been evaluated
I've isolated the error to enquo:
>> lapply(tbl_vars(sprkTbl),
... function(col_nme) {
... print(col_nme)
... # Enquoe column name
... quo_col_nme <- enquo(col_nme)
... # print(quo_col_nme)
...
... # sprkTbl%>%
... # select(!!quo_col_nme) %>%
... # # do stuff
... # collect -> dta_res
... }) -> l_res
[1] "first_column_in_spark"
(and then the same error)
Error in (function (x, strict = TRUE) : the argument has
already been evaluated
I want to understand why enquo can't be used like that. tbl_vars returns an ordinary character vector, shouldn't the col_name be a string? I would envisage for the syntax to work in the same manner as in:
mtcars %>% select(!!enquote("am")) %>% head(2)
am
Mazda RX4 1
Mazda RX4 Wag 1
but, clearly this is not the case, when called from within lapply.
Edit
leaving the sparklyr aspect on side, a better and more reproducible example can be provided:
lapply(names(mtcars),function(x) {
col_enq <- enquo(x)
mtcars %>%
select(!!col_enq) %>%
head(2)
})
produces identical error.
Desired results
The old _-based syntax works
lapply(names(mtcars),function(x) {
# col_enq <- enquo(x)
mtcars %>%
select_(x) %>%
head(2)
})
In a word, I want to achieve the same functionality by jumping to Spark table columns and I would prefer not use deprecated select_.

Answer 1

Do I understand your question correctly that you are interested in this result? Or are you bound to use enquo instead of quo?
library(dplyr)
lapply(names(mtcars),function(x) {
col_enq <- quo(x)
mtcars %>%
select(!!col_enq) %>%
head(2)
})
#> [[1]]
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
#>
#> [[2]]
#> cyl
#> Mazda RX4 6
#> Mazda RX4 Wag 6
#>
#> [[3]]
#> disp
#> Mazda RX4 160
#> Mazda RX4 Wag 160
#>
#> [[4]]
#> hp
#> Mazda RX4 110
#> Mazda RX4 Wag 110
#>
#> [[5]]
#> drat
#> Mazda RX4 3.9
#> Mazda RX4 Wag 3.9
#>
#> [[6]]
#> wt
#> Mazda RX4 2.620
#> Mazda RX4 Wag 2.875
#>
#> [[7]]
#> qsec
#> Mazda RX4 16.46
#> Mazda RX4 Wag 17.02
#>
#> [[8]]
#> vs
#> Mazda RX4 0
#> Mazda RX4 Wag 0
#>
#> [[9]]
#> am
#> Mazda RX4 1
#> Mazda RX4 Wag 1
#>
#> [[10]]
#> gear
#> Mazda RX4 4
#> Mazda RX4 Wag 4
#>
#> [[11]]
#> carb
#> Mazda RX4 4
#> Mazda RX4 Wag 4