Can you use dynamic column names with tidyeval in an anonymous function?

Question

We can define a function that uses a dynamic column name, then use it in map. Note how the tibbles from ListCol have the column names a and b, taken from the Letter column in the original tibble.

library(tidyverse)

myfunc <- function(col_name) {
  tibble(!!sym(col_name) := c(1, 2))
}

tibble(
  Letter = letters[1:2],
  ListCol = map(Letter, myfunc)
) %>%
  pull(ListCol)
#> [[1]]
#> # A tibble: 2 x 1
#>       a
#>   <dbl>
#> 1     1
#> 2     2
#> 
#> [[2]]
#> # A tibble: 2 x 1
#>       b
#>   <dbl>
#> 1     1
#> 2     2

However, using the same code from the previous myfunc function in an anonymous function gives an error.

tibble(
  Letter = letters[1:2],
  ListCol = map(Letter, function(col_name) {
    tibble(!!sym(col_name) := c(1, 2))
  })
) %>%
  pull(ListCol)
#> Error in is_symbol(x): object 'col_name' not found

^{Created on 2020-01-12 by the reprex package (v0.3.0)}

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Not sure what is happening there. It works separately though, `df <- tibble(Letter = letters[1:2])` and `map(df$Letter, ~tibble(!!sym(.x) := c(1, 2)))` — Ronak Shah, Jan 12 '20 at 12:58
I think this post is referring to the same point https://stackoverflow.com/questions/51902438/rlangsym-in-anonymous-functions/ — Ronak Shah, Jan 12 '20 at 13:17
Yes, [this comment](https://stackoverflow.com/questions/51902438/rlangsym-in-anonymous-functions/#comment90764125_51902438) from Lionel Henry seems to explain the issue — Greg, Jan 12 '20 at 13:23
do you think it is safe to mark this post as a duplicate of that one? Does it answer your question? — Ronak Shah, Jan 12 '20 at 13:25
I'm not sure whether it's a duplicate because in the other question, `!!` is being used on the right-hand side of a `mutate` rather than the left-hand side. Unfortunately if you try using `eval`, you get `Error: The LHS of := must be a string or a symbol` — Greg, Jan 12 '20 at 13:28

Greg · Answer 1 · 2020-01-12T13:56:12.717

0

Thanks to Ronak Shah for finding this related question in which Lionel Henry explains the issue:

Unquoting is not a function call: it always takes effect at the very first, outermost, quoting function. That's why you have to be a bit careful with anonymous functions. Unquoting happens immediately while anonymous functions denote a scope that is created later on, so there's a timing problem.

This is also linked from Lionel and Hadley Wickham's online book on tidyeval.

Unfortunately the answer to my original question seems to be that it's not possible – you need to create a named function and call this as in the first example.

Unlike in the related question, you cannot use eval(sym(col_name)) to fix the issue.

edited Jan 12 '20 at 13:56

answered Jan 12 '20 at 13:32

Greg

487
5
15

I think there might be alternative `map_*` functions that negate the need for tidy eval and/or an anon. – NelsonGon Jan 12 '20 at 13:33
I think these are for returning different types of object e.g. `map_chr` returns a character vector. Here we want to return a tibble so we use `map` – Greg Jan 12 '20 at 13:35
Yes but I think you can still get your result without using an anonymous function, maybe by using "canonical" `~` syntax. – NelsonGon Jan 12 '20 at 13:38
1

Unfortunately replacing the anonymous function with `~ tibble(!!(sym(col_name)) := c(1, 2))` gives the same error: `Error in is_symbol(x) : object 'col_name' not found` – Greg Jan 12 '20 at 13:40
We plan to fix this in rlang 0.5 by preventing `!!` from crossing `~` and `function` calls (unless qq is explicit like with `expr()`). This will be a breaking change but will help a lot in cases like these. See https://github.com/r-lib/rlang/issues/845. – Lionel Henry Jan 13 '20 at 08:55

score 0 · Answer 2 · answered Apr 28 '20 at 20:17

You have a very good handle on the source of issue. I just wanted to point out that one alternative solution is to rewrite your expression in a way that doesn't use !! unquoting for variables that are internal to the anonymous function (and thus not initialized before the unquoting takes place):

tibble(
  Letter = letters[1:2],
  ListCol = map(Letter, function(col_name) {
      do.call(tibble, set_names(list(c(1,2)), col_name))
  })
) %>%
  pull(ListCol)

Can you use dynamic column names with tidyeval in an anonymous function?

2 Answers2