how to unquote (!!) inside `map` inside `mutate`

This seems to be a feature/bug (not sure, see linked GitHub issue below) of how !! works within mutate and map. The solution is to define a custom function, in which case the unquoting works as expected.

library(tidyverse)

custom_mutate <- function(df, name, string = "anything")
    mutate(df, !!name := string)

foo <- mtcars %>%
  group_by(gear) %>%
  nest %>%
  mutate(name = c("one", "two", "three")) %>%
  mutate(data = map2(data, name, ~
      custom_mutate(.x, .y)))

foo[1,2] %>% unnest %>% names %>% .[11] == "one"
#[1] TRUE

You find more details on GitHub under issue #541: map2() call in dplyr::mutate() error while standalone map2() call works; note that the issue has been closed in September 2018, so I am assuming this is intended behaviour.

---

An alternative might be to use group_split instead of nest, in which case we avoid the unquoting issue

nms <- c("one", "two", "three")

mtcars %>%
    group_split(gear) %>%
    map2(nms, ~.x %>% mutate(!!.y := "anything"))

This is because of the timing of unquoting. Nesting tidy eval functions can be a bit tricky because it is the very first tidy eval function that processes the unquoting operators.

Let's rewrite this:

mutate(data = map2(data, name, ~ mutate(.x, !!.y := "anything")))

to

mutate(data = map2(data, name, function(x, y) mutate(x, !!y := "anything")))

The x and y bindings are only created when the function is called by map2(). So when the first mutate() runs, these bindings don't exist yet and you get an object not found error. With the formula it's a bit harder to see but the formula expands to a function taking .x and .y arguments so we have the same problem.

In general, it's better to avoid complex nested logic in your code because it makes it harder to read. With tidy eval that's even more complexity, so best do things in steps. As an added bonus, doing things in steps requires creating intermediate variables which, if well named, help understand what the function is doing.