using purrr to map dplyr::select

Question

I have a data frame with a bunch of nested data-frames within it, and I'd like to apply dplyr::select to each of those nested data frames. Here's an example

 library(tidyverse)

 mtcars %>%
 group_by(cyl) %>%
 nest %>%
 mutate(data2 = ~map(data, dplyr::select(.,-mpg)))

I would think that this would result in a data frame with three columns. cyl: the number of cylinders, data: the nested data, data2: the same as data except each element would not have the mpg column.

Instead R crashes:

 *** caught segfault ***
address 0x7ffc1e445000, cause 'memory not mapped'

Traceback:
 1: .Call(`_dplyr_mutate_impl`, df, dots)
 2: mutate_impl(.data, dots)
 3: mutate.tbl_df(., data2 = ~map(data, dplyr::select(., -mpg)))
 4: mutate(., data2 = ~map(data, dplyr::select(., -mpg)))
 5: function_list[[k]](value)
 6: withVisible(function_list[[k]](value))
 7: freduce(value, `_function_list`)
 8: `_fseq`(`_lhs`)
 9: eval(quote(`_fseq`(`_lhs`)), env, env)
10: eval(quote(`_fseq`(`_lhs`)), env, env)
11: withVisible(eval(quote(`_fseq`(`_lhs`)), env, env))
12: mtcars %>% group_by(cyl) %>% nest %>% mutate(data2 = ~map(data,     dplyr::select(., -mpg)))

Possible actions:
1: abort (with core dump, if enabled)
2: normal R exit
3: exit R without saving workspace
4: exit R saving workspace

I realize I could get the columns I wanted if I apply the select operation before the nesting, but this would be less analogous with my real problem. Could somebody please explain to me what I am doing wrong here? Thanks for any advice.

Answer 1

You need to move ~ from map to select; or use the comment as @Russ; ~ is used when the function (in this case purrr::map) accepts a formula as argument:

mtcars %>%
    group_by(cyl) %>%
    nest %>%
    mutate(data2 = map(data, ~ select(., -mpg)))

# A tibble: 3 x 3
#    cyl data               data2            
#  <dbl> <list>             <list>           
#1     6 <tibble [7 × 10]>  <tibble [7 × 9]> 
#2     4 <tibble [11 × 10]> <tibble [11 × 9]>
#3     8 <tibble [14 × 10]> <tibble [14 × 9]>