split apply combine exploration, adding more to this post https://coolbutuseless.bitbucket.io/2018/03/03/split-apply-combine-my-search-for-a-replacement-for-group_by---do/

pmap-and-map2-also-useful.R

library(tidyverse)

## approximates big_df in the post
big_df <- mtcars %>% 
  select(cyl, mpg, disp) %>% 
  arrange(cyl) %>% 
  slice(17:22) %>% 
  rename(ID = cyl)

## dummy function that needs access to ID and data
complex_func2 <- function(ID, data) {
  tibble(
    ID = ID,
    half = ID / 2,
    N = nrow(data)
  )
}

big_df %>% 
  group_by(ID) %>%
  nest() %>%
  pmap_dfr(complex_func2)
#> # A tibble: 2 x 3
#>      ID  half     N
#>   <dbl> <dbl> <int>
#> 1    6.    3.     2
#> 2    8.    4.     4

big_df_nested <- big_df %>% 
  group_by(ID) %>%
  nest()
map2_dfr(big_df_nested$ID, big_df_nested$data, complex_func2)
#> # A tibble: 2 x 3
#>      ID  half     N
#>   <dbl> <dbl> <int>
#> 1    6.    3.     2
#> 2    8.    4.     4

## OP says:

## "this requires writing a version of "complex_func" to be explicitly aware of
## the grouping variable. When grouping changes you will need to update the
## function signature *and* the pmap call.  High cognitive overhead?"

## This arises in my work when I've been using "ID" as group var in one project.
## In similar project, IDs have been re-used within GROUPs. so now group_by(ID,
## GROUP) is necessary to get data.frames for individual entities.
## complex_func doesn't change, but grouping does.

## JB: So all approaches will need a new grouping statement. And the body for
## complex_func needs or change (or not) for all approaches as well. So if this
## is just about the signature of complex_func ... this is getting very
## split-y but w/o the replication of the key vars

## dummy function that needs access to key vars and data
complex_func3 <- function(x) {
  tibble(
    ID = x$ID,
    half = x$ID / 2,
    N = nrow(x$data)
  )
}

big_df %>% 
  group_by(ID) %>% 
  nest() %>% 
  transpose() %>% 
  map_dfr(complex_func3)
#> # A tibble: 2 x 3
#>      ID  half     N
#>   <dbl> <dbl> <int>
#> 1    6.    3.     2
#> 2    8.    4.     4

#' Created on 2018-03-03 by the [reprex package](http://reprex.tidyverse.org) (v0.2.0).