Davis Vaughan

I have a feeling this would be in the verb itself if we did this, like: ```r df %>% mutate(pct = x/sum(x), .groups = y) ```

Alternative argument name would be `.by`

In my brain, if we were to add it to one of the major verbs, then we would add it to all of them (Oops accidentally closed sorry)

@hadley do you have any idea how it would support rowwise? Maybe it wouldn’t? It’s a little tricky because `.by` would either be `NULL` or a tidyselect expression of columns...

I'm also not sure how to support `dplyr::group_by(.drop = FALSE)` with `.by`. Like this? Adding `.drop` everywhere feels very clunky. ```r df %>% summarise(avg = mean(x), .by = fct, .drop...

I'm not sure what is so bad about ```r data %>% summarize( mu = mean(x), sigma = sd(x), .group_by = c(var1, var2, subject_id) ) ``` I feel like it nicely...

Oh I didn't realize you meant that `c()` was the extra level of nesting. Yea that looks "flat" to me, not as another layer of nesting. Also FWIW the IDE...

Note to change your initial comment in the PR to "Closes ..."

I have never actually needed `dplyr_col_modify()` (`dplyr_reconstruct()` was always enough for me), but tsibble does. They call the data frame method by converting the tsibble input to a tibble and...

As for setting it up, like mentioned above, could we do something like this? Like we do in `dplyr_reconstruct()` and in vctrs? ```r dplyr_col_modify