How to perform a rolling coalesce of columns in R

I’ve got a dataframe like this one:

stage1 stage2 stage3 stage4
a        NA     b      c
NA       d      NA     e
NA       NA     f      g
NA       NA     NA     h

Where each column is a stage from a process. What I want to do is to coalesce each column based on the previous columns:

stage1 stage2 stage3 stage4 
a        a      a      a
NA       d      d      d
NA       NA     f      f
NA       NA     NA     h

The actual values don’t really matter, this could also be a logical dataframe, where each string from the output is TRUE and each NA is FALSE .

I’ve written this function that lets me coalesce through a selection of columns:

coacross <- function(...) {
  coalesce(!!!across(...))
}

df <- df %>%
  mutate(total_stages = coacross(everything()))

Which basically creates stage4 column of my desired output. Is there any way to iteratively run this, ideally without a for loop? So I can do the same for stage2 and stage3? Else, is there another way to do this?

Thanks a lot.

Edit:

This works:

for(col in names(df %>% select(-stage1))){
  print(col)
  df = df %>%
    mutate({{col}} := coacross(stage1:{{col}}))
  
}

But any more elegant solutions are greatly appreciated

>Solution :

You can use across() with an assist from cur_column():

library(dplyr)

df %>%
  mutate(across(everything(), \(x) coacross(stage1:cur_column())))

  stage1 stage2 stage3 stage4
1      a      a      a      a
2   <NA>      d      d      d
3   <NA>   <NA>      f      f
4   <NA>   <NA>   <NA>      h