flag argument ?

[moodymudskipper]

It seems to be a common request to be able to identify the matched observation or the source, is flag a good name ?

flag would be a character vector of length 1 to 3 containing either :

• length 1 : The name of a boolean column that would be TRUE for matched observation
• length 2 : The names of columns containing TRUE when the observation was matched respectively in .x or .y
• length 3 : The latter + the former

flag would be NULL by default

Implementation wise we mutate some columns on each side, named as the first 2 elements of flag, or unique temp names from the unique flag element, then join, fill with FALSE, and combine accordingly.

Common use case would probably be :

left_join(x, y, flag ="matched")

left_join(
  x,
  y |> select_keys_and(), 
  flag ="matched"
)

In the latter case select_keys_and() is a bit awkwar, it is worth having select_keys() which would do the same but be more limited in scope and look better ?