Forget me not - dplyr::ungroup()

If you forget to ungroup() an operation after a group_by() it can lead to unwanted behaviour.

Question: What if we didn’t have to remember to ungroup after every chain?

My “just be annoying” non-solution - Part 2

Since I almost never want to leave grouped data in my workspace, just make a noisy version of the pipe operator (%>%) which

1. warns if you try and operate on a dataset which is already grouped.
2. warns if you the end result of an operation still has groups.

library(magrittr)
library(dplyr)

#-----------------------------------------------------------------------------
# Define a new version of the pipe that always checks for grouped data.
# This is a slightly modified clone of `magrittr::%>%`
#-----------------------------------------------------------------------------
'%>%' <- function (lhs, rhs) {
  parent <- parent.frame()
  env <- new.env(parent = parent)
  chain_parts <- magrittr:::split_chain(match.call(), env = env)
  pipes <- chain_parts[["pipes"]]
  rhss <- chain_parts[["rhss"]]
  lhs <- chain_parts[["lhs"]]
  env[["_function_list"]] <- lapply(
    1:length(rhss),
    function(i) magrittr:::wrap_function(rhss[[i]], pipes[[i]], parent)
  )
  env[["_fseq"]] <- `class<-`(eval(quote(
    function(value) freduce(value, `_function_list`)),
    env, env), c("fseq", "function")
  )
  env[["freduce"]] <- freduce
  if (magrittr:::is_placeholder(lhs)) {
    env[["_fseq"]]
  } else {
    env[["_lhs"]] <- eval(lhs, parent, parent)
    if (!is.null(groups(env[["_lhs"]])) || inherits(env[['_lhs']], 'rowwise_df')) {
      warning("This data.frame already has groups - did you forget to call `ungroup()` earlier?", call. = FALSE)
    }
    result <- withVisible(eval(quote(`_fseq`(`_lhs`)), env, env))
    if (magrittr:::is_compound_pipe(pipes[[1L]])) {
      eval(call("<-", lhs, result[["value"]]), parent, parent)
    }  else {
      if (!is.null(groups(result[["value"]])) || inherits(result[["value"]], 'rowwise_df')) {
        warning("The end result of this operation still has groups - did you mean to call `ungroup()` as well?", call. = FALSE)
      }
      if (result[["visible"]])
        result[["value"]]
      else
        invisible(result[["value"]])
    }
  }
}

#-----------------------------------------------------------------------------
# Use this new pipe instead of the original magrittr pipe and you get 
# a warning if end result is ungrouped
#-----------------------------------------------------------------------------
res1 <- mtcars %>%
  group_by(cyl) 

Warning: The end result of this operation still has groups - did you mean to
call `ungroup()` as well?

#-----------------------------------------------------------------------------
# Get a warning when the initial data.frame is already grouped
#-----------------------------------------------------------------------------
res2 <- res1 %>%
  mutate(mean_mpg = mean(mpg)) %>%
  select(cyl, mean_mpg) %>%
  ungroup()

Warning: This data.frame already has groups - did you forget to call `ungroup()`
earlier?

#-----------------------------------------------------------------------------
# But don't get any warning if it isn't grouped input or output
#-----------------------------------------------------------------------------
res3 <- mtcars %>%
  mutate(mean_mpg = mean(mpg)) %>%
  select(cyl, mean_mpg) %>%
  ungroup()