#' Flatten a list
#'
#' Flattening a list removes a single layer of internal hierarchy,
#' i.e. it inlines elements that are lists leaving non-lists alone.
#'
#' @param x A list.
#' @param name_spec If both inner and outer names are present, control
#' how they are combined. Should be a glue specification that uses
#' variables `inner` and `outer`.
#' @param name_repair One of `"minimal"`, `"unique"`, `"universal"`, or
#' `"check_unique"`. See [vctrs::vec_as_names()] for the meaning of these
#' options.
#' @inheritParams rlang::args_dots_empty
#' @return A list of the same type as `x`. The list might be shorter
#' if `x` contains empty lists, the same length if it contains lists
#' of length 1 or no sub-lists, or longer if it contains lists of
#' length > 1.
#' @export
#' @examples
#' x <- list(1, list(2, 3), list(4, list(5)))
#' x |> list_flatten() |> str()
#' x |> list_flatten() |> list_flatten() |> str()
#'
#' # Flat lists are left as is
#' list(1, 2, 3, 4, 5) |> list_flatten() |> str()
#'
#' # Empty lists will disappear
#' list(1, list(), 2, list(3)) |> list_flatten() |> str()
#'
#' # Another way to see this is that it reduces the depth of the list
#' x <- list(
#' list(),
#' list(list())
#' )
#' x |> pluck_depth()
#' x |> list_flatten() |> pluck_depth()
#'
#' # Use name_spec to control how inner and outer names are combined
#' x <- list(x = list(a = 1, b = 2), y = list(c = 1, d = 2))
#' x |> list_flatten() |> names()
#' x |> list_flatten(name_spec = "{outer}") |> names()
#' x |> list_flatten(name_spec = "{inner}") |> names()
list_flatten <- function(
x,
...,
name_spec = "{outer}_{inner}",
name_repair = c("minimal", "unique", "check_unique", "universal")
) {
vec_check_list(x)
check_dots_empty()
check_string(name_spec)
# Take the proxy as we restore on exit
proxy <- vec_proxy(x)
# Unclass S3 lists to avoid their coercion methods. Wrap atoms in a
# list of size 1 so the elements can be concatenated in a single list.
proxy <- map_if(proxy, vec_is_list, unclass, .else = list)
out <- list_unchop(
proxy,
ptype = list(),
name_spec = name_spec,
name_repair = name_repair,
error_arg = x,
error_call = current_env()
)
# Preserve input type
vec_restore(out, x)
}
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