映射嵌套列表时dplyr::select报NULL错误的排查与解决

之前用类似代码处理小嵌套列表能正常工作，现在处理包含89个元素的大嵌套列表时，同一逻辑的代码抛出错误。

嵌套列表结构 #

str(df_raw_comments_threads)
List of 89
 $ tgg001              :List of 2
  ..$ comments:'data.frame':    992 obs. of  12 variables:
  .. ..$ date_utc      : chr [1:992] "2019-02-05" "2019-02-05" "2019-02-05" "2019-02-05" ...
  .. ..$ timestamp     : num [1:992] 1.55e+09 1.55e+09 1.55e+09 1.55e+09 1.55e+09 ...
  .. ..$ subreddit     : chr [1:992] "hardwareswap" "hardwareswap" "hardwareswap" "hardwareswap" ...
  .. ..$ thread_author : chr [1:992] "aagarwal82" "aagarwal82" "aagarwal82" "[deleted]" ...
  .. ..$ comment_author: chr [1:992] "tgg001" "tgg001" "tgg001" "tgg001" ...
  .. ..$ thread_title  : chr [1:992] "[USA-NC] [H] NVIDIA GTX 1080 TI MSI Duke [W] PayPal" "[USA-NC] [H] NVIDIA GTX 1080 TI MSI Duke [W] PayPal" "[USA-NC] [H] NVIDIA GTX 1080 TI MSI Duke [W] PayPal" "[USA-VA] [H] G.Skill Trident Z RGB 32GB 4 x 8GB DDR4-3200, Enermax Liqtech TR4 II 280mm, Asus X399 Zenith Extre"| __truncated__ ...
  .. ..$ comment       : chr [1:992] "Yes I paid for it" "SOLD TO ME W\017" "check pm" "Well I need the other 16gb.... OP can we split this?" ...
  .. ..$ score         : num [1:992] 1 1 1 1 1 1 1 1 1 2 ...
  .. ..$ up            : num [1:992] 1 1 1 1 1 1 1 1 1 2 ...
  .. ..$ downs         : num [1:992] 0 0 0 0 0 0 0 0 0 0 ...
  .. ..$ golds         : num [1:992] 0 0 0 0 0 0 0 0 0 0 ...
  ..$ threads :'data.frame':    62 obs. of  11 variables:
  .. ..$ date_utc : chr [1:62] "2019-01-22" "2019-01-21" "2019-01-21" "2018-11-29" ...
  .. ..$ timestamp: num [1:62] 1.55e+09 1.55e+09 1.55e+09 1.54e+09 1.54e+09 ...
  .. ..$ subreddit: chr [1:62] "FortniteCompetitive" "FortNiteBR" "FortNiteBR" "Kanye" ...
  .. ..$ author   : chr [1:62] "tgg001" "tgg001" "tgg001" "tgg001" ...
  .. ..$ title    : chr [1:62] "Well rip my 4th consecutive solo win streak today. he had 1 kill and 50 health... clearly the better player" "15 kills later basically solo squad and this is how I die. Epic can we vault trees?" "I went AFK for 20 seconds and I came back at the perfect time lol" "=«>â Is he wavy? >\024 If not try to roast him with Kanye lyric-related comments" ...
  .. ..$ text     : chr [1:62] "" "" "" "" ...
  .. ..$ golds    : num [1:62] 0 0 0 0 0 0 0 0 0 0 ...
  .. ..$ score    : num [1:62] 0 3289 3115 0 41 ...
  .. ..$ ups      : num [1:62] 0 3289 3115 0 41 ...
  .. ..$ downs    : num [1:62] 0 0 0 0 0 0 0 0 0 0 ...

需求 #

从每个列表元素下的comments和threads两个数据框中，提取subreddit和date_utc字段，最终保留原嵌套结构。

第一次尝试的代码及错误 #

df_map_date_sub <- map2( 
  df_raw_comments_threads |> map(~ .x$comments |> dplyr::select("subreddit", "date_utc")), 
  df_raw_comments_threads |> map(~ .x$threads |> dplyr::select("subreddit", "date_utc")), 
  ~ list(comments = .x, threads = .y) )

错误信息：

Error in UseMethod("select") : 
  no applicable method for 'select' applied to an object of class "NULL"

排查尝试 #

• 重启R，问题依旧
• 检查列表中是否存在NULL值，执行以下代码：

group_errs = df_map_comments_threads %>%  map(~ .x$comments |> 
     keep(~is.null(.x) ) ) %>%
     names()
> length(group_errs)
[1] 89

结果显示有89个元素，这让我很困惑，不知道问题出在哪。

第二次尝试的代码及错误 #

换了另一种写法，还是报错：

df_map_date_sub <- map2( 
   df_raw_comments_threads |> map(.x$comments, ~ dplyr::select(.,subreddit, date_utc)), 
   df_raw_comments_threads |> map(.x$threads, ~ dplyr::select(.,subreddit, date_utc)), 
   ~ list(comments = .x, threads = .y) )

错误信息：

Error in as_mapper(.f, ...) : object '.x' not found

内容的提问来源于stack exchange，提问作者LuCama