用 T 建立副作用

R 中的某些函式產生副作用（即儲存，列印，繪圖等）並且不總是返回有意義或期望的值。

%T>%（tee operator）允許你將值轉換為副作用生成函式，同時保持原始 lhs 值不變。換句話說：tee 運算子就像%>%一樣工作，除了返回值是 lhs 本身，而不是 rhs 函式/表示式的結果。

示例：建立，管道，寫入和返回物件。如果在本例中使用%>%代替%T>%，則變數 all_letters 將包含 NULL 而不是 sort 物件的值。

all_letters <- c(letters, LETTERS) %>%
    sort %T>% 
    write.csv(file = "all_letters.csv")

read.csv("all_letters.csv") %>% head()
#   x
# 1 a
# 2 A
# 3 b
# 4 B
# 5 c
# 6 C

警告：將未命名的物件管道傳輸到 save() 將在使用 load() 載入到工作區時生成名為 . 的物件。但是，可以使用輔助函式進行解決方法（也可以作為匿名函式內聯編寫）。

all_letters <- c(letters, LETTERS) %>%
    sort %T>% 
    save(file = "all_letters.RData")

load("all_letters.RData", e <- new.env())

get("all_letters", envir = e)
# Error in get("all_letters", envir = e) : object 'all_letters' not found

get(".", envir = e)
#  [1] "a" "A" "b" "B" "c" "C" "d" "D" "e" "E" "f" "F" "g" "G" "h" "H" "i" "I" "j" "J" 
# [21] "k" "K" "l" "L" "m" "M" "n" "N" "o" "O" "p" "P" "q" "Q" "r" "R" "s" "S" "t" "T" 
# [41] "u" "U" "v" "V" "w" "W" "x" "X" "y" "Y" "z" "Z"

# Work-around
save2 <- function(. = ., name, file = stop("'file' must be specified")) {
  assign(name, .)
  call_save <- call("save", ... = name, file = file)
  eval(call_save)
}

all_letters <- c(letters, LETTERS) %>%
    sort %T>%
    save2("all_letters", "all_letters.RData")