[英]Haskell: Question about Partial Application
我正在閱讀“為了大好而學習你的哈斯克爾!”這本書。 由Miran Lipovaca撰寫並在第5章中學習高階函數。
其中一個示例涉及以下功能:
applyTwice :: (a -> a) -> a -> a
applyTwice f x = f (f x)
以下是函數輸出的示例:
ghci> applyTwice (++ " HAHA") "HEY"
"HEY HAHA HAHA"
ghci> applyTwice ("HAHA " ++) "HEY"
"HAHA HAHA HEY"
對於第一個示例,我理解字符串是通過以下方式使用串聯運算符生成的:
"HEY" ++ " HAHA"
"HEY HAHA" ++ " HAHA"
"HEY HAHA HAHA"
但是,我不明白連接運算符在第二個示例中是如何工作的。 輸出字符串“HAHA HAHA HEY”是如何產生的? 任何見解都表示贊賞。
對於第一個示例,我理解字符串是通過以下方式使用串聯運算符生成的:
"HEY" ++ " HAHA" "HEY HAHA" ++ " HAHA" "HEY HAHA HAHA"
而不是直接跳轉到中綴表達式(即++
介於其間),如果您根據函數進行思考,它會有所幫助。
(++ " HAHA") :: [Char] -> [Char] -- #1 this is a function (++ is partially applied)
"HEY" :: [Char]
(++ " HAHA") "HEY" -- apply "HEY" as an argument to #1
-- same as "HEY" ++ " HAHA"
(+) :: (Num a) => a -> a -> a -- #2 a binary function
(+) 1 2 -- #3 apply 1 and 2 as arguments to #2
-- same as 1 + 2
-- technically, #3 is curried as
-- ((+) 1) 2 -- i.e. (+) 1 is a partially applied function, which is then applied to 2
如果你將(++ " HAHA")
替換為applyTwice
的定義,你就得到了
applyTwice f x = f (f x)
applyTwice (++ " HAHA") "HEY" = (++ " HAHA") ((++ " HAHA") "HEY")
= (++ " HAHA") ("HEY" ++ " HAHA")
= (++ " HAHA") ("HEY HAHA")
= "HEY HAHA" ++ " HAHA"
= "HEY HAHA HAHA"
現在使用applyTwice ("HAHA " ++) "HEY"
。
applyTwice f x = f (f x)
applyTwice ("HAHA " ++) "HEY" = ("HAHA " ++) (("HAHA " ++) "HEY")
= ("HAHA " ++) ("HAHA " ++ "HEY")
= ("HAHA " ++) ("HAHA HEY")
= "HAHA " ++ "HAHA HEY"
= "HAHA HAHA HEY"
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