[英]Haskell - How do I write a tuple list to a text file
我从 .txt 获取文本,对其进行处理。 我从文本中得到元音和它们的数量。 我无法将元组列表 [(Char, Int)] 写入文本文件。 我想让每一行都有一个字母和它的编号,但我根本不会写。
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import Data.List
import Char
add :: Eq a => a -> [(a, Int)] -> [(a, Int)]
add x [] = [(x, 1)]
add x ((y, n):rest) = if x == y
then (y, n+1) : rest
else (y, n) : add x rest
count :: Eq a => [a] -> [(a, Int)]
count = sortBy f . foldr add [] where
f (_, x) (_, y) = compare y x
ff x = filter (\x->elem (fst x) "aeyioAEYIO") x
fff x = ff (count x)
main :: IO ()
main = do
src <- readFile "input.txt"
writeFile "output.txt" (operate src)
operate :: [(Char, Int)] -> String
operate = fff
它给出了一个错误:
*** Term : operate
*** Type : [Char] -> [(Char,Int)]
*** Does not match : [(Char,Int)] -> String
操作类型错误,因为 fff 的类型为[Char] -> [(Char, Int)]
operate :: [(Char, Int)] -> String
operate = fff
类型推断建议[Char] -> [(Char, Int)]
,
很好,但是返回类型仍然需要是[(String, Int)]
如果我们想把这个函数的输出提供给formatOne
formatOne :: Show a => (String, a) -> String
formatOne (s, i) = s ++ " : " ++ show i
operate :: String -> [(String, Int)]
operate = map (\(x,y) -> (x:"", y)) . fff
-- (\(x,y) -> (x:"", y)) this lambda turns first element of tuple from Char to String
-- unlines joins elements of list into string while separating elements with \n
formatAll = unlines . map formatOne
main :: IO ()
main = do
src <- readFile "input.txt"
writeFile "output.txt" (formatAll (operate src))
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