haskell - 如何在haskell的一行中输入多个值

Question

例如，我想编写一个程序，它将从命令行输入 3 个整数。我学到的函数是readLn从整行读取值。但是readLn似乎将整行解析为单个值。如何使用haskell获得一行的三个值？

Answer 1

阅读一行 getLine ，将其拆分为 words , 和 read每个:

readInts :: IO [Int]
readInts = fmap (map read.words) getLine

它读取任意数量的 Int:

ghci> readInts
1 2 3
[1,2,3]
ghci> readInts
1 2 3 4 5 6
[1,2,3,4,5,6]

您可以限制为三个:

read3Ints :: IO [Int]
read3Ints = do
     ints <- readInts
     if length ints == 3 then return ints else do
         putStrLn ("sorry, could you give me exactly three integers, "
                  ++ "separated by spaces?")
         read3Ints

看起来像这样:

ghci> read3Ints
1 2
sorry, could you give me exactly three integers, separated by spaces?
1 23 , 5, 6
sorry, could you give me exactly three integers, separated by spaces?
1,2,3
sorry, could you give me exactly three integers, separated by spaces?
1 3 6

fmap的 secret
fmap有点像 map ，但您可以更广泛地使用它:

ghci> fmap (*10) [1,2,3,4]
[10,20,30,40]
ghci> fmap (*10) (Just 5)
Just 50
ghci> map (fmap (*10)) [Left 'b', Right 4, Left 'c', Right 7]
[Left 'b',Right 40,Left 'c',Right 70]
ghci> fmap words getLine
Hello there me hearties!
["Hello","there","me","hearties!"]

在 getInts ，我做了 fmap (map read.words)按空格分割行，然后 map read把每一个变成一个 Int .编译器知道我想要 Int由于类型签名 - 如果我省略它，我会得到一个错误。