递归IO出错

Question

为什么这段代码不起作用？

xMal 0 _ = return ()
xMal n text =
 do
  text
    xMal (n-1) text
main = xMal (putStrLn t <- getLine) (n <- readLn)

我希望能够得到一个数字和字符串作为输入.

Answer 1

Haskell不会那样工作,使用你的<-表达式,你进行monadic调用,这些应该放在正确的顺序上.你可能想要:

xMal 0 _ = []
xMal n text = text : xMal (n-1) text

main = do
    n <- readLn
    t <- getLine
    putStrLn $ show $ xMal n t

这xMal是一个纯函数:xMal :: (Eq a, Num a) => a -> t -> [t]复制给定的t对象a时间.

main是一个IO从stdin读取一行并将其作为整数解析的monad,然后它将一行读作一个字符串,最后它显示复制的字符串.就像是:

*Main> main
15
Hello
["Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello","Hello"]

但请注意,您xMal已经存在:replicate :: Int -> a -> [a]已经这样做了.