Haskell 方式将 [IO String] 加入 IO String

Question

我的目标是编写 Haskell 函数，该函数从输入中读取 N 行并将它们连接到一个字符串中。下面是第一次尝试:

readNLines :: Int -> IO String
readNLines n = do
  let rows = replicate n getLine
  let rowsAsString = foldl ++ [] rows 
  return rowsAsString  

这里haskell 投诉 foldl :

Couldn't match expected type [a]'
against inferred type(a1 -> b -> a1) -> a1 -> [b] -> a1'

据我了解，行类型是 [IO String] ，是否有可能将这样的列表加入单个 IO String ?

Answer 1

您正在寻找 sequence :: (Monad m) => [m a] -> m [a] .

(加上 liftM :: Monad m => (a1 -> r) -> m a1 -> m r 和 unlines :: [String] -> String ，可能。)