1

我收到以下错误：

Couldn't match expected type `IO b' with actual type `Bool'
In the expression:
  upCheck
    ["########", "#11xxx3#", "#xx2xx3", "#002xx3", ....] 2 [2, 3, 4]
In the expression:
  do { s <- readFile "projekt-board.txt";
       let content = lines s;
       printTable content;
       printHelp;
       .... }
In an equation for `gameFunc':
    gameFunc
      = do { s <- readFile "projekt-board.txt";
             let content = ...;
             printTable content;
             .... }

定义为gameFunc：

gameFunc = do
  s <- readFile "projekt-board.txt"
  let content = lines s
  printTable content
  printHelp
  upCheck ["########","#11xxx3#","#xx2xx3","#002xx3","#7x2x44#"] 2 [2,3,4]

和功能upCheck：

upCheck :: [[Char]] -> Int -> [Int] -> Bool
upCheck tableArr number posArr = do
  --posArr saadakse findBlock funktsiooniga
  let arv = findVIndex (findVertIndex tableArr number)
  let str = tableArr !! (posArr !! 0)
  if ((posArr !! 0) == 1)
  then False
  else if arv == (-1)
    then False
    else if isHorizontal str number
      then False
      else if (tableArr !! ((posArr !! 0)-1)) !! arv /= 'x'
        then False
        else True

所以基本上这里发生的upCheck是一个检查某些条件是否匹配的函数，当我从 ghci 调用它的方式与我从函数调用它的方式相同时gameFunc，我会得到一个结果True。但是，如果我从函数中调用它，则会gameFunc收到上述错误。任何想法出了什么问题？

4

1 回答 1

2

问题是您的函数gameFunc使用 do 表达式，它是monad（在本例中为IOmonad）的语法糖。return您可以使用以下函数将任何非单子结果转换为单子结果：

return :: Monad m => a -> m a

所以你可以重写你gameFunc的（见最后一行）：

gameFunc = do
  s <- readFile "projekt-board.txt"
  let content = lines s
  printTable content
  printHelp
  return $ upCheck ["########","#11xxx3#","#xx2xx3","#002xx3","#7x2x44#"] 2 [2,3,4]

现在gameFunc有了类型IO Bool。您可以gameFunc在其他单子环境中使用，例如：

main = do
    b <- gameFunc
    print b

为了打印结果gameFunc。

于 2016-01-05T09:40:35.810 回答