Haskell IO 代码不进行类型检查

Question

我是 Haskell 的初学者，在理解一些代码时遇到困难。我需要做什么才能在代码的此 IO 部分获得正确的类型？

提前致谢。

loadPeople :: FilePath -> IO [Person]
loadPeople file = do
   lines <- getLines file
   map parsePerson lines

getLines :: FilePath -> IO [String]
getLines = liftM lines . readFile

parsePerson :: String -> Person
parsePerson line = ...........

map 在 Leksah 中用红色下划线表示，我收到的编译错误是:

src\Main.hs:13:3:
    Couldn't match expected type `IO [Person]'
           against inferred type `[Person]'
    In the expression: map parsePerson lines
    In the expression:
        do { lines <- getLines file;
             map parsePerson lines }
    In the definition of `loadPeople':
        loadPeople file
                     = do { lines <- getLines file;
                            map parsePerson lines }

Answer 1

map parsePersonlines 的类型为 [Person]，但由于您需要 loadPeople 的结果类型为 IO [Person] ，您需要使用return将其包装在IO中:

return $ map parsePerson lines