Haskell IO代码没有进行类型检查

Question

我是Haskell的初学者,我很难搞清楚一些代码.我需要做什么才能在我的代码的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中用红色加下划线,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 parsePerson lines有类型[Person],但由于你需要结果类型loadPeople是IO [Person],你需要将其包装IO使用return:

return $ map parsePerson lines