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Haskell IO代码不进行类型检查

typechecking monads io haskell

3

Alex Baranosky · 技术社区 · 14 年前

提前谢谢。

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

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 }

1 回复 | 直到 12 年前

1

9

sepp2k 14 年前

map parsePerson lines 具有类型 [Person] ,但由于您需要 loadPeople 是 IO [Person] ,你得把它包起来 IO return :

return $ map parsePerson lines