Haskell IO code doesn't typecheck

Question

I'm a beginner with Haskell and am having trouble figuring out some code. What do I need to do to get the types right on this IO section of my code?

Thanks in advance.

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 is underlined in red in Leksah, and the compile error I am receiving is:

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 }

You could eliminate the "getLines" function by using Functors (see: learnyouahaskell.com/functors-applicative-functors-and-monoids). You can do: "ls <- fmap lines $ readFile file" — Daniel
– Daniel, Commented Nov 9, 2010 at 17:34

sepp2k · Accepted Answer · 2010-11-09 03:11:29Z

9

map parsePerson lines has type [Person], but since you need the result type of loadPeople is IO [Person], you need to wrap it in IO using return:

return $ map parsePerson lines

answered Nov 9, 2010 at 3:11

sepp2k

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2 Comments

Alex Baranosky Over a year ago

YES! Thanks. That makes total sense, now that I think of it.

alternative Over a year ago

Seriously late comment: Better to write this as liftM (parsePerson lines) $ getLines file. Keep it mind that whenever you have a two line monadic function theres usually a better way to write it.

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