Question

I have this:

data SomeData = SomeData Int Int

getDataFromUser :: SomeData
getDataFromUser = do
{
    read (getLine)::SomeData;
}

This doesnt compile : Expected type String Actual type IO String

How I can fix it? I need this data deserialization…

Answer 1

You’re trying to treat getLine as a String, but it’s an IO String — an IO action that, when executed, produces a string. You can execute it and get the resulting value from inside a do block by using <-, but since getDataFromUser does IO, its type has to be IO SomeData:

getDataFromUser :: IO SomeData
getDataFromUser = do
  line <- getLine
  return $ read line

More broadly, I would recommend reading a tutorial on IO in Haskell, like Learn You a Haskell’s chapter on IO; it’s very different from the IO facilities of most other languages, and it can take some time to get used to how things fit together; it’s hard to convey a full understanding with answers to specific questions like this 🙂