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...
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 :)
You need to read up more on how Haskell IO works and make sure you understand it.
A couple of points on your example. If you want to use read to deserialize to SomeData, you need to provide a Read instance for the type. You can use the default one:
data SomeData = SomeData Int Int deriving (Read)
Second: getLine is an IO action that returns a String, not a String; since read wants a String, this is the cause of your error. This is closer to what you want:
getDataFromUser :: IO SomeData
getDataFromUser = do str <- getLine
return (read str)
This can be simplified to the following, but make sure you understand the above example before you worry too much about this:
getDataFromUser :: IO SomeData
getDataFromUser = liftM read getLine
getDataFromUser :: IO SomeData; getDataFromUser = readLn.