Haskell : moving from 'type' to 'data'

Question

I'm writing a parser

And it's working as it is :

type Parser a = String -> Maybe (a ,String)

parseInt :: Parser Int
parseInt "" =  Nothing 
parseInt s = case reads s ::[(Int, String)] of 
            [(x, s)]-> Just(x, s)
            _ -> Nothing


parseChar :: Char -> Parser Char
parseChar a  str | a == head str = Just (a, tail str)
                 |  otherwise = Nothing

But i need too change type Parser a = String -> Maybe (a ,String)

to

data Parser a = Parser { runParser :: String -> Maybe (a , String ) }

As soon as I do, it every function i wrote do not compile anymore, what are the step to do this change, and, what is happening ?

Answer 1

You will need to use the Parser data constructor and thus wrap the function into that data constructor, so:

parseInt :: Parser Int
parseInt = Parser f
  where f "" =  Nothing 
        f s = case reads s ::[(Int, String)] of 
                [(x, s)]-> Just(x, s)
                _ -> Nothing

parseChar :: Char -> Parser Char
parseChar a = Parser f
  where f str | a == head str = Just (a, tail str)
              |  otherwise = Nothing