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 ?

do yourself a favor and use data Parser a = MkParser { runParser :: String -> Maybe (a , String ) } instead. it will be much less confusing. you can always remove the Mks if you have to, afterwards. — Will Ness
– Will Ness, Commented Oct 2, 2021 at 20:38
to use a parser, you get at its function via pattern matching, or by using runParser. read more about it by searching for "Haskell record syntax". — Will Ness
– Will Ness, Commented Oct 2, 2021 at 21:18

willeM_ Van Onsem · Accepted Answer · 2021-10-02 18:58:30Z

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

answered Oct 2, 2021 at 18:58

willeM_ Van Onsem

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