Hello, Andre Nathan schrieb: > so I'm wondering what else I need to do for the "do" > notation to work.
> import Prelude hiding ((>>=), return) You explicitly ask for the well-known and standard functions >>= and return to be hidden away, because you want to define your own versions. > p :: Parser (Char, Char) > p = do x <- item > item > y <- item > return (x, y) The do-notation is desugared to some calls of the well-known and standard functions >>= and return, defined in the Prelude. Wich are different from your own functions >>= and return. So you have to connect your definitions of >>= and return to the well-known functions used by the do notation. To do so, you have to declare your Parser type an instance of the somewhat magic do-notation-enableing typeclass Monad. Fortunately, this is not too hard, because you can reuse your functions >>= and return. First, change your Parser type to be a proper data type instead of a type synonym (and change your code to correctly pack / unpack Parser values): > data Parser a = Parser (String -> [(a, String)]) Second, move >>= and return inside an instance declaration: > instance Monad Parser where > (Parser p) >>= f = ... > return v = Parser (\inp -> [(v, inp)]) Third, stop hiding Prelude's >>= and return. > import Prelude Now you should be able to use do notation with your own Parser type. Tillmann _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
