[Haskell-cafe] Help with Programming in Haskell example

[Tillmann Rendel]

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