Arindam Roy wrote:
I'm trying to learn Haskell using Hutton's book and Hugs (Sep 06), and am trying to work the parsing examples (chapter 8, Functional parsers), but I can't understand the error I'm seeing when I try to load the program into Hugs.
Following the book, I've defined a Parser type, then written the basic parsers, then a sequencing operator >>=, followed by the example p parser (bottom of page 77). However, trying to load this file in Hugs generates an error (caused by the definition of p).
To make do notation work, you have to (1) use a data type for Parser (instead of a type synonym) and (2) declare that data type an instance of the Monad typeclass. Your code should look something like this: data Parser = Parser (...) instance Monad Parser where return v = ... Parser p >>= f = ... ...
(>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v, out)] -> parse (f v) out
What happens if p returns more then one result? Tillmann