[Haskell-beginners] Help with monad syntax?

Tillmann Rendel rendel at daimi.au.dk
Mon Aug 25 10:29:08 EDT 2008

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?


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