[Haskell-beginners] Help with monad syntax?

[Arindam Roy]

> 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 = ...
> 
>    ...


Thanks, I'll try this. I haven't read upto the data declaration and inheritance
yet, but I'll try it as soon as I have :-)

> 
> > (>>=)   ::      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?
> 


The way Parsers are designed in the book, it can't. Parsers always return a list
with one element. The only reason the return value is a list is to use the empty
list as an error indication.

Arindam