How to make Parser explicitly "fail"?

[Leonid Bouriakovsky]

If you are using the ParseLib from the Hugs distribution,
the function you are looking for is mzero:

C:\Hugs98\lib\hugs\ParseLib.hs:

instance MonadPlus Parser where
   -- mzero            :: Parser a
   mzero                = P (\inp -> [])

   -- mplus            :: Parser a -> Parser a -> Parser a
   (P p) `mplus` (P q)  = P (\inp -> (p inp ++ q inp))

For the explanations on how this function works and
why it has such a name you can take a look at the
following paper:

Monadic parser combinators
Graham Hutton and Erik Meijer. Technical Report NOTTCS-TR-96-4,
Department of Computer Science, University of Nottingham, 1996.

http://www.cs.nott.ac.uk/Department/Staff/gmh/bib.html#monparsing

Regards,

Leonid Bouriakovsky


> Hi, there. I am glad to join Haskell community. :)
> 
> Here I have a question about Parser: how to make a
> parser explicitly "fail"?
> 
> My question arises from the syntax below:
> 
> <statement>::=<keyword><seperator><variable>
> <keyword>::="define"
> <seperator>::=':'
> <variable>::=identifier other than <keyword>
> <identifier>::=(<letter>)+
> <letter>::=a..z|A..Z
> 
> Now I am thinking of writing a Parser for variable. My
> thought is:
> 
> variable::Parser String
> variable = do s<-identifier
>               if not (s=="define") then return s else
> --let parser fail
> 
> I will be very happy if there is some mechanism to
> implement the "else" branch, for I have thought of it
> for long. :)
> 
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