[Haskell-cafe] Re: Scheme in Haskell, Parsec Example, how to add scheme comments

[Berlin Brown]

On Nov 18, 2007 8:01 PM, Thomas Schilling <nominolo at googlemail.com> wrote:
> On Sun, 2007-11-18 at 19:37 -0500, Berlin Brown wrote:
> > On Nov 18, 2007 7:32 PM, Berlin Brown <berlin.brown at gmail.com> wrote:
> > > I am sure many of you have looked at the scheme in haskell example that
> > > is on the web by Jonathan Tang. If you are familiar with the code, I
> > > need a little help trying to add scheme style comments:
> > >
> > > "; This is my comment"
>
> The preferred way to do that is to use a token helper function:
>
>   token :: P a -> P a
>   token p = do r <- p
>                whiteSpace
>                return r
>
>   -- or, if you add a Control.Applicative instance for your
>   -- parser type, this is just: token p = p <* whiteSpace
>
> Then you handle comments as whitespace:
>
>   whiteSpace :: P ()
>   whiteSpace = skipMany $
>         spaces
>     <|> (char ';' >> skipMany (satisfy (/='\n')))
>
> Then you just use that like this:
>
>   symbol :: P String
>   symbol = token $ many1 $ satisfy $ not . (`elem` "()[]; ")
>
> See also Parsec's TokenParser.
>
>

token :: Parser -> Parser String
token p = do r <- p
                  whiteSpace
                  return $ String r


I know I am being lazy, but what am I missing in your pseudo code: I
tried playing with your example but kept getting these errors:

Parsec3.hs:23:13:
    The last statement in a 'do' construct must be an expression

at the token line.



-- 
Berlin Brown
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