[Haskell-cafe] Parsec question

[C K Kashyap]

Thanks Roman .. I'll try and implement laziness to retain the whole list.
Regards,
Kashyap


On Thu, Jul 25, 2013 at 3:41 AM, Roman Cheplyaka <roma at ro-che.info> wrote:

> To construct such an example, you have to ask yourself: when can we get a
> list of more than one element?
>
> Consider this:
>
>   a = char 'a'
>
>   ((a >> a) <|> a) >> a
>
> Suppose that our input is "aa". The result of ((a >> a) <|> a) would be
> the list
>
>   [('a', ""), ('a', "a")]
>
> If you proceed with the first element of the list, the overall parse
> will fail. It can only succeed if you then try the second element.
>
> By the way, you shouldn't confuse Parsec (the library) with the general
> concept of parser combinators or the implementation from the paper you
> reference. The above parse would fail in Parsec as well, despite the
> fact that Parsec allows backtracking.
>
> Roman
>
> * Kashyap CK <ckkashyap at gmail.com> [2013-07-24 08:38:53-0700]
> > There is reference in the paper that empty list indicates failure...so
> > could we just use it like Maybe? I'd like it very much if I could get
> > an example of a missed match by not using the complete match.
> >
> > regards,
> > Kashyap
> >
> > Sent from my Windows Phone
> > From: Roman Cheplyaka
> > Sent: 24/07/2013 8:19 PM
> > To: C K Kashyap
> > Cc: Haskell Cafe
> > Subject: Re: [Haskell-cafe] Parsec question
> > Think about this: if you always take only the first element, why do you
> > need lists at all?
> >
> > Roman
> >
> > * C K Kashyap <ckkashyap at gmail.com> [2013-07-24 19:56:29+0530]
> > > Dear Cafe,
> > >
> > > I am trying to implement[1] parsec in go using the "Monadic Parser
> > > Combinators" paper [2] . I've been able to implement "plus" "bind" and
> > > "many"
> > > While doing the implementation - I looked at bind closely
> > >
> > > bind :: Parser a -> (a -> Parser b) -> Parser b
> > > p `bind` f = \inp -> concat [f v inp' | (v,inp') <- p inp]
> > >
> > > I wondered if the result needs the complete list - wouldn't just the
> first
> > > successful value suffice?
> > > Perhaps -
> > > p `bind` f = \inp -> take 1 $ concat [f v inp' | (v,inp') <- p inp]
> > >
> > > Will this miss out matches?
> > >
> > >
> > > Regards,
> > > Kashyap
> > >
> > > [1] https://github.com/ckkashyap/parsec/blob/master/parsec.go
> > > [2] Monadic Parser Combinators: Graham Hutton, Erik Meijer
> >
> > > _______________________________________________
> > > Haskell-Cafe mailing list
> > > Haskell-Cafe at haskell.org
> > > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
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