[Haskell-cafe] Consider adding recoverability to the vocabulary of parser combinators

Compl Yue compl.yue at gmail.com
Wed Oct 28 13:48:37 UTC 2020

(sorry I forgot the text color again, maybe https://darkreader.org is
to blame, anyway plain text mode I should use)

Hi Li-yao,

I appreciate your help especially the PR as a working fix.

But personally I don't like the overall method of your solution, I can
see Monad Transformers are powerful enough to tackle similar problems,
but I'm not satisfied by the ergonomics in composing monads with
transformers. `ParsecT` had already caused me much pain to get started
in the beginning, and I'm still not fluent (comfortable) in
transforming monads, especially I'm afraid I will have to transform
much of the standard combinator functions, in order to get the real
case parser working, as its resulting AST is much more complex.

I still have faith in the improvement of megaparsec as a well known
parser combinator library (I regard it as the best for engineering
needs among other libraries), and I must admit megaparsec already
elegantly works 99% out of my current use cases, and the very issue we
are talking about  is a nice-to-have rather than must-to-have, so I
would think we still have time to anticipate more options to come out.

And I particularly like to see parser combinators have this issue
addressed in its own design space.

Thanks again with best regards,

On Wed, Oct 28, 2020 at 8:19 PM Li-yao Xia <lysxia at gmail.com> wrote:
> Hi Compl,
> At least, for the example you gave on this list, it can be fixed by
> returning Nothing instead of using the facility for failure baked into
> (mega)parsec. (Proposed diff for reference:
> https://github.com/complyue/dcp/pull/3)
> "Returning Nothing" can be seen as adding a new channel for errors,
> turning the Parser monad into `MaybeT Parser`. Then `return Nothing` is
> how `empty` is defined by `MaybeT`, allowing that error to be caught and
> recovered from at the point where it was thrown, no backtracking. (And
> the original failure mode of Parser becomes `lift empty`.)
> Does that address your problem?
> Cheers,
> Li-yao
> On 10/28/2020 5:18 AM, Compl Yue wrote:
> > (sorry for repost, seems GMail's html processing on my last email has
> > rendered it barely readable, so again with plain text here)
> >
> > Dear Cafe,
> >
> > I'm still not fully clear about the confusion regarding megaparsec's
> > behavior that I posted lately here. But now comes to my mind that it may
> > have some problem rooted in the lacking of recoverability semantic with
> > respect to parser combinators, some quoting from
> > http://hackage.haskell.org/package/parser-combinators/docs/Control-Applicative-Combinators.html
> >
> > The  *A note on backtracking* section
> >
> >  > Combinators in this module are defined in terms Applicative and
> > Alternative operations.
> >
> > And `empty`'s doc:
> >
> >  > This parser fails unconditionally without providing any information
> > about the cause of the failure.
> >
> > Clearly `empty` is used to express failure, but there is seemingly no
> > device to explicitly express whether a failure is recoverable. Then I
> > observed megaparsec's implicit rule as currently implemented is like:
> >
> > *) a failure with no input consumed can be recovered by rest parsers
> > *) a failure with some input consumed can not be recovered by rest parsers
> >
> > This works to great extent, but I would think the expressiveness can be
> > further extended for a parser from the application, to tell the library
> > that some input induces recoverable failure.
> >
> > I have no expertise to suggest whether `MonadPlus` and/or `MonadFail`
> > are suitable devices to be considered, but as megaparsec has implemented
> > instances for them, I do feel some tweaks would be possible and meaningful.
> >
> > Best regards,
> > Compl
> >
> >
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