Consider adding recoverability to the vocabulary of parser combinators

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Consider adding recoverability to the vocabulary of parser combinators

Compl Yue
(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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Re: Consider adding recoverability to the vocabulary of parser combinators

Li-yao Xia-2
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
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
>
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Re: Consider adding recoverability to the vocabulary of parser combinators

Compl Yue
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,
Compl


On Wed, Oct 28, 2020 at 8:19 PM Li-yao Xia <[hidden email]> 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
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
>

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Re: Consider adding recoverability to the vocabulary of parser combinators

Compl Yue
In reply to this post by Li-yao Xia-2
(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,
Compl

On Wed, Oct 28, 2020 at 8:19 PM Li-yao Xia <[hidden email]> 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
> >
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > To (un)subscribe, modify options or view archives go to:
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> > Only members subscribed via the mailman list are allowed to post.
> >
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