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

[Li-yao Xia]

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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