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-Applicati... The *A note on backtracking* section

Combinators in this module are defined in terms Applicative http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.ht... and Alternative http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.ht... operations.

This parser fails unconditionally without providing any information about

And `empty`'s doc: 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