Multiple Error Messages

[j-mie6]

Is your feature request related to a problem? Please describe. Occasionally, after having generated one syntax error, it may be possible to resume the parse and generate further syntax errors. As these go on, they may become less and less useful, but this is up for the end-user to appreciate for themselves.

Describe the solution you'd like It would be nice if Parsley could produce multiple syntax errors from a parser. To accomplish this, recovery points need to be added into parsing flow, to indicate where a catastrophically failed parser could resume from. Such recovery points are places in the grammar that the parse can resume, and must know first predicate of that grammar position. This can be done via a combinator, which marks a specific parser as a continuation point, and tells it the set of characters it may resume from: these are provided explicitly. When a failure occurs, the parse will try and resume afresh from the next recovery point, dropping input until one of the provided character set is reached.

Describe alternatives you've considered It is possible to establish the first predicate of a parser via static analysis: this, however, would be prohibitively expensive, so a user-directed approach is favourable.

Additional context Recovery combinators either need to provide dummy values to allow for the parser to continue to construct values, or need to force the parser to run in some cheaper error generating only mode. The former is easier but requires more user annotation, the latter requires extra evaluation semantics for each instruction, which may be annoying to implement.