Error reporting

[robrix]

We don’t have any right now! It’s just pass/fail. That sucks.

[jspahrsummers]

The current lack of error reporting will hit Carthage pretty hard, because any deviation from OGDL syntax will result in the whole file failing to parse, without any indication as to why.

[robrix]

So, we need:

• [x] the current index of parsing at every point
• [x] error location data for the productions
• [ ] some way to pretty-print subsets of the grammar
• [ ] as part of that, some map from individual parsers into Printable identifiers, such that
  • [ ] we can terminate recursive printing, and
  • [ ] the caller can produce human-readable strings explaining the use of the identifiers to their users
• [x] a composite pattern Error type, to pass the errors back up the stack in an Either or Result
• [x] nice combinations of Error for failed alternatives (e.g. “expected x or y”)
• [ ] heuristics as to which error(s) to present (these may belong to the caller, but they’ll probably be defined in terms of Madness’ primitives)

My intuition is that the latter is essentially the same problem as ambiguity, cf #69.