Error messages non-descriptive

[edsko]

Usually an error message is just "empty grammar". This might not be such an easy problem to tackle though.

[MedeaMelana]

Currently error messages are straight from Aeson which doesn't have very interesting error reporting. It'd be really useful to have good error reporting especially as we have the whole grammar available. I looked into this in the previous version of JsonGrammar, but it was really difficult there as I didn't have a deep embedding.

[mgsloan]

Note that this error message comes from JsonGrammar, when it fails to parse the Value yielded by aeson. I've also had issues with this being uninformative.

I had an initial swing at adding error context: mgsloan@f952cca

However, I've found that in practice this is nearly useless, because attoparsec has no way to fail in a way that prevents backtracking. So, I always just get the "empty grammar" message with the label of the top level type being parsed.

See https://mail.haskell.org/pipermail/haskell-cafe/2013-March/107004.html

Yes, I've found it impossible to force attoparsec to fail a parse.
I think that is intended as a feature.

So, I believe in order to add this, attoparsec would need to support failure without backtracking, and JsonGrammar would need a new constructor in Grammar exposing this capability.