What is the strategy for adding error productions to a grammar?

Question

How are error productions typically added? I'm encountering the issue that my error productions are too shallow: when the parser starts popping states on an error in a statement, it pops until it hits the error production for the section in which it is located, and prints out an invalid error message.

Is it a good idea to just add some descriptive error production to every nonterminal?

Answer 1

Error productions are about recovering from an error in order to attempt to continue processing the input, not about printing reasonable or useful error messages. Therefor they should be used at points in the grammar where its likely that you can recognize and resynchronize the input stream properly. For example, if your language consists of a sequence of constructs ending with ; characters, a good error production is something like construct: error ';', which will recover from errors in a construct (whatever that is) by skipping forward in the input to a ; and attempting to go on from there.

Putting many error recovery rules is generaly a bad idea, since the parser will only recover to the closest one, and its often the most global ones at the top level that are most likely to be useful and trying to use a finer granularity will just lead to error cascades as the error recovery rules can't resync with the input properly.