- for pretty-printing parse errors thrown from the parsing of excerpts
of the source text as if they were thrown from the parsing of the
source text itself
That code fails to compile with ghc-8.6.1 because the instance is undecidable.
I suppose we could enable the appropriate compiler extension to support it, but
I've found that simply removing the instance causes no problems whatsoever: the
entire repository still compiles fine and it passes all test suites, too.
We previously had another parser type, 'type ErroringJournalParser =
ExceptT String ...' for throwing parse errors without the possibility of
backtracking. This parser type was removed under the assumption that it
would be possible to write our parser without this capability. However,
after a hairy backtracking bug, we would now prefer to have the option
to prevent backtracking.
- Define a 'FinalParseError' type specifically for the 'ExceptT' layer
- Any parse error can be raised as a "final" parse error
- Tracks the stack of include files for parser errors, anticipating the
removal of the tracking of stacks of include files in megaparsec 7
- Although a stack of include files is also tracked in the 'StateT
Journal' layer of the parser, it seems easier to guarantee correct
error messages in the 'ExceptT FinalParserError' layer
- This does not make the 'StateT Journal' stack redundant because the
'ExceptT FinalParseError' stack cannot be used to detect cycles of
include files
- Don't immediately throw custom parse errors into 'ParsecT'; rather,
just construct and return them
- This anticipates the re-implementation of an 'ExceptT' layer of the
parser, which should be able throw custom parse errors
