- Parse errors encountered in include files are treated as "final" parse
errors in the parent file, preventing backtracking and fixing an issue
in #853
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
- In anticipation of megaparsec 7, which removes support for stacks of
include files (as far as I can tell)
- Intended for the 'StateT Journal' layer of the parser
- A stack of include files would be better in a 'ReaderT' layer, but I
don't want to add another layer to the parser
- Intended for detecting cycles of include files
- Potential issue: for proper error messages for include file cycles,
we must remember to provide the filepath of the root journal file via
the initial journal state passed to a 'JournalParser'; I imagine
that we may forget to do so because in all other cases it is okay
not to do so.
