Copyright | © 2015–2018 Megaparsec contributors |
---|---|
License | FreeBSD |
Maintainer | Mark Karpov <markkarpov92@gmail.com> |
Stability | experimental |
Portability | portable |
Safe Haskell | None |
Language | Haskell2010 |
Debugging helpers.
Since: megaparsec-7.0.0
Documentation
:: (Stream s, ShowErrorComponent e, Show a) | |
=> String | Debugging label |
-> ParsecT e s m a | Parser to debug |
-> ParsecT e s m a | Parser that prints debugging messages |
parser works exactly like dbg
label pp
, but when it's evaluated
it also prints information useful for debugging. The label
is only used
to refer to this parser in the debugging output. This combinator uses the
trace
function from Debug.Trace under the hood.
Typical usage is to wrap every sub-parser in misbehaving parser with
dbg
assigning meaningful labels. Then give it a shot and go through the
print-out. As of current version, this combinator prints all available
information except for hints, which are probably only interesting to
the maintainer of Megaparsec itself and may be quite verbose to output in
general. Let me know if you would like to be able to see hints in the
debugging output.
The output itself is pretty self-explanatory, although the following abbreviations should be clarified (they are derived from the low-level source code):
COK
—“consumed OK”. The parser consumed input and succeeded.CERR
—“consumed error”. The parser consumed input and failed.EOK
—“empty OK”. The parser succeeded without consuming input.EERR
—“empty error”. The parser failed without consuming input.
Finally, it's not possible to lift this function into some monad
transformers without introducing surprising behavior (e.g. unexpected
state backtracking) or adding otherwise redundant constraints (e.g.
Show
instance for state), so this helper is only available for
ParsecT
monad, not any instance of MonadParsec
in general.