Copyright	© 2015–present Megaparsec contributors
License	FreeBSD
Maintainer	Mark Karpov <markkarpov92@gmail.com>
Stability	experimental
Portability	portable
Safe Haskell	Unsafe
Language	Haskell2010

Text.Megaparsec.Debug

Description

Debugging helpers.

Since: 7.0.0

Synopsis

class MonadParsec e s m => MonadParsecDbg e s (m :: Type -> Type) where
- dbg :: Show a => String -> m a -> m a
dbg' :: MonadParsecDbg e s m => String -> m a -> m a

Documentation

class MonadParsec e s m => MonadParsecDbg e s (m :: Type -> Type) where Source #

Type class describing parser monads that can trace during evaluation.

Since: 9.3.0

Methods

dbg Source #

Arguments

:: Show a
=> String	Debugging label
-> m a	Parser to debug
-> m a	Parser that prints debugging messages

The dbg label p parser works exactly like p, but when it's evaluated it 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 of a misbehaving parser with dbg, assigning meaningful labels. Then give it a shot and go through the print-out. As of the 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.

Note: up until version 9.3.0 this was a non-polymorphic function that worked only in ParsecT. It was first introduced in version 7.0.0.

Instances

Instances details

MonadParsecDbg e s m => MonadParsecDbg e s (IdentityT m) Source #
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> IdentityT m a -> IdentityT m a Source #
MonadParsecDbg e s m => MonadParsecDbg e s (ReaderT r m) Source #
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> ReaderT r m a -> ReaderT r m a Source #
(Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (StateT st m) Source #	`dbg (p :: StateT st m)` prints state after running `p`: `>>>` `p = modify succ >> dbg "a" (single 'a' >> modify succ)``>>>` `parseTest (runStateT p 0) "a"`a> IN: 'a' a> MATCH (COK): 'a' a> VALUE: () (STATE: 2) ((),2)
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> StateT st m a -> StateT st m a Source #
(Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (StateT st m) Source #	`dbg (p :: StateT st m)` prints state after running `p`: `>>>` `p = modify succ >> dbg "a" (single 'a' >> modify succ)``>>>` `parseTest (runStateT p 0) "a"`a> IN: 'a' a> MATCH (COK): 'a' a> VALUE: () (STATE: 2) ((),2)
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> StateT st m a -> StateT st m a Source #
(Monoid w, Show w, MonadParsecDbg e s m) => MonadParsecDbg e s (WriterT w m) Source #	`dbg (p :: WriterT st m)` prints only log produced by `p`: `>>>` `p = tell [0] >> dbg "a" (single 'a' >> tell [1])``>>>` `parseTest (runWriterT p) "a"`a> IN: 'a' a> MATCH (COK): 'a' a> VALUE: () (LOG: [1]) ((),[0,1])
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> WriterT w m a -> WriterT w m a Source #
(Monoid w, Show w, MonadParsecDbg e s m) => MonadParsecDbg e s (WriterT w m) Source #	`dbg (p :: WriterT st m)` prints only log produced by `p`: `>>>` `p = tell [0] >> dbg "a" (single 'a' >> tell [1])``>>>` `parseTest (runWriterT p) "a"`a> IN: 'a' a> MATCH (COK): 'a' a> VALUE: () (LOG: [1]) ((),[0,1])
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> WriterT w m a -> WriterT w m a Source #
(VisualStream s, ShowErrorComponent e) => MonadParsecDbg e s (ParsecT e s m) Source #
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> ParsecT e s m a -> ParsecT e s m a Source #
(Monoid w, Show w, Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (RWST r w st m) Source #	`RWST` works like `StateT` inside a `WriterT`: the subparser's log and its final state are printed: `>>>` `p = tell [0] >> modify succ >> dbg "a" (single 'a' >> tell [1] >> modify succ)``>>>` `parseTest (runRWST p () 0) "a"`a> IN: 'a' a> MATCH (COK): 'a' a> VALUE: () (STATE: 2) (LOG: [1]) ((),2,[0,1])
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> RWST r w st m a -> RWST r w st m a Source #
(Monoid w, Show w, Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (RWST r w st m) Source #	`RWST` works like `StateT` inside a `WriterT`: the subparser's log and its final state are printed: `>>>` `p = tell [0] >> modify succ >> dbg "a" (single 'a' >> tell [1] >> modify succ)``>>>` `parseTest (runRWST p () 0) "a"`a> IN: 'a' a> MATCH (COK): 'a' a> VALUE: () (STATE: 2) (LOG: [1]) ((),2,[0,1])
Instance details Defined in Text.Megaparsec.Debug Methods dbg :: Show a => String -> RWST r w st m a -> RWST r w st m a Source #

dbg' Source #

Arguments

:: MonadParsecDbg e s m
=> String	Debugging label
-> m a	Parser to debug
-> m a	Parser that prints debugging messages

Just like dbg, but doesn't require the return value of the parser to be Show-able.

Since: 9.1.0