Text.Megaparsec.Debug

Debugging helpers.

Since: 7.0.0

Documentation

class MonadParsec e s m => MonadParsecDbg e s m where

Type class describing parser monads that can trace during evaluation.

Since: 9.3.0

Methods

dbg

Arguments

:: Show a

=> String

Debugging label

-> m a

Parser to debug

-> m a

Parser that prints debugging messages

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 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.

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

Instances

MonadParsecDbg e s m => MonadParsecDbg e s (IdentityT m)

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> IdentityT m a -> IdentityT m a

MonadParsecDbg e s m => MonadParsecDbg e s (ReaderT r m)

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> ReaderT r m a -> ReaderT r m a

(Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (StateT st m)

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)

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> StateT st m a -> StateT st m a

(Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (StateT st m)

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)

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> StateT st m a -> StateT st m a

(Monoid w, Show w, MonadParsecDbg e s m) => MonadParsecDbg e s (WriterT w m)

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])

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> WriterT w m a -> WriterT w m a

(Monoid w, Show w, MonadParsecDbg e s m) => MonadParsecDbg e s (WriterT w m)

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])

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> WriterT w m a -> WriterT w m a

(VisualStream s, ShowErrorComponent e) => MonadParsecDbg e s (ParsecT e s m)

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> ParsecT e s m a -> ParsecT e s m a

(Monoid w, Show w, Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (RWST r w st m)

RWST works like StateT inside a WriterT: subparser's log and its final state is 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])

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> RWST r w st m a -> RWST r w st m a

(Monoid w, Show w, Show st, MonadParsecDbg e s m) => MonadParsecDbg e s (RWST r w st m)

RWST works like StateT inside a WriterT: subparser's log and its final state is 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])

Defined in Text.Megaparsec.Debug

Methods

dbg :: Show a => String -> RWST r w st m a -> RWST r w st m a

dbg'

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