megaparsec/Text/Parsec/Prim.hs
Roman Cheplyaka 860764939c Track current position in the 'tokens' function
Example

Before:

  Prelude Text.Parsec> parseTest (string "abcd") "abbe"
  parse error at (line 1, column 1):
  unexpected "b"
  expecting "abcd"

After:

  *Main> parseTest (string "abcd") "abbe"
  parse error at (line 1, column 3):
  unexpected "b"
  expecting "cd"

Before this patch, 'b' was reported to be found at 1:1, which is clearly not the
case. After this patch, we correctly report the location of the wrong token, and
also report missing tokens starting from that position.
2014-04-04 01:28:21 +03:00

755 lines
26 KiB
Haskell

-----------------------------------------------------------------------------
-- |
-- Module : Text.Parsec.Prim
-- Copyright : (c) Daan Leijen 1999-2001, (c) Paolo Martini 2007
-- License : BSD-style (see the LICENSE file)
--
-- Maintainer : derek.a.elkins@gmail.com
-- Stability : provisional
-- Portability : portable
--
-- The primitive parser combinators.
--
-----------------------------------------------------------------------------
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleContexts,
UndecidableInstances, ScopedTypeVariables #-}
{-# OPTIONS_HADDOCK not-home #-}
module Text.Parsec.Prim
( unknownError
, sysUnExpectError
, unexpected
, ParsecT
, runParsecT
, mkPT
, Parsec
, Consumed(..)
, Reply(..)
, State(..)
, parsecMap
, parserReturn
, parserBind
, mergeErrorReply
, parserFail
, parserZero
, parserPlus
, (<?>)
, (<|>)
, label
, labels
, lookAhead
, Stream(..)
, tokens
, try
, token
, tokenPrim
, tokenPrimEx
, many
, skipMany
, manyAccum
, runPT
, runP
, runParserT
, runParser
, parse
, parseTest
, getPosition
, getInput
, setPosition
, setInput
, getParserState
, setParserState
, updateParserState
, getState
, putState
, modifyState
, setState
, updateState
) where
import qualified Control.Applicative as Applicative ( Applicative(..), Alternative(..) )
import Control.Monad()
import Control.Monad.Trans
import Control.Monad.Identity
import Control.Monad.Reader.Class
import Control.Monad.State.Class
import Control.Monad.Cont.Class
import Control.Monad.Error.Class
import Text.Parsec.Pos
import Text.Parsec.Error
unknownError :: State s u -> ParseError
unknownError state = newErrorUnknown (statePos state)
sysUnExpectError :: String -> SourcePos -> Reply s u a
sysUnExpectError msg pos = Error (newErrorMessage (SysUnExpect msg) pos)
-- | The parser @unexpected msg@ always fails with an unexpected error
-- message @msg@ without consuming any input.
--
-- The parsers 'fail', ('<?>') and @unexpected@ are the three parsers
-- used to generate error messages. Of these, only ('<?>') is commonly
-- used. For an example of the use of @unexpected@, see the definition
-- of 'Text.Parsec.Combinator.notFollowedBy'.
unexpected :: (Stream s m t) => String -> ParsecT s u m a
unexpected msg
= ParsecT $ \s _ _ _ eerr ->
eerr $ newErrorMessage (UnExpect msg) (statePos s)
-- | ParserT monad transformer and Parser type
-- | @ParsecT s u m a@ is a parser with stream type @s@, user state type @u@,
-- underlying monad @m@ and return type @a@. Parsec is strict in the user state.
-- If this is undesirable, simply used a data type like @data Box a = Box a@ and
-- the state type @Box YourStateType@ to add a level of indirection.
newtype ParsecT s u m a
= ParsecT {unParser :: forall b .
State s u
-> (a -> State s u -> ParseError -> m b) -- consumed ok
-> (ParseError -> m b) -- consumed err
-> (a -> State s u -> ParseError -> m b) -- empty ok
-> (ParseError -> m b) -- empty err
-> m b
}
-- | Low-level unpacking of the ParsecT type. To run your parser, please look to
-- runPT, runP, runParserT, runParser and other such functions.
runParsecT :: Monad m => ParsecT s u m a -> State s u -> m (Consumed (m (Reply s u a)))
runParsecT p s = unParser p s cok cerr eok eerr
where cok a s' err = return . Consumed . return $ Ok a s' err
cerr err = return . Consumed . return $ Error err
eok a s' err = return . Empty . return $ Ok a s' err
eerr err = return . Empty . return $ Error err
-- | Low-level creation of the ParsecT type. You really shouldn't have to do this.
mkPT :: Monad m => (State s u -> m (Consumed (m (Reply s u a)))) -> ParsecT s u m a
mkPT k = ParsecT $ \s cok cerr eok eerr -> do
cons <- k s
case cons of
Consumed mrep -> do
rep <- mrep
case rep of
Ok x s' err -> cok x s' err
Error err -> cerr err
Empty mrep -> do
rep <- mrep
case rep of
Ok x s' err -> eok x s' err
Error err -> eerr err
type Parsec s u = ParsecT s u Identity
data Consumed a = Consumed a
| Empty !a
data Reply s u a = Ok a !(State s u) ParseError
| Error ParseError
data State s u = State {
stateInput :: s,
statePos :: !SourcePos,
stateUser :: !u
}
instance Functor Consumed where
fmap f (Consumed x) = Consumed (f x)
fmap f (Empty x) = Empty (f x)
instance Functor (Reply s u) where
fmap f (Ok x s e) = Ok (f x) s e
fmap _ (Error e) = Error e -- XXX
instance Functor (ParsecT s u m) where
fmap f p = parsecMap f p
parsecMap :: (a -> b) -> ParsecT s u m a -> ParsecT s u m b
parsecMap f p
= ParsecT $ \s cok cerr eok eerr ->
unParser p s (cok . f) cerr (eok . f) eerr
instance Applicative.Applicative (ParsecT s u m) where
pure = return
(<*>) = ap -- TODO: Can this be optimized?
instance Applicative.Alternative (ParsecT s u m) where
empty = mzero
(<|>) = mplus
instance Monad (ParsecT s u m) where
return x = parserReturn x
p >>= f = parserBind p f
fail msg = parserFail msg
instance (MonadIO m) => MonadIO (ParsecT s u m) where
liftIO = lift . liftIO
instance (MonadReader r m) => MonadReader r (ParsecT s u m) where
ask = lift ask
local f p = mkPT $ \s -> local f (runParsecT p s)
-- I'm presuming the user might want a separate, non-backtracking
-- state aside from the Parsec user state.
instance (MonadState s m) => MonadState s (ParsecT s' u m) where
get = lift get
put = lift . put
instance (MonadCont m) => MonadCont (ParsecT s u m) where
callCC f = mkPT $ \s ->
callCC $ \c ->
runParsecT (f (\a -> mkPT $ \s' -> c (pack s' a))) s
where pack s a= Empty $ return (Ok a s (unknownError s))
instance (MonadError e m) => MonadError e (ParsecT s u m) where
throwError = lift . throwError
p `catchError` h = mkPT $ \s ->
runParsecT p s `catchError` \e ->
runParsecT (h e) s
parserReturn :: a -> ParsecT s u m a
parserReturn x
= ParsecT $ \s _ _ eok _ ->
eok x s (unknownError s)
parserBind :: ParsecT s u m a -> (a -> ParsecT s u m b) -> ParsecT s u m b
{-# INLINE parserBind #-}
parserBind m k
= ParsecT $ \s cok cerr eok eerr ->
let
-- consumed-okay case for m
mcok x s err =
let
-- if (k x) consumes, those go straigt up
pcok = cok
pcerr = cerr
-- if (k x) doesn't consume input, but is okay,
-- we still return in the consumed continuation
peok x s err' = cok x s (mergeError err err')
-- if (k x) doesn't consume input, but errors,
-- we return the error in the 'consumed-error'
-- continuation
peerr err' = cerr (mergeError err err')
in unParser (k x) s pcok pcerr peok peerr
-- empty-ok case for m
meok x s err =
let
-- in these cases, (k x) can return as empty
pcok = cok
peok x s err' = eok x s (mergeError err err')
pcerr = cerr
peerr err' = eerr (mergeError err err')
in unParser (k x) s pcok pcerr peok peerr
-- consumed-error case for m
mcerr = cerr
-- empty-error case for m
meerr = eerr
in unParser m s mcok mcerr meok meerr
mergeErrorReply :: ParseError -> Reply s u a -> Reply s u a
mergeErrorReply err1 reply -- XXX where to put it?
= case reply of
Ok x state err2 -> Ok x state (mergeError err1 err2)
Error err2 -> Error (mergeError err1 err2)
parserFail :: String -> ParsecT s u m a
parserFail msg
= ParsecT $ \s _ _ _ eerr ->
eerr $ newErrorMessage (Message msg) (statePos s)
instance MonadPlus (ParsecT s u m) where
mzero = parserZero
mplus p1 p2 = parserPlus p1 p2
-- | @parserZero@ always fails without consuming any input. @parserZero@ is defined
-- equal to the 'mzero' member of the 'MonadPlus' class and to the 'Control.Applicative.empty' member
-- of the 'Control.Applicative.Applicative' class.
parserZero :: ParsecT s u m a
parserZero
= ParsecT $ \s _ _ _ eerr ->
eerr $ unknownError s
parserPlus :: ParsecT s u m a -> ParsecT s u m a -> ParsecT s u m a
{-# INLINE parserPlus #-}
parserPlus m n
= ParsecT $ \s cok cerr eok eerr ->
let
meerr err =
let
neok y s' err' = eok y s' (mergeError err err')
neerr err' = eerr $ mergeError err err'
in unParser n s cok cerr neok neerr
in unParser m s cok cerr eok meerr
instance MonadTrans (ParsecT s u) where
lift amb = ParsecT $ \s _ _ eok _ -> do
a <- amb
eok a s $ unknownError s
infix 0 <?>
infixr 1 <|>
-- | The parser @p \<?> msg@ behaves as parser @p@, but whenever the
-- parser @p@ fails /without consuming any input/, it replaces expect
-- error messages with the expect error message @msg@.
--
-- This is normally used at the end of a set alternatives where we want
-- to return an error message in terms of a higher level construct
-- rather than returning all possible characters. For example, if the
-- @expr@ parser from the 'try' example would fail, the error
-- message is: '...: expecting expression'. Without the @(\<?>)@
-- combinator, the message would be like '...: expecting \"let\" or
-- letter', which is less friendly.
(<?>) :: (ParsecT s u m a) -> String -> (ParsecT s u m a)
p <?> msg = label p msg
-- | This combinator implements choice. The parser @p \<|> q@ first
-- applies @p@. If it succeeds, the value of @p@ is returned. If @p@
-- fails /without consuming any input/, parser @q@ is tried. This
-- combinator is defined equal to the 'mplus' member of the 'MonadPlus'
-- class and the ('Control.Applicative.<|>') member of 'Control.Applicative.Alternative'.
--
-- The parser is called /predictive/ since @q@ is only tried when
-- parser @p@ didn't consume any input (i.e.. the look ahead is 1).
-- This non-backtracking behaviour allows for both an efficient
-- implementation of the parser combinators and the generation of good
-- error messages.
(<|>) :: (ParsecT s u m a) -> (ParsecT s u m a) -> (ParsecT s u m a)
p1 <|> p2 = mplus p1 p2
-- | A synonym for @<?>@, but as a function instead of an operator.
label :: ParsecT s u m a -> String -> ParsecT s u m a
label p msg
= labels p [msg]
labels :: ParsecT s u m a -> [String] -> ParsecT s u m a
labels p msgs =
ParsecT $ \s cok cerr eok eerr ->
let eok' x s' error = eok x s' $ if errorIsUnknown error
then error
else setExpectErrors error msgs
eerr' err = eerr $ setExpectErrors err msgs
in unParser p s cok cerr eok' eerr'
where
setExpectErrors err [] = setErrorMessage (Expect "") err
setExpectErrors err [msg] = setErrorMessage (Expect msg) err
setExpectErrors err (msg:msgs)
= foldr (\msg' err' -> addErrorMessage (Expect msg') err')
(setErrorMessage (Expect msg) err) msgs
-- TODO: There should be a stronger statement that can be made about this
-- | An instance of @Stream@ has stream type @s@, underlying monad @m@ and token type @t@ determined by the stream
--
-- Some rough guidelines for a \"correct\" instance of Stream:
--
-- * unfoldM uncons gives the [t] corresponding to the stream
--
-- * A @Stream@ instance is responsible for maintaining the \"position within the stream\" in the stream state @s@. This is trivial unless you are using the monad in a non-trivial way.
class (Monad m) => Stream s m t | s -> t where
uncons :: s -> m (Maybe (t,s))
instance (Monad m) => Stream [tok] m tok where
uncons [] = return $ Nothing
uncons (t:ts) = return $ Just (t,ts)
{-# INLINE uncons #-}
tokens :: forall u s m t .
(Stream s m t, Eq t)
=> ([t] -> String) -- Pretty print a list of tokens
-> (SourcePos -> [t] -> SourcePos)
-> [t] -- List of tokens to parse
-> ParsecT s u m [t]
{-# INLINE tokens #-}
tokens _ _ []
= ParsecT $ \s _ _ eok _ ->
eok [] s $ unknownError s
tokens showTokens nextposs tts@(tok:toks)
= ParsecT $ \(State input pos0 u) cok cerr eok eerr ->
let
nextpos :: SourcePos -> t -> SourcePos
nextpos pos t = nextposs pos [t]
errEof pos ts =
(setErrorMessage (Expect (showTokens ts))
(newErrorMessage (SysUnExpect "") pos))
errExpect pos ts x =
(setErrorMessage (Expect (showTokens ts))
(newErrorMessage (SysUnExpect (showTokens [x])) pos))
-- 'pos' is the position of the first token in the stream
walk [] pos rs = ok pos rs
walk allTs@(t:ts) pos rs = do
sr <- uncons rs
case sr of
Nothing -> cerr $ errEof pos allTs
Just (x,xs) | t == x -> walk ts (nextpos pos x) xs
| otherwise -> cerr $ errExpect pos allTs x
ok pos rs =
let s' = State rs pos u
in cok tts s' (newErrorUnknown pos)
in do
sr <- uncons input
case sr of
Nothing -> eerr $ errEof pos0 tts
Just (x,xs)
| tok == x -> walk toks (nextpos pos0 x) xs
| otherwise -> eerr $ errExpect pos0 tts x
-- | The parser @try p@ behaves like parser @p@, except that it
-- pretends that it hasn't consumed any input when an error occurs.
--
-- This combinator is used whenever arbitrary look ahead is needed.
-- Since it pretends that it hasn't consumed any input when @p@ fails,
-- the ('<|>') combinator will try its second alternative even when the
-- first parser failed while consuming input.
--
-- The @try@ combinator can for example be used to distinguish
-- identifiers and reserved words. Both reserved words and identifiers
-- are a sequence of letters. Whenever we expect a certain reserved
-- word where we can also expect an identifier we have to use the @try@
-- combinator. Suppose we write:
--
-- > expr = letExpr <|> identifier <?> "expression"
-- >
-- > letExpr = do{ string "let"; ... }
-- > identifier = many1 letter
--
-- If the user writes \"lexical\", the parser fails with: @unexpected
-- \'x\', expecting \'t\' in \"let\"@. Indeed, since the ('<|>') combinator
-- only tries alternatives when the first alternative hasn't consumed
-- input, the @identifier@ parser is never tried (because the prefix
-- \"le\" of the @string \"let\"@ parser is already consumed). The
-- right behaviour can be obtained by adding the @try@ combinator:
--
-- > expr = letExpr <|> identifier <?> "expression"
-- >
-- > letExpr = do{ try (string "let"); ... }
-- > identifier = many1 letter
try :: ParsecT s u m a -> ParsecT s u m a
try p =
ParsecT $ \s cok _ eok eerr ->
unParser p s cok eerr eok eerr
-- | @lookAhead p@ parses @p@ without consuming any input.
--
-- If @p@ fails and consumes some input, so does @lookAhead@. Combine with 'try'
-- if this is undesirable.
lookAhead :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m a
lookAhead p = do{ state <- getParserState
; x <- p'
; setParserState state
; return x
}
where
p' = ParsecT $ \s cok cerr eok eerr ->
unParser p s eok cerr eok eerr
-- | The parser @token showTok posFromTok testTok@ accepts a token @t@
-- with result @x@ when the function @testTok t@ returns @'Just' x@. The
-- source position of the @t@ should be returned by @posFromTok t@ and
-- the token can be shown using @showTok t@.
--
-- This combinator is expressed in terms of 'tokenPrim'.
-- It is used to accept user defined token streams. For example,
-- suppose that we have a stream of basic tokens tupled with source
-- positions. We can than define a parser that accepts single tokens as:
--
-- > mytoken x
-- > = token showTok posFromTok testTok
-- > where
-- > showTok (pos,t) = show t
-- > posFromTok (pos,t) = pos
-- > testTok (pos,t) = if x == t then Just t else Nothing
token :: (Stream s Identity t)
=> (t -> String) -- ^ Token pretty-printing function.
-> (t -> SourcePos) -- ^ Computes the position of a token.
-> (t -> Maybe a) -- ^ Matching function for the token to parse.
-> Parsec s u a
token showToken tokpos test = tokenPrim showToken nextpos test
where
nextpos _ tok ts = case runIdentity (uncons ts) of
Nothing -> tokpos tok
Just (tok',_) -> tokpos tok'
-- | The parser @tokenPrim showTok nextPos testTok@ accepts a token @t@
-- with result @x@ when the function @testTok t@ returns @'Just' x@. The
-- token can be shown using @showTok t@. The position of the /next/
-- token should be returned when @nextPos@ is called with the current
-- source position @pos@, the current token @t@ and the rest of the
-- tokens @toks@, @nextPos pos t toks@.
--
-- This is the most primitive combinator for accepting tokens. For
-- example, the 'Text.Parsec.Char.char' parser could be implemented as:
--
-- > char c
-- > = tokenPrim showChar nextPos testChar
-- > where
-- > showChar x = "'" ++ x ++ "'"
-- > testChar x = if x == c then Just x else Nothing
-- > nextPos pos x xs = updatePosChar pos x
tokenPrim :: (Stream s m t)
=> (t -> String) -- ^ Token pretty-printing function.
-> (SourcePos -> t -> s -> SourcePos) -- ^ Next position calculating function.
-> (t -> Maybe a) -- ^ Matching function for the token to parse.
-> ParsecT s u m a
{-# INLINE tokenPrim #-}
tokenPrim showToken nextpos test = tokenPrimEx showToken nextpos Nothing test
tokenPrimEx :: (Stream s m t)
=> (t -> String)
-> (SourcePos -> t -> s -> SourcePos)
-> Maybe (SourcePos -> t -> s -> u -> u)
-> (t -> Maybe a)
-> ParsecT s u m a
{-# INLINE tokenPrimEx #-}
tokenPrimEx showToken nextpos Nothing test
= ParsecT $ \(State input pos user) cok cerr eok eerr -> do
r <- uncons input
case r of
Nothing -> eerr $ unexpectError "" pos
Just (c,cs)
-> case test c of
Just x -> let newpos = nextpos pos c cs
newstate = State cs newpos user
in seq newpos $ seq newstate $
cok x newstate (newErrorUnknown newpos)
Nothing -> eerr $ unexpectError (showToken c) pos
tokenPrimEx showToken nextpos (Just nextState) test
= ParsecT $ \(State input pos user) cok cerr eok eerr -> do
r <- uncons input
case r of
Nothing -> eerr $ unexpectError "" pos
Just (c,cs)
-> case test c of
Just x -> let newpos = nextpos pos c cs
newUser = nextState pos c cs user
newstate = State cs newpos newUser
in seq newpos $ seq newstate $
cok x newstate $ newErrorUnknown newpos
Nothing -> eerr $ unexpectError (showToken c) pos
unexpectError msg pos = newErrorMessage (SysUnExpect msg) pos
-- | @many p@ applies the parser @p@ /zero/ or more times. Returns a
-- list of the returned values of @p@.
--
-- > identifier = do{ c <- letter
-- > ; cs <- many (alphaNum <|> char '_')
-- > ; return (c:cs)
-- > }
many :: ParsecT s u m a -> ParsecT s u m [a]
many p
= do xs <- manyAccum (:) p
return (reverse xs)
-- | @skipMany p@ applies the parser @p@ /zero/ or more times, skipping
-- its result.
--
-- > spaces = skipMany space
skipMany :: ParsecT s u m a -> ParsecT s u m ()
skipMany p
= do manyAccum (\_ _ -> []) p
return ()
manyAccum :: (a -> [a] -> [a])
-> ParsecT s u m a
-> ParsecT s u m [a]
manyAccum acc p =
ParsecT $ \s cok cerr eok eerr ->
let walk xs x s' err =
unParser p s'
(seq xs $ walk $ acc x xs) -- consumed-ok
cerr -- consumed-err
manyErr -- empty-ok
(\e -> cok (acc x xs) s' e) -- empty-err
in unParser p s (walk []) cerr manyErr (\e -> eok [] s e)
manyErr = error "Text.ParserCombinators.Parsec.Prim.many: combinator 'many' is applied to a parser that accepts an empty string."
-- < Running a parser: monadic (runPT) and pure (runP)
runPT :: (Stream s m t)
=> ParsecT s u m a -> u -> SourceName -> s -> m (Either ParseError a)
runPT p u name s
= do res <- runParsecT p (State s (initialPos name) u)
r <- parserReply res
case r of
Ok x _ _ -> return (Right x)
Error err -> return (Left err)
where
parserReply res
= case res of
Consumed r -> r
Empty r -> r
runP :: (Stream s Identity t)
=> Parsec s u a -> u -> SourceName -> s -> Either ParseError a
runP p u name s = runIdentity $ runPT p u name s
-- | The most general way to run a parser. @runParserT p state filePath
-- input@ runs parser @p@ on the input list of tokens @input@,
-- obtained from source @filePath@ with the initial user state @st@.
-- The @filePath@ is only used in error messages and may be the empty
-- string. Returns a computation in the underlying monad @m@ that return either a 'ParseError' ('Left') or a
-- value of type @a@ ('Right').
runParserT :: (Stream s m t)
=> ParsecT s u m a -> u -> SourceName -> s -> m (Either ParseError a)
runParserT = runPT
-- | The most general way to run a parser over the Identity monad. @runParser p state filePath
-- input@ runs parser @p@ on the input list of tokens @input@,
-- obtained from source @filePath@ with the initial user state @st@.
-- The @filePath@ is only used in error messages and may be the empty
-- string. Returns either a 'ParseError' ('Left') or a
-- value of type @a@ ('Right').
--
-- > parseFromFile p fname
-- > = do{ input <- readFile fname
-- > ; return (runParser p () fname input)
-- > }
runParser :: (Stream s Identity t)
=> Parsec s u a -> u -> SourceName -> s -> Either ParseError a
runParser = runP
-- | @parse p filePath input@ runs a parser @p@ over Identity without user
-- state. The @filePath@ is only used in error messages and may be the
-- empty string. Returns either a 'ParseError' ('Left')
-- or a value of type @a@ ('Right').
--
-- > main = case (parse numbers "" "11, 2, 43") of
-- > Left err -> print err
-- > Right xs -> print (sum xs)
-- >
-- > numbers = commaSep integer
parse :: (Stream s Identity t)
=> Parsec s () a -> SourceName -> s -> Either ParseError a
parse p = runP p ()
-- | The expression @parseTest p input@ applies a parser @p@ against
-- input @input@ and prints the result to stdout. Used for testing
-- parsers.
parseTest :: (Stream s Identity t, Show a)
=> Parsec s () a -> s -> IO ()
parseTest p input
= case parse p "" input of
Left err -> do putStr "parse error at "
print err
Right x -> print x
-- < Parser state combinators
-- | Returns the current source position. See also 'SourcePos'.
getPosition :: (Monad m) => ParsecT s u m SourcePos
getPosition = do state <- getParserState
return (statePos state)
-- | Returns the current input
getInput :: (Monad m) => ParsecT s u m s
getInput = do state <- getParserState
return (stateInput state)
-- | @setPosition pos@ sets the current source position to @pos@.
setPosition :: (Monad m) => SourcePos -> ParsecT s u m ()
setPosition pos
= do updateParserState (\(State input _ user) -> State input pos user)
return ()
-- | @setInput input@ continues parsing with @input@. The 'getInput' and
-- @setInput@ functions can for example be used to deal with #include
-- files.
setInput :: (Monad m) => s -> ParsecT s u m ()
setInput input
= do updateParserState (\(State _ pos user) -> State input pos user)
return ()
-- | Returns the full parser state as a 'State' record.
getParserState :: (Monad m) => ParsecT s u m (State s u)
getParserState = updateParserState id
-- | @setParserState st@ set the full parser state to @st@.
setParserState :: (Monad m) => State s u -> ParsecT s u m (State s u)
setParserState st = updateParserState (const st)
-- | @updateParserState f@ applies function @f@ to the parser state.
updateParserState :: (State s u -> State s u) -> ParsecT s u m (State s u)
updateParserState f =
ParsecT $ \s _ _ eok _ ->
let s' = f s
in eok s' s' $ unknownError s'
-- < User state combinators
-- | Returns the current user state.
getState :: (Monad m) => ParsecT s u m u
getState = stateUser `liftM` getParserState
-- | @putState st@ set the user state to @st@.
putState :: (Monad m) => u -> ParsecT s u m ()
putState u = do updateParserState $ \s -> s { stateUser = u }
return ()
-- | @updateState f@ applies function @f@ to the user state. Suppose
-- that we want to count identifiers in a source, we could use the user
-- state as:
--
-- > expr = do{ x <- identifier
-- > ; updateState (+1)
-- > ; return (Id x)
-- > }
modifyState :: (Monad m) => (u -> u) -> ParsecT s u m ()
modifyState f = do updateParserState $ \s -> s { stateUser = f (stateUser s) }
return ()
-- XXX Compat
-- | An alias for putState for backwards compatibility.
setState :: (Monad m) => u -> ParsecT s u m ()
setState = putState
-- | An alias for modifyState for backwards compatibility.
updateState :: (Monad m) => (u -> u) -> ParsecT s u m ()
updateState = modifyState