    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Language.C.Syntax.ParserMonad
    -- Copyright   :  (c) [1999..2004] Manuel M T Chakravarty
    --                (c) 2005-2007 Duncan Coutts
    -- License     :  BSD-style
    -- Maintainer  :  benedikt.huber@gmail.com
    -- Portability :  portable
    --
    -- Monad for the C lexer and parser
    --
    --  This monad has to be usable with Alex and Happy. Some things in it are
    --  dictated by that, eg having to be able to remember the last token.
    --
    --  The monad also provides a unique name supply (via the Name module)
    --
    --  For parsing C we have to maintain a set of identifiers that we know to be
    --  typedef'ed type identifiers. We also must deal correctly with scope so we
    --  keep a list of sets of identifiers so we can save the outer scope when we
    --  enter an inner scope.
    module Language.C.Parser.ParserMonad (
      P,
      execParser,
      failP,
      getNewName,        -- :: P Name
      addTypedef,        -- :: Ident -> P ()
      shadowTypedef,     -- :: Ident -> P ()
      isTypeIdent,       -- :: Ident -> P Bool
      enterScope,        -- :: P ()
      leaveScope,        -- :: P ()
      setPos,            -- :: Position -> P ()
      getPos,            -- :: P Position
      getInput,          -- :: P String
      setInput,          -- :: String -> P ()
      getLastToken,      -- :: P CToken
      setLastToken,      -- :: CToken -> P ()
      ParseError(..),
      ) where
    import Language.C.Data.Error (internalErr, showErrorInfo,ErrorInfo(..),ErrorLevel(..))
    import Language.C.Data.Position  (Position(..))
    import Language.C.Data.InputStream
    import Language.C.Data.Name    (Name)
    import Language.C.Data.Ident    (Ident)
    import Language.C.Parser.Tokens (CToken)

    import Data.Set  (Set)
    import qualified Data.Set as Set (fromList, insert, member, delete)

    newtype ParseError = ParseError ([String],Position)
    instance Show ParseError where
        show (ParseError (msgs,pos)) = showErrorInfo "Syntax Error !" (ErrorInfo LevelError pos msgs)

    data ParseResult a
      = POk !PState a
      | PFailed [String] Position   -- The error message and position

    data PState = PState {
            curPos     :: !Position,        -- position at current input location
            curInput   :: !InputStream,      -- the current input
            prevToken  ::  CToken,          -- the previous token
            namesupply :: ![Name],          -- the name unique supply
            tyidents   :: !(Set Ident),     -- the set of typedef'ed identifiers
            scopes     :: ![Set Ident]      -- the tyident sets for outer scopes
         }

    newtype P a = P { unP :: PState -> ParseResult a }

    instance Monad P where
      return = returnP
      (>>=) = thenP
      fail m = getPos >>= \pos -> failP pos [m]

    -- | execute the given parser on the supplied input stream.
    --   returns 'ParseError' if the parser failed, and a pair of
    --   result and remaining name supply otherwise
    --
    -- Synopsis: @execParser parser inputStream initialPos predefinedTypedefs uniqNameSupply@
    execParser :: P a -> InputStream -> Position -> [Ident] -> [Name]
               -> Either ParseError (a,[Name])
    execParser (P parser) input pos builtins names =
      case parser initialState of
        PFailed message errpos -> Left (ParseError (message,errpos))
        POk st result -> Right (result, namesupply st)
      where initialState = PState {
              curPos = pos,
              curInput = input,
              prevToken = internalErr "CLexer.execParser: Touched undefined token!",
              namesupply = names,
              tyidents = Set.fromList builtins,
              scopes   = []
            }

    {-# INLINE returnP #-}
    returnP :: a -> P a
    returnP a = P $ \s -> POk s a

    {-# INLINE thenP #-}
    thenP :: P a -> (a -> P b) -> P b
    (P m) `thenP` k = P $ \s ->
            case m s of
                    POk s' a        -> (unP (k a)) s'
                    PFailed err pos -> PFailed err pos

    failP :: Position -> [String] -> P a
    failP pos msg = P $ \_ -> PFailed msg pos

    getNewName :: P Name
    getNewName = P $ \s@PState{namesupply=(n:ns)} -> POk s{namesupply=ns} n

    setPos :: Position -> P ()
    setPos pos = P $ \s -> POk s{curPos=pos} ()

    getPos :: P Position
    getPos = P $ \s@PState{curPos=pos} -> POk s pos

    addTypedef :: Ident -> P ()
    addTypedef ident = (P $ \s@PState{tyidents=tyids} ->
                                 POk s{tyidents = ident `Set.insert` tyids} ())

    shadowTypedef :: Ident -> P ()
    shadowTypedef ident = (P $ \s@PState{tyidents=tyids} ->
                                 -- optimisation: mostly the ident will not be in
                                 -- the tyident set so do a member lookup to avoid
                                 --  churn induced by calling delete
                                 POk s{tyidents = if ident `Set.member` tyids
                                                    then ident `Set.delete` tyids
                                                    else tyids } ())

    isTypeIdent :: Ident -> P Bool
    isTypeIdent ident = P $ \s@PState{tyidents=tyids} ->
                                 POk s $! Set.member ident tyids

    enterScope :: P ()
    enterScope = P $ \s@PState{tyidents=tyids,scopes=ss} ->
                         POk s{scopes=tyids:ss} ()

    leaveScope :: P ()
    leaveScope = P $ \s@PState{scopes=ss} ->
                         case ss of
                           []          -> error "leaveScope: already in global scope"
                           (tyids:ss') -> POk s{tyidents=tyids, scopes=ss'} ()

    getInput :: P InputStream
    getInput = P $ \s@PState{curInput=i} -> POk s i

    setInput :: InputStream -> P ()
    setInput i = P $ \s -> POk s{curInput=i} ()

    getLastToken :: P CToken
    getLastToken = P $ \s@PState{prevToken=tok} -> POk s tok

    setLastToken :: CToken -> P ()
    setLastToken tok = P $ \s -> POk s{prevToken=tok} ()