    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Text.Parsec.Error
    -- 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
    --
    -- Parse errors
    --
    -----------------------------------------------------------------------------

    module Text.Parsec.Error
        ( Message ( SysUnExpect, UnExpect, Expect, Message )
        , messageString
        , ParseError, errorPos, errorMessages, errorIsUnknown
        , showErrorMessages
        , newErrorMessage, newErrorUnknown
        , addErrorMessage, setErrorPos, setErrorMessage
        , mergeError
        ) where

    import Data.List ( nub, sort )

    import Text.Parsec.Pos

    -- | This abstract data type represents parse error messages. There are
    -- four kinds of messages:
    --
    -- >  data Message = SysUnExpect String
    -- >               | UnExpect String
    -- >               | Expect String
    -- >               | Message String
    --
    -- The fine distinction between different kinds of parse errors allows
    -- the system to generate quite good error messages for the user. It
    -- also allows error messages that are formatted in different
    -- languages. Each kind of message is generated by different combinators:
    --
    --     * A 'SysUnExpect' message is automatically generated by the
    --       'Text.Parsec.Combinator.satisfy' combinator. The argument is the
    --       unexpected input.
    --
    --     * A 'UnExpect' message is generated by the 'Text.Parsec.Prim.unexpected'
    --       combinator. The argument describes the
    --       unexpected item.
    --
    --     * A 'Expect' message is generated by the 'Text.Parsec.Prim.<?>'
    --       combinator. The argument describes the expected item.
    --
    --     * A 'Message' message is generated by the 'fail'
    --       combinator. The argument is some general parser message.

    data Message = SysUnExpect !String -- @ library generated unexpect
                 | UnExpect    !String -- @ unexpected something
                 | Expect      !String -- @ expecting something
                 | Message     !String -- @ raw message

    instance Enum Message where
        fromEnum (SysUnExpect _) = 0
        fromEnum (UnExpect    _) = 1
        fromEnum (Expect      _) = 2
        fromEnum (Message     _) = 3
        toEnum _ = error "toEnum is undefined for Message"

    -- < Return 'True' only when 'compare' would return 'EQ'.

    instance Eq Message where

        m1 == m2 = fromEnum m1 == fromEnum m2

    -- < Compares two error messages without looking at their content. Only
    -- the constructors are compared where:
    --
    -- > 'SysUnExpect' < 'UnExpect' < 'Expect' < 'Message'

    instance Ord Message where
        compare msg1 msg2 = compare (fromEnum msg1) (fromEnum msg2)

    -- | Extract the message string from an error message

    messageString :: Message -> String
    messageString (SysUnExpect s) = s
    messageString (UnExpect    s) = s
    messageString (Expect      s) = s
    messageString (Message     s) = s

    -- | The abstract data type @ParseError@ represents parse errors. It
    -- provides the source position ('SourcePos') of the error
    -- and a list of error messages ('Message'). A @ParseError@
    -- can be returned by the function 'Text.Parsec.Prim.parse'. @ParseError@ is an
    -- instance of the 'Show' class.

    data ParseError = ParseError !SourcePos [Message]

    -- | Extracts the source position from the parse error

    errorPos :: ParseError -> SourcePos
    errorPos (ParseError pos _msgs)
        = pos

    -- | Extracts the list of error messages from the parse error

    errorMessages :: ParseError -> [Message]
    errorMessages (ParseError _pos msgs)
        = sort msgs

    errorIsUnknown :: ParseError -> Bool
    errorIsUnknown (ParseError _pos msgs)
        = null msgs

    -- < Create parse errors

    newErrorUnknown :: SourcePos -> ParseError
    newErrorUnknown pos
        = ParseError pos []

    newErrorMessage :: Message -> SourcePos -> ParseError
    newErrorMessage msg pos
        = ParseError pos [msg]

    addErrorMessage :: Message -> ParseError -> ParseError
    addErrorMessage msg (ParseError pos msgs)
        = ParseError pos (msg:msgs)

    setErrorPos :: SourcePos -> ParseError -> ParseError
    setErrorPos pos (ParseError _ msgs)
        = ParseError pos msgs

    setErrorMessage :: Message -> ParseError -> ParseError
    setErrorMessage msg (ParseError pos msgs)
        = ParseError pos (msg : filter (msg /=) msgs)

    mergeError :: ParseError -> ParseError -> ParseError
    mergeError e1@(ParseError pos1 msgs1) e2@(ParseError pos2 msgs2)
        -- prefer meaningful errors
        | null msgs2 && not (null msgs1) = e1
        | null msgs1 && not (null msgs2) = e2
        | otherwise
        = case pos1 `compare` pos2 of
            -- select the longest match
            EQ -> ParseError pos1 (msgs1 ++ msgs2)
            GT -> e1
            LT -> e2

    instance Show ParseError where
        show err
            = show (errorPos err) ++ ":" ++
              showErrorMessages "or" "unknown parse error"
                                "expecting" "unexpected" "end of input"
                               (errorMessages err)

    -- Language independent show function

    --  TODO
    -- < The standard function for showing error messages. Formats a list of
    --    error messages in English. This function is used in the |Show|
    --    instance of |ParseError <#ParseError>|. The resulting string will be
    --    formatted like:
    --
    --    |unexpected /{The first UnExpect or a SysUnExpect message}/;
    --    expecting /{comma separated list of Expect messages}/;
    --    /{comma separated list of Message messages}/

    showErrorMessages ::
        String -> String -> String -> String -> String -> [Message] -> String
    showErrorMessages msgOr msgUnknown msgExpecting msgUnExpected msgEndOfInput msgs
        | null msgs = msgUnknown
        | otherwise = concat $ map ("\n"++) $ clean $
                     [showSysUnExpect,showUnExpect,showExpect,showMessages]
        where
          (sysUnExpect,msgs1) = span ((SysUnExpect "") ==) msgs
          (unExpect,msgs2)    = span ((UnExpect    "") ==) msgs1
          (expect,messages)   = span ((Expect      "") ==) msgs2

          showExpect      = showMany msgExpecting expect
          showUnExpect    = showMany msgUnExpected unExpect
          showSysUnExpect | not (null unExpect) ||
                            null sysUnExpect = ""
                          | null firstMsg    = msgUnExpected ++ " " ++ msgEndOfInput
                          | otherwise        = msgUnExpected ++ " " ++ firstMsg
              where
                  firstMsg  = messageString (head sysUnExpect)

          showMessages      = showMany "" messages

          -- helpers
          showMany pre msgs = case clean (map messageString msgs) of
                                [] -> ""
                                ms | null pre  -> commasOr ms
                                   | otherwise -> pre ++ " " ++ commasOr ms

          commasOr []       = ""
          commasOr [m]      = m
          commasOr ms       = commaSep (init ms) ++ " " ++ msgOr ++ " " ++ last ms

          commaSep          = seperate ", " . clean

          seperate   _ []     = ""
          seperate   _ [m]    = m
          seperate sep (m:ms) = m ++ sep ++ seperate sep ms

          clean             = nub . filter (not . null)