    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Text.Parsec.Pos
    -- 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
    --
    -- Textual source positions.
    --
    -----------------------------------------------------------------------------

    module Text.Parsec.Pos
        ( SourceName, Line, Column
        , SourcePos
        , sourceLine, sourceColumn, sourceName
        , incSourceLine, incSourceColumn
        , setSourceLine, setSourceColumn, setSourceName
        , newPos, initialPos
        , updatePosChar, updatePosString
        ) where

    import Data.Data (Data)
    import Data.Typeable (Typeable)

    -- < Source positions: a file name, a line and a column
    -- upper left is (1,1)

    type SourceName = String
    type Line       = Int
    type Column     = Int

    -- | The abstract data type @SourcePos@ represents source positions. It
    -- contains the name of the source (i.e. file name), a line number and
    -- a column number. @SourcePos@ is an instance of the 'Show', 'Eq' and
    -- 'Ord' class.

    data SourcePos  = SourcePos SourceName !Line !Column
        deriving ( Eq, Ord, Data, Typeable)

    -- | Create a new 'SourcePos' with the given source name,
    -- line number and column number.

    newPos :: SourceName -> Line -> Column -> SourcePos
    newPos name line column
        = SourcePos name line column

    -- | Create a new 'SourcePos' with the given source name,
    -- and line number and column number set to 1, the upper left.

    initialPos :: SourceName -> SourcePos
    initialPos name
        = newPos name 1 1

    -- | Extracts the name of the source from a source position.

    sourceName :: SourcePos -> SourceName
    sourceName (SourcePos name _line _column) = name

    -- | Extracts the line number from a source position.

    sourceLine :: SourcePos -> Line
    sourceLine (SourcePos _name line _column) = line

    -- | Extracts the column number from a source position.

    sourceColumn :: SourcePos -> Column
    sourceColumn (SourcePos _name _line column) = column

    -- | Increments the line number of a source position.

    incSourceLine :: SourcePos -> Line -> SourcePos
    incSourceLine (SourcePos name line column) n = SourcePos name (line+n) column

    -- | Increments the column number of a source position.

    incSourceColumn :: SourcePos -> Column -> SourcePos
    incSourceColumn (SourcePos name line column) n = SourcePos name line (column+n)

    -- | Set the name of the source.

    setSourceName :: SourcePos -> SourceName -> SourcePos
    setSourceName (SourcePos _name line column) n = SourcePos n line column

    -- | Set the line number of a source position.

    setSourceLine :: SourcePos -> Line -> SourcePos
    setSourceLine (SourcePos name _line column) n = SourcePos name n column

    -- | Set the column number of a source position.

    setSourceColumn :: SourcePos -> Column -> SourcePos
    setSourceColumn (SourcePos name line _column) n = SourcePos name line n

    -- | The expression @updatePosString pos s@ updates the source position
    -- @pos@ by calling 'updatePosChar' on every character in @s@, ie.
    -- @foldl updatePosChar pos string@.

    updatePosString :: SourcePos -> String -> SourcePos
    updatePosString pos string
        = foldl updatePosChar pos string

    -- | Update a source position given a character. If the character is a
    -- newline (\'\\n\') or carriage return (\'\\r\') the line number is
    -- incremented by 1. If the character is a tab (\'\t\') the column
    -- number is incremented to the nearest 8'th column, ie. @column + 8 -
    -- ((column-1) \`mod\` 8)@. In all other cases, the column is
    -- incremented by 1.

    updatePosChar   :: SourcePos -> Char -> SourcePos
    updatePosChar (SourcePos name line column) c
        = case c of
            '\n' -> SourcePos name (line+1) 1
            '\t' -> SourcePos name line (column + 8 - ((column-1) `mod` 8))
            _    -> SourcePos name line (column + 1)

    instance Show SourcePos where
      show (SourcePos name line column)
        | null name = showLineColumn
        | otherwise = "\"" ++ name ++ "\" " ++ showLineColumn
        where
          showLineColumn    = "(line " ++ show line ++
                              ", column " ++ show column ++
                              ")"