    {-# OPTIONS_GHC -fglasgow-exts #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Text.Regex.Base.RegexLike
    -- Copyright   :  (c) Chris Kuklewicz 2006
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org, textregexlazy@personal.mightyreason.com
    -- Stability   :  experimental
    -- Portability :  non-portable (MPTC+FD)
    --
    -- Classes and instances for Regex matching.
    --
    --
    -- All the classes are declared here, and some common type aliases, and
    -- the MatchResult data type.
    --
    -- The only instances here are for Extract String and Extract ByteString.
    -- There are no data values.  The 'RegexContext' instances are in
    -- "Text.Regex.Base.Context", except for ones which run afoul of a
    -- repeated variable (RegexContext regex a a), which are defined in each
    -- modules' String and ByteString modules.
    -----------------------------------------------------------------------------

    module Text.Regex.Base.RegexLike (
      -- ** Type aliases
      MatchOffset,
      MatchLength,
      MatchArray,
      MatchText,
      -- ** Data types
      MatchResult(..),
      -- ** Classes
      RegexOptions(..),
      RegexMaker(..),
      RegexLike(..),
      RegexContext(..),
      Extract(..),
      ) where

    import Data.Array(Array,(!))
    import Data.Maybe(isJust)
    import Data.ByteString(ByteString)
    import qualified Data.ByteString as B (take,drop,empty)

    -- | 0 based index from start of source, or (-1) for unused
    type MatchOffset = Int
    -- | non-negative length of a match
    type MatchLength = Int
    -- | 0 based array, with 0th index indicating the full match.  If the
    -- full match location is not available, represent as (0,0).
    type MatchArray = Array Int (MatchOffset,MatchLength)
    type MatchText source = Array Int (source,(MatchOffset,MatchLength))

    -- | This is the same as the type from JRegex.
    data MatchResult a = MR {
        mrBefore :: a,
        mrMatch  :: a,
        mrAfter  :: a,
        mrSubList :: [a],
        mrSubs   :: Array Int a
    }

    ----------------
    -- | Rather than carry them around spearately, the options for how to
    -- execute a regex are kept as part of the regex.  There are two types
    -- of options.  Those that can only be specified at compilation time
    -- and never changed are CompOpt.  Those that can be changed later and
    -- affect how matching is performed are ExecOpt.  The actually types
    -- for these depend on the backend.
    class RegexOptions regex compOpt execOpt
      | regex->compOpt execOpt, compOpt->regex execOpt, execOpt->regex compOpt where
      blankCompOpt :: compOpt    -- ^ no options set at all in the backend
      blankExecOpt :: execOpt    -- ^ no options set at all in the backend
      defaultCompOpt :: compOpt  -- ^ reasonable options (extended,caseSensitive,multiline regex)
      defaultExecOpt :: execOpt  -- ^ reasonable options (extended,caseSensitive,multiline regex)
      setExecOpts :: execOpt -> regex -> regex
      -- ^ forget old flags and use new ones
      getExecOpts :: regex -> execOpt
      -- ^ retrieve the current flags

    ----------------
    -- | RegexMaker captures the creation of the compiled regular
    -- expression from a source type and an option type.  The 'makeRegex'
    -- function has a default implementation that depends on makeRegexOpts
    -- and used 'defaultCompOpt' and 'defaultExecOpt'.
    class (RegexOptions regex compOpt execOpt) => RegexMaker regex compOpt execOpt source
      | regex -> compOpt execOpt, compOpt -> regex execOpt, execOpt -> regex compOpt where
      -- | make using the defaultCompOpt and defaultExecOpt
      makeRegex :: source -> regex
      -- | Specify your own options
      makeRegexOpts :: compOpt -> execOpt -> source -> regex

      makeRegex = makeRegexOpts defaultCompOpt defaultExecOpt

    ----------------
    -- | RegexLike is parametrized on a regular expression type and a
    -- source type to run the matching on.
    --
    -- There are default implementations: matchTest and matchOnceText
    -- using matchOnce; matchCount and matchAllText using
    -- matchAll. matchOnce uses matchOnceText and matchAll uses
    -- matchAllText. So a minimal complete instance need to provide
    -- (matchOnce or matchOnceText) and (matchAll or matchAllText).
    class (Extract source)=> RegexLike regex source where
      matchAll   :: regex -> source-> [MatchArray]
      -- | This can return an array of (offset,length) index pairs for the
      -- match and captured substrings.
      matchOnce  :: regex -> source-> Maybe MatchArray
      matchCount :: regex -> source-> Int
      matchTest  :: regex -> source-> Bool
      matchAllText  :: regex -> source-> [MatchText source]
      -- | This can return a tuple of three items: the source before the
      -- match, an array of the match and captured substrings (with their
      -- indices), and the source after the match.
      matchOnceText :: regex -> source-> Maybe (source,MatchText source,source)

      matchAll regex source = map (fmap snd) (matchAllText regex source)
      matchOnce regex source = fmap (\(_,mt,_) -> fmap snd mt) (matchOnceText regex source)
      matchTest regex source = isJust (matchOnce regex source)
      matchCount regex source = length (matchAll regex source)
      matchOnceText regex source =
        fmap (\ma -> let (o,l) = ma!0
                     in (before o source
                        ,fmap (\ol -> (extract ol source,ol)) ma
                        ,after (o+l) source))
             (matchOnce regex source)
      matchAllText regex source =
        map (fmap (\ol -> (extract ol source,ol)))
            (matchAll regex source)

    ----------------
    -- | RegexContext is the polymorphic interface to do matching
    class (RegexLike regex source) => RegexContext regex source target where
      match :: regex -> source -> target
      matchM :: (Monad m) => regex -> source -> m target

    ----------------
    -- | Extract allows for indexing operations on String or ByteString.
    class Extract source where
      -- | before is a renamed "take"
      before :: Int -> source -> source
      -- | after is a renamed "drop"
      after :: Int -> source -> source
      -- | For when there is no match, this can construct an empty data value
      empty :: source
      -- | extract takes an offset and length and has a default
      -- implementation of @extract (off,len) source = before len (after
      -- off source)@
      extract :: (Int,Int) -> source -> source
      extract (off,len) source = before len (after off source)

    instance Extract String where
      before =  take; after = drop; empty = []

    instance Extract ByteString where
      before = B.take; after = B.drop; empty = B.empty