    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Version
    -- Copyright   :  (c) The University of Glasgow 2004
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (local universal quantification in ReadP)
    --
    -- A general library for representation and manipulation of versions.
    --
    -- Versioning schemes are many and varied, so the version
    -- representation provided by this library is intended to be a
    -- compromise between complete generality, where almost no common
    -- functionality could reasonably be provided, and fixing a particular
    -- versioning scheme, which would probably be too restrictive.
    --
    -- So the approach taken here is to provide a representation which
    -- subsumes many of the versioning schemes commonly in use, and we
    -- provide implementations of 'Eq', 'Ord' and conversion to\/from 'String'
    -- which will be appropriate for some applications, but not all.
    --
    -----------------------------------------------------------------------------

    module Data.Version (
            -- * The @Version@ type
            Version(..),
            -- * A concrete representation of @Version@
            showVersion, parseVersion,
      ) where

    import Prelude -- necessary to get dependencies right

    -- These #ifdefs are necessary because this code might be compiled as
    -- part of ghc/lib/compat, and hence might be compiled by an older version
    -- of GHC.  In which case, we might need to pick up ReadP from
    -- Distribution.Compat.ReadP, because the version in
    -- Text.ParserCombinators.ReadP doesn't have all the combinators we need.
    #if __GLASGOW_HASKELL__ || __HUGS__ || __NHC__
    import Text.ParserCombinators.ReadP
    #else
    import Distribution.Compat.ReadP
    #endif

    #if !__GLASGOW_HASKELL__
    import Data.Typeable    ( Typeable, TyCon, mkTyCon, mkTyConApp )
    #else
    import Data.Typeable    ( Typeable )
    #endif

    import Data.List        ( intersperse, sort )
    import Control.Monad    ( liftM )
    import Data.Char        ( isDigit, isAlphaNum )

    {- |
    A 'Version' represents the version of a software entity.

    An instance of 'Eq' is provided, which implements exact equality
    modulo reordering of the tags in the 'versionTags' field.

    An instance of 'Ord' is also provided, which gives lexicographic
    ordering on the 'versionBranch' fields (i.e. 2.1 > 2.0, 1.2.3 > 1.2.2,
    etc.).  This is expected to be sufficient for many uses, but note that
    you may need to use a more specific ordering for your versioning
    scheme.  For example, some versioning schemes may include pre-releases
    which have tags @\"pre1\"@, @\"pre2\"@, and so on, and these would need to
    be taken into account when determining ordering.  In some cases, date
    ordering may be more appropriate, so the application would have to
    look for @date@ tags in the 'versionTags' field and compare those.
    The bottom line is, don't always assume that 'compare' and other 'Ord'
    operations are the right thing for every 'Version'.

    Similarly, concrete representations of versions may differ.  One
    possible concrete representation is provided (see 'showVersion' and
    'parseVersion'), but depending on the application a different concrete
    representation may be more appropriate.
    -}
    data Version =
      Version { versionBranch :: [Int],
                    -- ^ The numeric branch for this version.  This reflects the
                    -- fact that most software versions are tree-structured; there
                    -- is a main trunk which is tagged with versions at various
                    -- points (1,2,3...), and the first branch off the trunk after
                    -- version 3 is 3.1, the second branch off the trunk after
                    -- version 3 is 3.2, and so on.  The tree can be branched
                    -- arbitrarily, just by adding more digits.
                    --
                    -- We represent the branch as a list of 'Int', so
                    -- version 3.2.1 becomes [3,2,1].  Lexicographic ordering
                    -- (i.e. the default instance of 'Ord' for @[Int]@) gives
                    -- the natural ordering of branches.

               versionTags :: [String]  -- really a bag
                    -- ^ A version can be tagged with an arbitrary list of strings.
                    -- The interpretation of the list of tags is entirely dependent
                    -- on the entity that this version applies to.
            }
      deriving (Read,Show
    #if __GLASGOW_HASKELL__
            ,Typeable
    #endif
            )

    #if !__GLASGOW_HASKELL__
    versionTc :: TyCon
    versionTc = mkTyCon "Version"

    instance Typeable Version where
      typeOf _ = mkTyConApp versionTc []
    #endif

    instance Eq Version where
      v1 == v2  =  versionBranch v1 == versionBranch v2
                    && sort (versionTags v1) == sort (versionTags v2)
                    -- tags may be in any order

    instance Ord Version where
      v1 `compare` v2 = versionBranch v1 `compare` versionBranch v2

    -- -----------------------------------------------------------------------------
    -- A concrete representation of 'Version'

    -- | Provides one possible concrete representation for 'Version'.  For
    -- a version with 'versionBranch' @= [1,2,3]@ and 'versionTags'
    -- @= [\"tag1\",\"tag2\"]@, the output will be @1.2.3-tag1-tag2@.
    --
    showVersion :: Version -> String
    showVersion (Version branch tags)
      = concat (intersperse "." (map show branch)) ++
         concatMap ('-':) tags

    -- | A parser for versions in the format produced by 'showVersion'.
    --
    #if __GLASGOW_HASKELL__ || __HUGS__
    parseVersion :: ReadP Version
    #elif __NHC__
    parseVersion :: ReadPN r Version
    #else
    parseVersion :: ReadP r Version
    #endif
    parseVersion = do branch <- sepBy1 (liftM read $ munch1 isDigit) (char '.')
                      tags   <- many (char '-' >> munch1 isAlphaNum)
                      return Version{versionBranch=branch, versionTags=tags}