    {-# LANGUAGE CPP, TypeFamilies, MultiParamTypeClasses, FunctionalDependencies,
          FlexibleContexts, FlexibleInstances, UndecidableInstances,
          TypeSynonymInstances, GeneralizedNewtypeDeriving #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  HSX.XMLGenerator
    -- Copyright   :  (c) Niklas Broberg 2008
    -- License     :  BSD-style (see the file LICENSE.txt)
    --
    -- Maintainer  :  Niklas Broberg, niklas.broberg@chalmers.se
    -- Stability   :  experimental
    -- Portability :  requires newtype deriving and MPTCs with fundeps
    --
    -- The class and monad transformer that forms the basis of the literal XML
    -- syntax translation. Literal tags will be translated into functions of
    -- the GenerateXML class, and any instantiating monads with associated XML
    -- types can benefit from that syntax.
    -----------------------------------------------------------------------------
    module HSX.XMLGenerator where

    import Control.Applicative (Applicative, Alternative)
    import Control.Monad.Trans (MonadTrans(lift), MonadIO)
    import Control.Monad.Cont  (MonadCont)
    import Control.Monad.Error (MonadError)
    import Control.Monad.Reader(MonadReader)
    import Control.Monad.Writer(MonadWriter)
    import Control.Monad.State (MonadState)
    import Control.Monad.RWS   (MonadRWS)
    import Control.Monad       (MonadPlus(..),liftM)

    ----------------------------------------------
    -- General XML Generation

    -- | The monad transformer that allows a monad to generate XML values.
    newtype XMLGenT m a = XMLGenT (m a)
      deriving (Applicative, Alternative, Monad, Functor, MonadIO, MonadPlus, MonadWriter w, MonadReader r,
                MonadState s, MonadRWS r w s, MonadCont, MonadError e)

    -- | un-lift.
    unXMLGenT :: XMLGenT m a -> m a
    unXMLGenT   (XMLGenT ma) =  ma

    -- | map the inner monad
    mapXMLGenT :: (m a -> n b) -> XMLGenT m a -> XMLGenT n b
    mapXMLGenT f (XMLGenT m) = XMLGenT (f m)

    instance MonadTrans XMLGenT where
     lift = XMLGenT

    type Name = (Maybe String, String)

    -- | Generate XML values in some XMLGenerator monad.
    class Monad m => XMLGen m where
     type XMLType m
     data ChildType m
     data AttributeType m
     genElement  :: Name -> [XMLGenT m [AttributeType m]] -> [XMLGenT m [ChildType m]] -> XMLGenT m (XMLType m)
     genEElement :: Name -> [XMLGenT m [AttributeType m]]                              -> XMLGenT m (XMLType m)
     genEElement n ats = genElement n ats []
     xmlToChild :: XMLType m -> ChildType m
     pcdataToChild :: String -> ChildType m

    -- | Type synonyms to avoid writing out the XMLnGenT all the time
    type GenXML m           = XMLGenT m (XMLType m)
    type GenXMLList m       = XMLGenT m [XMLType m]
    type GenChild m         = XMLGenT m (ChildType m)
    type GenChildList m     = XMLGenT m [ChildType m]
    type GenAttribute m     = XMLGenT m (AttributeType m)
    type GenAttributeList m = XMLGenT m [AttributeType m]

    -- | Embed values as child nodes of an XML element. The parent type will be clear
    -- from the context so it is not mentioned.
    class XMLGen m => EmbedAsChild m c where
     asChild :: c -> GenChildList m

    #if __GLASGOW_HASKELL__ >= 610
    instance (EmbedAsChild m c, m ~ n) => EmbedAsChild m (XMLGenT n c) where
     asChild m = asChild =<< m
    #else
    instance (EmbedAsChild m c, TypeCastM m1 m) => EmbedAsChild m (XMLGenT m1 c) where
     asChild (XMLGenT m1a) = do
                a <- XMLGenT $ typeCastM m1a
                asChild a
    #endif

    instance EmbedAsChild m c => EmbedAsChild m [c] where
     asChild = liftM concat . mapM asChild

    instance XMLGen m => EmbedAsChild m (ChildType m) where
     asChild = return . return

    #if __GLASGOW_HASKELL__ >= 610
    instance (XMLGen m,  XMLType m ~ x) => EmbedAsChild m x where
    #else
    instance (XMLGen m) => EmbedAsChild m (XMLType m) where
    #endif
     asChild = return . return . xmlToChild

    instance XMLGen m => EmbedAsChild m String where
     asChild = return . return . pcdataToChild

    instance XMLGen m => EmbedAsChild m () where
     asChild _ = return []

    -- | Similarly embed values as attributes of an XML element.
    class XMLGen m => EmbedAsAttr m a where
     asAttr :: a -> GenAttributeList m

    instance (XMLGen m, EmbedAsAttr m a) => EmbedAsAttr m (XMLGenT m a) where
     asAttr ma = ma >>= asAttr

    instance (EmbedAsAttr m (Attr a v), TypeCastM m1 m) => EmbedAsAttr m (Attr a (XMLGenT m1 v)) where
     asAttr (a := (XMLGenT m1a)) = do
                v <- XMLGenT $ typeCastM m1a
                asAttr (a := v)

    instance XMLGen m => EmbedAsAttr m (AttributeType m) where
     asAttr = return . return

    instance EmbedAsAttr m a => EmbedAsAttr m [a] where
     asAttr = liftM concat . mapM asAttr

    class (XMLGen m,
           SetAttr m (XMLType m),
           AppendChild m (XMLType m),
           EmbedAsChild m (XMLType m),
           EmbedAsChild m [XMLType m],
           EmbedAsChild m String,
           EmbedAsChild m Char, -- for overlap purposes
           EmbedAsChild m (),
           EmbedAsAttr m (Attr String String),
           EmbedAsAttr m (Attr String Int),
           EmbedAsAttr m (Attr String Bool)
           ) => XMLGenerator m

    {-
    -- This is certainly true, but we want the various generators to explicitly state it,
    -- in order to get the error messages right.
    instance (XMLGen m,
           SetAttr m (XMLType m),
           AppendChild m (XMLType m),
           EmbedAsChild m (XMLType m),
           EmbedAsChild m [XMLType m],
           EmbedAsChild m String,
           EmbedAsChild m Char,
           EmbedAsChild m (),
           EmbedAsAttr m (Attr String String),
           EmbedAsAttr m (Attr String Int),
           EmbedAsAttr m (Attr String Bool)
           ) => XMLGenerator m
    -}

    data Attr n a = n := a
      deriving Show

    -------------------------------------
    -- Setting attributes

    -- | Set attributes on XML elements
    class XMLGen m => SetAttr m elem where
     setAttr :: elem -> GenAttribute m     -> GenXML m
     setAll  :: elem -> GenAttributeList m -> GenXML m
     setAttr e a = setAll e $ liftM return a

    (<@), set :: (SetAttr m elem, EmbedAsAttr m attr) => elem -> attr -> GenXML m
    set xml attr = setAll xml (asAttr attr)
    (<@) = set

    (<<@) :: (SetAttr m elem, EmbedAsAttr m a) => elem -> [a] -> GenXML m
    xml <<@ ats = setAll xml (liftM concat $ mapM asAttr ats)

    instance (TypeCastM m1 m, SetAttr m x) =>
            SetAttr m (XMLGenT m1 x) where
     setAll (XMLGenT m1x) ats = (XMLGenT $ typeCastM m1x) >>= (flip setAll) ats

    -------------------------------------
    -- Appending children

    class XMLGen m => AppendChild m elem where
     appChild :: elem -> GenChild m     -> GenXML m
     appAll   :: elem -> GenChildList m -> GenXML m
     appChild e c = appAll e $ liftM return c

    (<:), app :: (AppendChild m elem, EmbedAsChild m c) => elem -> c -> GenXML m
    app xml c = appAll xml $ asChild c
    (<:) = app

    (<<:) :: (AppendChild m elem, EmbedAsChild m c) => elem -> [c] -> GenXML m
    xml <<: chs = appAll xml (liftM concat $ mapM asChild chs)

    instance (AppendChild m x, TypeCastM m1 m) =>
            AppendChild m (XMLGenT m1 x) where
     appAll (XMLGenT m1x) chs = (XMLGenT $ typeCastM m1x) >>= (flip appAll) chs

    -------------------------------------
    -- Names

    -- | Names can be simple or qualified with a domain. We want to conveniently
    -- use both simple strings or pairs wherever a Name is expected.
    class Show n => IsName n where
     toName :: n -> Name

    -- | Names can represent names, of course.
    instance IsName Name where
     toName = id

    -- | Strings can represent names, meaning a simple name with no domain.
    instance IsName String where
     toName s = (Nothing, s)

    -- | Pairs of strings can represent names, meaning a name qualified with a domain.
    instance IsName (String, String) where
     toName (ns, s) = (Just ns, s)

    ---------------------------------------
    -- TypeCast, in lieu of ~ constraints

    -- literally lifted from the HList library
    class TypeCast   a b   | a -> b, b -> a      where typeCast   :: a -> b
    class TypeCast'  t a b | t a -> b, t b -> a  where typeCast'  :: t->a->b
    class TypeCast'' t a b | t a -> b, t b -> a  where typeCast'' :: t->a->b
    instance TypeCast'  () a b => TypeCast a b   where typeCast x = typeCast' () x
    instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast''
    instance TypeCast'' () a a where typeCast'' _ x  = x

    class TypeCastM   ma mb   | ma -> mb, mb -> ma      where typeCastM   :: ma x -> mb x
    class TypeCastM'  t ma mb | t ma -> mb, t mb -> ma  where typeCastM'  :: t -> ma x -> mb x
    class TypeCastM'' t ma mb | t ma -> mb, t mb -> ma  where typeCastM'' :: t -> ma x -> mb x
    instance TypeCastM'  () ma mb => TypeCastM ma mb   where typeCastM mx = typeCastM' () mx
    instance TypeCastM'' t ma mb => TypeCastM' t ma mb where typeCastM' = typeCastM''
    instance TypeCastM'' () ma ma where typeCastM'' _ x  = x