content: {-# LANGUAGE CPP, TypeFamilies, MultiParamTypeClasses, FunctionalDependencies,
        FlexibleContexts, FlexibleInstances, UndecidableInstances, OverlappingInstances,
        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 T4809_XMLGenerator where
  
  import Control.Applicative
  import Control.Monad
  import Control.Monad.Trans
  import Control.Monad.Cont  (MonadCont)
  import Control.Monad.Except(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 (Monad, Functor, Applicative, MonadIO, MonadPlus, MonadWriter w,
              MonadReader r, MonadState s, MonadRWS r w s, MonadCont,
              MonadError e)
  
  instance Monad m => Alternative (XMLGenT m) where
  
  -- | un-lift.
  unXMLGenT :: XMLGenT m a -> m a
  unXMLGenT   (XMLGenT ma) =  ma
  
  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 XML m
   data Child m
   genElement  :: Name -> [XMLGenT m [Int]] -> [XMLGenT m [Child m]] -> XMLGenT m (XML m)
   genEElement :: Name -> [XMLGenT m [Int]]                          -> XMLGenT m (XML m)
   genEElement n ats = genElement n ats []
  
  -- | 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 -> XMLGenT m [Child m]
  
  instance (MonadIO m, EmbedAsChild m c, m ~ n) => EmbedAsChild m (XMLGenT n c) where
   asChild m = do
        liftIO $ putStrLn "EmbedAsChild m (XMLGenT n c)"
        a <- m
        asChild a
  
  instance (MonadIO m, EmbedAsChild m c) => EmbedAsChild m [c] where
    asChild cs =
        do liftIO $ putStrLn "EmbedAsChild m [c]"
           liftM concat . mapM asChild $ cs
  
  instance (MonadIO m, XMLGen m) => EmbedAsChild m (Child m) where
   asChild c =
       do liftIO $ putStrLn "EmbedAsChild m (Child m)"
          return . return $ c
  