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{-# LANGUAGE FlexibleContexts, FlexibleInstances, GeneralizedNewtypeDeriving, OverlappingInstances, UndecidableInstances #-}
module Main where
class (Monad m) => MonadIO m where
-- | Lift a computation from the 'IO' monad.
liftIO :: IO a -> m a
instance MonadIO IO where
liftIO = id
class XMLGenerator m where
genElement :: (Maybe String, String) -> m ()
newtype IdentityT m a = IdentityT { runIdentityT :: m a }
deriving (Functor, Applicative, Monad, MonadIO)
instance (MonadIO m) => (XMLGenerator (IdentityT m)) where
genElement _ = liftIO $ putStrLn "in genElement"
main :: IO ()
main =
do runIdentityT web
putStrLn "done."
class (Widgets x) => MonadRender x
class (XMLGenerator m) => Widgets m
-- instance Widgets (IdentityT IO) -- if you uncomment this, it will work
instance (XMLGenerator m, MonadRender m) => Widgets m
instance MonadRender (IdentityT IO)
web :: ( MonadIO m
, Widgets m
, XMLGenerator m
) => m ()
web =
do liftIO $ putStrLn "before"
genElement (Nothing, "p")
return ()
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