[Haskell-cafe] How to understand such a newtype ?

[zaxis]

newtype X a = X (ReaderT XConf (StateT XState IO) a)
#ifndef __HADDOCK__
    deriving (Functor, Monad, MonadIO, MonadState XState, MonadReader XConf,
Typeable)
#endif

In `X (ReaderT XConf (StateT XState IO) a)`, X is a type constructor, how to
understand `(ReaderT XConf (StateT XState IO) a)` ?  And why use `#ifndef
__HADDOCK__` ?

Sincerely!

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fac n = foldr (*) 1 [1..n]
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