[Haskell-cafe] Indentation Creep

[Dan Doel]

In addition to what's already been pointed out, note that this:

>  do t <- readTVar p
>     case t of
>         Empty -> return Nothing
>         Trie l m r -> do <stuff>

Is a case of the (non-existent) MaybeT transformer:

> do Trie l m r <- readTVar p
>    <stuff with slight modifications>

The modifications being something like 'return . Just' => 'return', 
and 'return Nothing' => mzero.

> newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

> instance Functor f => Functor (MaybeT f) where
>     fmap f = MaybeT . fmap (fmap f) . runMaybeT

> instance Monad m => Monad (MaybeT m) where
>     return  = MaybeT . return . Just
>     m >>= f = MaybeT $ runMaybeT m >>= (runMaybeT . f)
>     fail _  = MaybeT $ return Nothing

> instance Monad m => MonadPlus (MaybeT m) where
>     mzero = MaybeT $ return Nothing
>     m1 `mplus` m2 = MaybeT $ liftM2 mplus (runMaybeT m1) (runMaybeT m2)

(I haven't tested the code, but that's approximately what it looks like; let 
me know if I did something wrong and you need it fixed; I suspect you won't, 
as I'm not sure it simplifies the remained of <stuff> any :))

-- Dan