[Haskell-cafe] Still stacking monad transformers

[Andrew Coppin]

Reid Barton wrote:
> It's not difficult: the operation is called
>
> mplus :: MyMonad a -> MyMonad a -> MyMonad a
>
> and already exists (assuming the author of ListT has not forgotten to
> write a MonadPlus instance).
>   

I see... I was under the impression that "mplus" is just any arbitrary 
binary operation over a given monad. How do you know what it does for a 
specific monad?

Anyway, utilising this trick, I now have my function working quite well. 
Implementing negation is the only hard part; I need to unwind everything 
down to the list level, and check whether the list is empty, and do 
something different depending on whether it is or it isn't:

  foo = do
    ...
    let x = run_some_moadic_action
    if null x
      then ...
      else ...

This obviously fails since "x" isn't a list, it's a StateT MyState 
(ListT (ErrorT MyError Ideneity)) x. I can't see a "nice" way to handle 
this. I found a way that works, but it's quite ugly...