How to circumvent existing instances (for Failure) in Haskell?

Question

I'm in a monad transformer with IO and I'd like to define my own instance for Failure.

Because Failure already defines instances for IO and for MonadTrans, I cannot even build my own overlapping instance.

As far as I know, I have four options left:

1. Newtyping IO:
   This is a little bit awkward, I would need to derive all that I want to change, and redefine the rest.
2. Hacking Failure to separate the class itself into it's own Module:
   I would move the class definition into a submodule Control.Failure.Class
3. Creating a Simple wrapper module for Failure without reexporting the instances
4. Removing MonadTrans from my Monad Transformer

Do you know any other option? What do you think?

Answer 1

A newtype wrapper is the standard approach to such a problem. The GeneralizedNewtypeDeriving extension makes optionally deriving instances from the wrapped monad a breeze.

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

-- | Your custom monad transformer
newtype YourMonadT e m r = 
  YourMonadT (EitherT e m r)
  -- Easily derive the instances using the GeneralizedNewtypeDeriving
  deriving (Functor, Applicative, Monad, MonadIO)

instance Failure e (YourMonadT e m) where
  failure = error "TODO: implement me however you want"