Typeclass for monads that can be unlifted to arbitrary base monads:

class (MonadBase b m) => MonadUnlift b m
  where
    withRunInBase :: ((forall a . m a -> b a) -> b x) -> m x

This typeclass is helpful when writing code that is polymorphic over the base monad, so later you can select a different base monad for each specific use-case.

Common usages include:

1. Concurrent testing with dejafu.
2. Working with code over polymorphic PrimMonad.