I'm farther from an expert than you. I tried (and failed) to find a less confusing way...

One thing I see people doing when they have real gnarly type signature is to concentrate the horror in one place and paint a veneer of simplicity elsewhere:

    type Get stateType returnType
      = forall monad.
      (SpockState monad ~ stateType, MonadIO monad, HasSpock monad)
      => monad returnType

Then elsewhere:

    getBlockChain :: Get BlockChainState [Block]
    getLatestBlock :: Get BlockChainState Block

This may only help superficially as type errors will still be hard-to-understand.