Is there a way to ask GHCI about where a particular instance is defined?

Question

For example, can I query "where is RWS defined as an instance of MonadState"? I would expect an answer like module (version), source file, line.

Answer 1

λ> :info MonadState
...
instance (Monad m, Monoid w) => MonadState s (RWST r w s m)
  -- Defined in `Control.Monad.State.Class'
λ> :i RWS
type RWS r w s = RWST r w s Identity
        -- Defined in `Control.Monad.Trans.RWS.Lazy'
λ> :i RWST
...
instance MonadState s m => MonadState s (Reader.ReaderT r m)
  -- Defined in `Control.Monad.State.Class'
...

You can't get line information, and it only shows modules that are currently loaded, but that's enough to hunt instances down.