From the homework description I've tried running:

dlee@morty ~/repos/cis194/week7 $ runhaskell StringBufEditor.hs

Editor.hs:11:8:
    Could not find module `Control.Monad.State'
    Perhaps you meant
      Control.Monad.ST (from base)
      Control.Monad.ST.Safe (from base)
      Control.Monad.Fix (from base)
    Use -v to see a list of the files searched for.