Do not tackle the bigger "abstractions around visibility, delay, blocking" etc. issue. This is a minimal fix:

• print code already deals with future as a special case
• should special-case promise as well
• make a definterface IPromiseImpl that you can switch on
  • right above promise in core.clj
  • has a method that gets you what you need to ask "has-value?"
• in print code
  • if has value, show it, if not, show :not-delivered
• make sure to fix print code in both core_print and pprint