[GHC] #14400: Make :sprint, :print, and :force work with cyclical structures

[GHC]

#14400: Make :sprint, :print, and :force work with cyclical structures
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  GHCi           |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 > t = "hi" : t
 > :force t
 }}}

 never completes, because GHC races around and around in a loop forever. It
 would seem better to keep a hashset of nodes that have already been
 visited to avoid getting stuck, and print out a reasonable representation
 of the graph.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14400>
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