[GHC] #10874: Implement `:type-at`, `:all-types`, `:loc-at` in GHCi

[GHC]

#10874: Implement `:type-at`, `:all-types`, `:loc-at` in GHCi
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  GHCi              |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1240
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"7997d6c0f0ba4560dab799cd87850917e0df5e2f/ghc"
 7997d6c0/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="7997d6c0f0ba4560dab799cd87850917e0df5e2f"
 Refactor GHCi Command type; allow "hidden" commands

 This transforms the 'Command' tuple into a record which is
 easier to extend.

 While at it, this refactoring turns the IDE `:complete` into a hidden
 command excluded from completion.

 The next obvious step is to add a summary text field for constructing
 the `:help` output (as well as allowing to get `:help <CMD>` for single
 commands.

 This is a preparatory refactoring for D1240 / #10874

 Reviewed By: thomie, bgamari

 Differential Revision: https://phabricator.haskell.org/D1590
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10874#comment:4>
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