[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:"dd56eb1efc11bcbd60ab0b77ca3e4f949d7d0844/ghc"
 dd56eb1e/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="dd56eb1efc11bcbd60ab0b77ca3e4f949d7d0844"
 Merge new commands from ghci-ng (re #10874)

 This adds the new commands `:all-types`, `:loc-at`, `:type-at`, and
 `:uses` designed for editor-integration (such as Emacs' `haskell-mode`).

 This was originally implemented by Chris Done on

   https://github.com/chrisdone/ghci-ng

 and has been in use by Emacs' `haskell-mode` for over a year already,
 and closely missed the GHC 7.10 release back then.

 I've squashed the commits, rebased to GHC HEAD, and heavily refactored and
 improved the patch.

 Tests will be added in a separate commit.

 Reviewed By: bgamari

 Differential Revision: https://phabricator.haskell.org/D1240
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10874#comment:5>
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