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

[GHC]

#10874: Implement `:type-at`, `:all-types`, `:loc-at` in GHCi
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        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:"fd3b845c01aa26b6e5cd12c00af59e5468e21b1b/ghc"
 fd3b845c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="fd3b845c01aa26b6e5cd12c00af59e5468e21b1b"
 Make HasDynFlags more transformers friendly

 Ideally, we'd have the more general

     instance (MonadTrans t, Monad m, HasDynFlags m) => HasDynFlags (t m)
 where
         getDynFlags = lift getDynFlags

 definition. However, that one would overlap with the `HasDynFlags (GhcT
 m)`
 instance. Instead we define instances for a couple of common Monad
 transformers explicitly in order to avoid nasty overlapping instances.

 This is a preparatory refactoring for #10874

 Reviewed By: austin

 Differential Revision: https://phabricator.haskell.org/D1581
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10874#comment:3>
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