[GHC] #12464: Add `instance Semigroup a => Semigroup (IO a)`

[GHC]

#12464: Add `instance Semigroup a => Semigroup (IO a)`
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.1
      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:D2588
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"8c6a3d68c0301bb985aa2a462936bbcf7584ae9c/ghc" 8c6a3d6/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8c6a3d68c0301bb985aa2a462936bbcf7584ae9c"
 Add missing Semigroup instances for Monoidal datatypes in base

 Summary:
 There are currently three datatypes that are exposed in `base` that have
 `Monoid` instances, but no `Semigroup` instances:

 * `IO`
 * `Event` (from `GHC.Event`)
 * `Lifetime` (from `GHC.Event`)

 (There is also `EventLifetime` in `GHC.Event.Internal`, but it is not
 exported
 directly, so I didn't bother with it.)

 Adding the `Semigroup` instances for these types directly in the modules
 in
 which they're defined resulted in some horrific import cycles, so I opted
 to
 take the easy approach of defining all of these instances in
 `Data.Semigroup`.
 (When `Semigroup` becomes a superclass of `Monoid`, these instances will
 have
 to be moved somehow.)

 Fixes #12464.

 Test Plan: It compiles

 Reviewers: hvr, ekmett, austin, bgamari

 Reviewed By: ekmett

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2588

 GHC Trac Issues: #12464
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12464#comment:2>
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