[GHC] #15178: Implement DerivingVia

[GHC]

#15178: Implement DerivingVia
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:  deriving,
                                     |  GHCProposal
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4684
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"8ed8b037fee9611b1c4ef49adb6cf50bbd929a27/ghc" 8ed8b03/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8ed8b037fee9611b1c4ef49adb6cf50bbd929a27"
 Introduce DerivingVia

 This implements the `DerivingVia` proposal put forth in
 https://github.com/ghc-proposals/ghc-proposals/pull/120.

 This introduces the `DerivingVia` deriving strategy. This is a
 generalization of `GeneralizedNewtypeDeriving` that permits the user
 to specify the type to `coerce` from.

 The major change in this patch is the introduction of the
 `ViaStrategy` constructor to `DerivStrategy`, which takes a type
 as a field. As a result, `DerivStrategy` is no longer a simple
 enumeration type, but rather something that must be renamed and
 typechecked. The process by which this is done is explained more
 thoroughly in section 3 of this paper
 ( https://www.kosmikus.org/DerivingVia/deriving-via-paper.pdf ),
 although I have inlined the relevant parts into Notes where possible.

 There are some knock-on changes as well. I took the opportunity to
 do some refactoring of code in `TcDeriv`, especially the
 `mkNewTypeEqn` function, since it was bundling all of the logic for
 (1) deriving instances for newtypes and
 (2) `GeneralizedNewtypeDeriving`
 into one huge broth. `DerivingVia` reuses much of part (2), so that
 was factored out as much as possible.

 Bumps the Haddock submodule.

 Test Plan: ./validate

 Reviewers: simonpj, bgamari, goldfire, alanz

 Subscribers: alanz, goldfire, rwbarton, thomie, mpickering, carter

 GHC Trac Issues: #15178

 Differential Revision: https://phabricator.haskell.org/D4684
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15178#comment:4>
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