[GHC] #15650: Add (or document if already exist) ability to derive custom typeclasses via source plugins

[GHC]

#15650: Add (or document if already exist) ability to derive custom typeclasses via
source plugins
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        Reporter:  chshersh          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:  source
                                     |  plugins,deriving,typeclass
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I am far from an expert on source plugins, so I can't say whether this is
 possible or not. But my initial reaction is: sure, why not? I could
 imagine an API like this:

 {{{#!hs
 type Derived a = a

 class Foo a where ...

 data Bar deriving (Derived Foo)
 }}}

 Here, the use of `Derived` is a syntactic clue to a source plugin to
 derive this using some custom functionality (instead of just trying to
 derive `Foo` normally).

 To make this robust, you'd likely need to borrow some of GHC's own logic
 for `deriving` type classes. Luckily, GHC already exposes much of this!
 See the `TcDeriv`, `TcGenDeriv`, and `TcDerivUtils` modules.

 I certainly don't have the time to try this out myself, but I'd be happy
 point any volunteers in the right direction.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15650#comment:1>
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