[GHC] #1830: Automatic derivation of Lift

[GHC]

#1830: Automatic derivation of Lift
-------------------------------------+-------------------------------------
        Reporter:  guest             |                   Owner:
                                     |  RyanGlScott
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  ⊥
       Component:  Template Haskell  |                 Version:  6.8.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D1168
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * owner:   => RyanGlScott
 * differential:   => Phab:D1168


Comment:

 I have a differential ( Phab:D1168 ) submitted which implements this
 feature, but I would like some feedback. I'll copy over my questions from
 the Phabricator page:


 1. What should we do if someone attempts to derive `Lift` on a datatype
 with no constructors? I (somewhat arbitrarily) chose it to generate code
 like this:

 {{{#!hs
 data Void deriving Lift

 instance Lift Void where
     lift _ = appE (varE 'error) (litE (stringL "Void lift"))
 }}}

   but {{{lift _ = error "Void lift"}}} would also work.

 2. How should primitive data types be handled? I adapted the approach
 `Show` takes, which has special cases for `Char#`, `Float#`, `Double#`,
 `Int#`, and `Word#`. Note that `template-haskell` also has a `StringPrimL`
 constructor, so we could handle `Addr#` as well, but there's not currently
 a function of type `String -> [Word8]` in `base` that I could use in
 conjunction with it
 ([http://haddock.stackage.org/lts-3.2/ghc-7.10.2/SMRep.html#v:stringToWord8s
 there is in] `ghc`, but I'm not sure if it's verboten to put GHC internals
 in derived code).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1830#comment:8>
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