[GHC] #14030: Implement the "Derive Lift instances for data types in template-haskell" proposal

[GHC]

#14030: Implement the "Derive Lift instances for data types in template-haskell"
proposal
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Template Haskell  |              Version:  8.3
      Resolution:                    |             Keywords:  deriving
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):

 Replying to [comment:8 harpocrates]:
 > the code that `Lift` will generate won't involve `conE`, `mkNameG_d`,
 etc. - it'll just be generating syntax for a bracket!

 I'm not so sure. Nominally, yes, the generated code will only use the
 bracket syntax. But one must ask: what does the bracket syntax desugar
 down to? The answer may surprise you:

 {{{
 λ> :set -ddump-simpl
 λ> data MyBool = F | T
 λ> deriving instance Lift MyBool

 ==================== Tidy Core ====================
 Result size of Tidy Core
   = {terms: 38, types: 12, coercions: 3, joins: 0/0}

 -- RHS size: {terms: 21, types: 2, coercions: 0, joins: 0/0}
 $clift_r4ul :: MyBool -> Q Exp
 [GblId, Arity=1, Unf=OtherCon []]
 $clift_r4ul
   = \ (ds_d4um :: MyBool) ->
       case ds_d4um of {
         F ->
           Language.Haskell.TH.Lib.Internal.conE
             (mkNameG_d
                (GHC.CString.unpackCString# "interactive"#)
                (GHC.CString.unpackCString# "Ghci4"#)
                (GHC.CString.unpackCString# "F"#));
         T ->
           Language.Haskell.TH.Lib.Internal.conE
             (mkNameG_d
                (GHC.CString.unpackCString# "interactive"#)
                (GHC.CString.unpackCString# "Ghci4"#)
                (GHC.CString.unpackCString# "T"#))
       }

 -- RHS size: {terms: 1, types: 0, coercions: 3, joins: 0/0}
 interactive:Ghci5.$fLiftMyBool [InlPrag=INLINE (sat-args=0)]
   :: Lift MyBool
 [GblId[DFunId(nt)], Arity=1, Unf=OtherCon []]
 interactive:Ghci5.$fLiftMyBool
   = $clift_r4ul
     `cast` (Sym (Language.Haskell.TH.Syntax.N:Lift[0] <MyBool>_N)
             :: (MyBool -> Q Exp) ~R# Lift MyBool)
 }}}

 It turns out that bracket syntax desugars down to code that, in fact, uses
 `conE` and `mkNameG_d`! It's a bit more indirect, but I worry that there's
 the same potential for fragility.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14030#comment:9>
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