[GHC] #14296: Add `Lift Exp` instance to `Language.Haskell.TH.Syntax`

[GHC]

#14296: Add `Lift Exp` instance to `Language.Haskell.TH.Syntax`
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        Reporter:  heisenbug         |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14030            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by heisenbug):

 Replying to [comment:4 RyanGlScott]:
 > <SNIP>
 >
 > But it's easy to subvert this with your proposed `Lift Exp` instance,
 since you can have, e.g.:
 >
 > {{{#!hs
 > uhOh :: Exp
 > uhOh = lift "uhOh"
 > }}}
 >

 You mean
 {{{#!hs
 uhOh :: ExpQ
 uhOh = lift "uhOh"
 }}}

 > Now `$(lift uhOh)` won't be an `Exp`, but rather a `String`. This is not
 at all what we want.

 Totally agree. My question is when this happens in a `[p|($uhOh ->
 binding)|]` and this ends up in a splice, ultimately being compiled by
 GHC, then I'll get a type error "Expected `Exp`, got: `String`", correct?

 >
 > Replying to [comment:3 heisenbug]:
 > > What about the derivation mechanism for `Lift` instances? Will it do
 ''the right thing'' in this case too?
 >
 > Right. Here is an except of the `-ddump-deriv` output from deriving a
 `Lift` instance for `Exp`:
 >
 > {{{#!hs
 > deriving instance Lift Exp
 >
 > ===>
 >
 > instance Lift Exp where
 >   lift (VarE a) = conE 'VarE `appE` lift a
 >   lift (ConE a) = conE 'ConE `appE` lift a
 >   lift (AppE f x) = conE 'AppE `appE` lift f `appE` lift x
 >   ...
 > }}}
 >
 > Now calling `lift` on an `Exp` will produce an `ExpQ` that represents an
 `Exp` value. It's quite meta, granted, but that's how it should be :)

 That's good. Thanks!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14296#comment:5>
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