Re: [GHC] #15961: TH 'Lift' instance for 'NonEmpty'

28 Nov 2018

      #15961: TH 'Lift' instance for 'NonEmpty'
-------------------------------------+-------------------------------------
        Reporter:  fr33domlover      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Template Haskell  |              Version:  8.6.2
      Resolution:                    |             Keywords:  lift,
                                     |  instance, nonempty
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by fr33domlover:

Old description:
...
I was using `deriving Lift` on a data type and the `DeriveLift`
extension:
{{{#!hs
import Data.List.NonEmpty
import Language.Haskell.TH
data T = T (NonEmpty String) Int
}}}
and I noticed I couldn't get an automatic instance because `NonEmpty`
doesn't have a `Lift` instance. I'm wondering if an instance can be added
to the `template-haskell` package (or elsewhere if that isn't the right
place? I'm assuming it is because `NonEmpty` is in `base` now)
Since `NonEmpty` has a `Data` instance, I suppose the following would be
enough?
{{{#!hs
instance Data a => Lift (NonEmpty a)
}}}
And without using `Data` it could be:
{{{#!hs
nonemptyConName :: Name
nonemptyConName = mkNameG DataName "base" "Data.List.NonEmpty" ":|"
instance Lift a => Lift (NonEmpty a) where
    lift (x :| xs) = do
        x' <- lift x
        xs' <- traverse lift xs
        return $ ConE nonemptyConName `AppE` x' `AppE` xs'
}}}
New description:

 I was using `deriving Lift` on a data type and the `DeriveLift` extension:

 {{{#!hs
 import Data.List.NonEmpty
 import Language.Haskell.TH

 data T = T (NonEmpty String) Int deriving Lift
 }}}

 and I noticed I couldn't get an automatic instance because `NonEmpty`
 doesn't have a `Lift` instance. I'm wondering if an instance can be added
 to the `template-haskell` package (or elsewhere if that isn't the right
 place? I'm assuming it is because `NonEmpty` is in `base` now)

 Since `NonEmpty` has a `Data` instance, I suppose the following would be
 enough?

 {{{#!hs
 instance Data a => Lift (NonEmpty a)
 }}}

 And without using `Data` it could be:

 {{{#!hs
 nonemptyConName :: Name
 nonemptyConName = mkNameG DataName "base" "Data.List.NonEmpty" ":|"

 instance Lift a => Lift (NonEmpty a) where
     lift (x :| xs) = do
         x' <- lift x
         xs' <- traverse lift xs
         return $ ConE nonemptyConName `AppE` x' `AppE` xs'
 }}}

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15961#comment:1
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