A.hs

module A

makeEtherHasLens :: Name -> DecsQ
makeEtherHasLens name = do
  TyConI dec <- reify name
  fields <- case dec of
    DataD _ctx _name _tvars _kind [dataCon] _derive -> case dataCon of
      RecC _conName fs -> return $ (\(n, _bang, ty) -> (n, ty)) <$> fs
      _ -> fail $ show name ++ ": hust have prefix constructor with named fields"
    _ -> fail $ show name ++ ": must be one constructor record with named fields"
  genHasLens (VarT name) fields

genHasLens
  :: Type
  -- ^ Struct type
  -> [(Name, Type)]
  -- ^ Fields of struct
  -> DecsQ
genHasLens outer fields = fmap mconcat $ for fields $ \(nm, typ) -> do
  let
    setLens = do
      s <- newName "s"
      a <- newName "a"
      let upd = RecUpdE (VarE s) [(nm, VarE a)]
      [e| ( \ $(varP s) $(varP a) -> $(pure upd) ) |]
  [d|instance E.HasLens $(pure typ) $(pure outer) $(pure typ) where
       lensOf = lens $(varE nm) $(setLens)|]

module B

import A

data Rec = Rec
  { _rInt    :: Int
  , _rString :: String
  }

makeEtherHasLens ''Rec

Illegal type variable name: ‘Rec’
When splicing a TH declaration:
  instance Ether.Internal.HasLens.HasLens GHC.Types.Int
                                    EtherCompat.Rec
                                    GHC.Types.Int
where Ether.Internal.HasLens.lensOf = Control.Lens.Lens.lens EtherCompat._rInt (\s_0 a_1 -> s_0{EtherCompat._rInt = a_1})

|
40 | makeEtherHasLens ''Rec
| ^^^^^^^^^^^^^^^^^^^^^^
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