content: {-# LANGUAGE DataKinds, DeriveGeneric, TypeApplications #-}
  
  module Test24 where
  
  -- Test case from #24, comments preserved
  import Optics.Core
  import Data.Generics.Product.Fields
  import Data.Generics.Product.Positions
  import GHC.Generics
  
  data Foo a b = Foo { x1 :: a, x2 :: b } deriving (Generic, Show)
  
  data Bar a b = Bar { x3 :: Foo a b, x4 :: Int } deriving (Generic, Show)
  
  tup :: ((Int, Char), Int)
  tup = ((1, 'a'), 2)
  tup2, tup3, tup4 :: ((Char, Char), Int)
  tup2 = tup & _1 % _1 %~ toEnum  -- Works.
  tup3 = tup & x %~ toEnum  -- Works also with type annotation.
    where x :: Lens ((Int, Char), Int) ((Char, Char), Int) Int Char
          x = _1 % _1
  -- Works.
  tup4 = tup & position @1 % position @1 %~ toEnum
  
  foo :: Foo Int Char
  foo = Foo 1 'a'
  foo2, foo3 :: Foo Char Char
  foo2 = foo & field @"x1" %~ toEnum  -- Works when there's just one 'field'.
  foo3 = foo & position @1 %~ toEnum -- Works when there's just one 'position'.
  
  bar :: Bar Int Char
  bar = Bar (Foo 1 'a') 2
  bar2, bar3, bar4 :: Bar Char Char
  -- Doesn't work, error at first 'field' (Couldn't match type ‘Int’ with ‘Char’ arising from a use of ‘field’).
  bar2 = bar & field @"x3" % field @"x1" %~ toEnum
  -- Type annotation doesn't help.
  bar3 = bar & l %~ toEnum
    where l :: Lens (Bar Int Char) (Bar Char Char) Int Char
          l = field @"x3" % field @"x1"
  -- Doesn't work, error at first 'position' (Couldn't match type ‘Int’ with ‘Char’ arising from a use of ‘position’).
  bar4 = bar & position @1 % position @1 %~ toEnum
  -- Works if we stick to simple Lens' (modify to the same type).
  bar5 :: Bar Int Char
  bar5 = bar & field @"x3" % field @"x1" %~ (+1)
  
  main :: IO ()
  main = print bar5
  