I am trying to generate lens for data type with a lens field.
data St st l = St {
_st_s :: String,
_st_lens :: Lens' st l
}
st_lens :: forall st l. Lens' (St st l) (Lens' st l)
st_lens = lens _st_lens (\s a -> s { _st_lens = a })
GHC 8.10.7 gives me an error:
Illegal polymorphic type:
forall (f1 :: * -> *). Functor f1 => (l -> f1 l) -> st -> f1 st
GHC doesn't yet support impredicative polymorphism
• In the expansion of type synonym ‘Lens’