Route from GHC generics to LR generics

Suppose a function such as

allEqualTo :: Eq a => a -> [a] -> Bool
allEqualTo x = all (== x)

is instead written as

allEqualTo :: (GHC.Generic a, GHC.GEq' (GHC.Rep a)) => a -> [a] -> Bool
allEqualTo x = all (GHC.geqdefault x)

where instead of using an indirection through an auxiliary type class Eq, it directly assumes GHC.Generics and uses a concrete generic implementation. Such design is arguably questionable, but for example beam-core contains many such deeply ingrained assumptions of the availability of GHC.Generics.

In order to be able to call such a function on a large record Foo, largeRecord will generate an instance

instance GHC.Generic Foo where
  type Rep Foo = ThroughLRGenerics Foo

  from = WrapThroughLRGenerics
  to   = unwrapThroughLRGenerics

For our running example, this instance makes it possible to call allEqualTo provided we then provide an instance

instance ( LR.Generic a
         , LR.Constraints a Eq
         ) => GHC.GEq' (ThroughLRGenerics a) where
  geq' = LR.geq `on` unwrapThroughLRGenerics

Effectively, ThroughLRGenerics can be used to redirect a function that uses GHC generics to a function that uses LR generics.

GHC generics metadata

TODO: Currently we provide metadata only for the record fields, not the constructor or type name