main :: IO ()
main = pure ()

class ExplExists m c where
  explExists :: Int -> m Bool
  
class ExplGet m c where
  explGet :: Int -> m c

data A = A Int
data B = B Word
data Foo = FooA A | FooB B

instance ExplExists IO Foo where
  explExists _ety = ...

instance ExplGet IO Foo where
  explGet _ety = ...
  
instance ExplExists IO A where
  explExists ety = do
    foo :: Foo <- explGet ety
    pure $ case foo of
      FooA _ -> True
      _ -> False
      
instance ExplExists IO B where
  explExists ety = do
    foo :: Foo <- explGet ety
    pure $ case foo of
      FooB _ -> True
      _ -> False
      
instance ExplGet IO A where
  explGet ety = do
    foo :: Foo <- explGet ety
    pure $ case foo of
      FooA a -> a
      _ -> error "accessing non-existent component"

instance ExplGet IO B where
  explGet ety = do
    foo :: Foo <- explGet ety
    pure $ case foo of
      FooB b -> b
      _ -> error "accessing non-existent component"