Depends on data-has

Foo.hs

{-# LANGUAGE FlexibleContexts, FlexibleInstances, UndecidableInstances #-}
import Data.Has
data Foo = Foo deriving Show
data Bar = Bar deriving Show

class HasFoo r where
  foo :: r -> Foo

instance HasFoo Foo where
  foo = id

instance Has Foo r => HasFoo r where
  foo = getter

useFoo :: HasFoo r => r -> Foo
useFoo = foo

{-
 I want to call this with `useFooAndBar (Foo, Bar)`, but there's no HasFoo instance for that, hence the `instance Has Foo r => HasFoo r`, but that gives:
Foo.hs:12:10: error:
    • The constraint ‘Has Foo r’ is no smaller than the instance head
      (Use UndecidableInstances to permit this)
    • In the instance declaration for ‘HasFoo r’
   |
12 | instance Has Foo r => HasFoo r where
   |                       ^^^^^^^^
-}

useFooAndBar :: (HasFoo r, Has Bar r) => r -> (Foo, Bar)
useFooAndBar x = (foo x, getter x)

--class HasBar r where
--  bar :: r -> Bar