RepPolyTuple.hs:11:9: error:
    • • The tuple argument in third position
        does not have a fixed runtime representation.
        Its type is:
          a :: TYPE r
      • The tuple argument in second position
        does not have a fixed runtime representation.
        Its type is:
          a :: TYPE r
      • The tuple argument in first position
        does not have a fixed runtime representation.
        Its type is:
          a :: TYPE r
    • In the expression: (# bar (), bar (), bar () #)
      In an equation for ‘foo’:
          foo _
            = (# bar (), bar (), bar () #)
            where
                bar :: forall (r :: RuntimeRep) (a :: TYPE r). () -> a
                bar _ = undefined