RepPolyMcBind.hs:26:16: error:
    • The first argument to (>>=),
      arising from the monad comprehension,
      does not have a fixed runtime representation:
        ma :: TYPE rep
    • In a stmt of a monad comprehension: x <- undefined :: ma
      In the expression: [() | x <- undefined :: ma]
      In an equation for ‘foo’: foo _ = [() | x <- undefined :: ma]