T12406.hs:20:7: error:
    • Couldn't match type ‘Ref m0’ with ‘IORef’
      Expected type: IO (Ref m0 (f0 ()))
        Actual type: IO (Ref IO (f0 ()))
      The type variable ‘m0’ is ambiguous
    • In the first argument of ‘(>>=)’, namely ‘newRef (pure ())’
      In the expression: newRef (pure ()) >>= join . readRef
      In an equation for ‘foo’: foo = newRef (pure ()) >>= join . readRef

T12406.hs:20:28: error:
    Variable not in scope: join :: m0 (f0 ()) -> IO ()