[Haskell-cafe] The rmonad package is mysterious

I tried to do something simple using the rmonad package (attached). I followed the example that's in haddocks (it is wrong, constraints has no arguments), grabbed the list of extensions from rmonad's source, but this program doesn't typecheck: Could not deduce (Hashable a) arising from a use of `HM.singleton' This seems to be an obvious use of this package ... am I missing something ?