[Haskell-cafe] Inverting a Monad

[Lutz Donnerhacke]

* Felipe Lessa wrote:
> Prelude Control.Monad> inv []
> [()]
> Prelude Control.Monad> inv [10]
> [()]

 inv m = if m == mzero then return () else mzero `asTypeOf` m

Yes, unfair.