You linked to the Trac issue [1], but for the purposes of motivating this proposal, I think it's important that we reiterate why you want this. First of all, I think we should be clear that you're proposing to add these in the modules: * Data.Functor.One * Data.Functor.Zero Right? Not literally "Data.Functor", as the title might mislead you into believing. Also, for a reason why you might want these - there are currently analogs in the GHC.Generics [2] module: * data U1 a = U1 -- isomorphic to One * data V1 a -- isomorphic to Zero But this is an unfortunate situation, since I'd rather not recommend that folks import a generics library to use what should be general-purpose datatypes. Now, as for the proposal itself: * I'm weakly +1 in favor of Zero. I think it'd be nice to have a proper * -> * counterpart to Void in base (that isn't V1, obviously). * I'm weakly -1 in favor of One, simply because we already have Data.Proxy, which came first. Ryan S. ----- [1] https://ghc.haskell.org/trac/ghc/ticket/13177 [2] http://hackage.haskell.org/package/base-4.9.1.0/docs/GHC-Generics.html