Re: [Haskell-cafe] tuple and HList

David Menendez wrote:

instance Functor ((,) a) where fmap f (x,y) = (x, f y)

If we get rid of '(,)' and redefine '(a,b)' as sugar for 'TCons a (TCons b HNil)' (or whatever), then there is no way to declare the above instance. I don't think that's a deal-killer, but it is a disadvantage.

You need to swap the arguments to TCons... data TCons l a = TCons !l a Then: instance Functor (TCons (TCons HNil a)) where fmap f (TCons (TCons HNil x) y) = TCons (TCons HNil (f x)) y) Keean.