proj :: (Functor f) => (f:*:f') (a,b) -> f a proj (Pair x _) = fmap fst x

Yes I think that was what I needed. ;-) It seems I also need fsplit :: (Functor f, Functor f') => (f a, f' b) -> (f :*: f') (a,b) for same data (f:*:f') o = Pair (f o) (f' o) Which takes (FA, F'B) into FxF' (A,B). I am confused by the fact of having two instances of F and somehow I need to map on both of them simultaneously. Med vänliga hälsningar / Yours Sincerely, Johan Glimming