Dear haskellers, currently the instances are defined as |instance (Monoid a,Monoid b) =>Monoid (a,b)where mempty = (mempty, mempty) (a1,b1) `mappend` (a2,b2) = (a1 `mappend` a2, b1 `mappend` b2)| However for some applications this isn't lazy enough, for example |-- | Build two lists by folding on a pair of `Endo` monoids. test = head $ appEndo (fst $ foldMap (f &&& f) [1..]) [] where f =Endo . (:)| never terminates because of the unnecessary pattern matching on the constructor |(,)| forces evaluation of the whole infinite list. I suggest to change all Monoid instances for tuples to be like | instance (Monoid a,Monoid b) =>Monoid (a,b)where mempty = (mempty, mempty) ~(a1,b1) `mappend` ~(a2,b2) = (a1 `mappend` a2, b1 `mappend` b2) -- ^^^ ^^^| which fixes the problem. Best regards, Petr