I found that the default implementation of foldr1 in Data.Foldable is too strict (and foldl1, too). It always evaluates the complete spine of an input list: -- taken from base-4.6:Data.Foldable.foldr1 foldr2 f xs = fromMaybe (P.error "foldr1: empty structure") (Fold.foldr mf Nothing xs) where mf x Nothing = Just x mf x (Just y) = Just (f x y) -- lazier version, only evaluates one item ahead foldr3 f xs = fromMaybe (P.error "foldr1: empty structure") (Fold.foldr mf Nothing xs) where mf x = Just . maybe x (f x) *Data.NonEmpty> foldr2 (P.++) $ "abc" : "def" : P.undefined "*** Exception: Prelude.undefined *Data.NonEmpty> foldr3 (P.++) $ "abc" : "def" : P.undefined "abc*** Exception: Prelude.undefined