Nested sums in Ruby and Haskell

nested_sum.hs

mapSum :: Num a => (a -> a) -> [a] -> a
mapSum f xs = sum $ map f xs

nestedSum :: Num a => [a] -> [a] -> (a -> a -> a) -> a
nestedSum xs ys f = mapSum (\ x -> mapSum (f x) ys) xs

nested_sum.rb

def nested_sum(outer, inner)
  outer.reduce(0) do |outer_sum, i|
    outer_sum + inner.reduce(0) do |inner_sum, j|
      inner_sum + yield(i, j)
    end
  end
end