cumulative_sum.rb

a = [1, 2, 3, 4, 5]

class Array
  def cumulative_sum(default = 0, &block)
    block ||= ->(n){n}
    a = [default]
    for i in 1 .. (self.length)
      a[i] = a[i - 1] + block.(self[i - 1])
    end
    return a
  end
  
  def scanl(default = nil, &block)
    if default.nil?
      a = [self[0]]
      for i in 1 .. self.length - 1
        a[i] = block.(a[i-1], self[i])
      end
    else
      a = [default]
      for i in 1 .. self.length
        a[i] = block.(a[i-1], self[i-1])
      end
    end
    return a
  end
end

a.cumulative_sum    # => [0, 1, 3, 6, 10, 15]
a.cumulative_sum(1) # => [1, 2, 4, 7, 11, 16]
a.cumulative_sum(0) { |x| x ** 2 }  # => [0, 1, 5, 14, 30, 55]

Haskellの scanl に相当する何かを生やせばよさそう。

• https://hackage.haskell.org/package/base-4.17.0.0/docs/Prelude.html#v:scanl
• https://hackage.haskell.org/package/base-4.17.0.0/docs/src/GHC.List.html#scanl