"Functional" approach to solve diamond kata

diamond.rb

class Diamond
  STARTING_CHARACTER = "A"

  def initialize(letter)
    @letter = letter
  end

  def to_s
    lines.join("\n")
  end

  private

  attr_reader :letter

  def lines
    go = ->(t, ch) {
      t = t.push(spacify(line, ch)).unshift(spacify(line, ch))
      if ch == STARTING_CHARACTER
        t
      else
        go[t, prec(ch)]
      end
    }

    go[[spacify(line, letter)], prec(letter)]
  end

  def line
    go = ->(str, ch) {
      str = ch + str + ch
      if ch == letter
        str
      else
        go[str, ch.succ]
      end
    }
    go[STARTING_CHARACTER, STARTING_CHARACTER.succ]
  end

  def spacify(str, ch)
    str.gsub(/[^#{ch}]/, " ")
  end

  def prec(ch)
    (ch.ord - 1).chr
  end
end

puts Diamond.new("D")