epsilon.rb

(i = ->(a, b, f) {
  (
    (opposite = ->(u, v){u < 0 && v > 0 || u > 0 && v < 0}).call(
      f_a = f.call(a),
      f_b = f.call(b)
    ) ? 
    (
      (f_mid = f.call(mid = (a + b) / 2.0)).abs < Float::EPSILON ?
        mid :
        i.call(
          (opposite.call(f_a, f_mid) ?
            a :
            mid),
          (opposite.call(f_b, f_mid) ?
            b :
            mid),
          f
        )
      ) :
    nil
  )
}).call(-5, 5, ->(x) {(x + 1)**3})