omit

application_helper.rb

module ApplicationHelper
  def omit(val = nil, &block)
    unless block_given?
      raise 'no parent block' if @_omit.empty?
      @_omit[-1] = false unless val.nil?
      return val
    end

    raise "Don't pass a value with block" if val
    @_omit ||= []

    @_omit.push true
    ret = capture(&block)
    return nil if @_omit.pop

    @_omit[-1] = false unless @_omit.empty?
    ret
  end
end

application_helper_spec.rb

RSpec.describe ApplicationHelper do
  describe 'omit' do
    subject { Haml::Engine.new(haml).def_method(self, :render); render }
    context 'empty block' do
      let(:haml) { <<HAML
= omit do
  %p
HAML
      }
      it { is_expected.to eq "\n" }
    end
    context 'block with nil' do
      let(:haml) { <<HAML
= omit do
  %p= omit nil
HAML
      }
      it { is_expected.to eq "\n" }
    end
    context 'block with non-nil value' do
      let(:haml) { <<HAML
= omit do
  %p= omit 'hoge'
HAML
      }
      it { is_expected.to eq "<p>hoge</p>\n" }
    end
    context 'block with empty block' do
      let(:haml) { <<HAML
= omit do
  = omit do
    %span= omit nil
  %p= omit nil
HAML
      }
      it { is_expected.to eq "\n" }
    end
    context 'block with non-empty block' do
      let(:haml) { <<HAML
= omit do
  = omit do
    %span= omit 'hoge'
  %p= omit nil
HAML
      }
      it { is_expected.to eq "<span>hoge</span><p></p>\n" }
    end
  end
end