Yielding Haml block will return 0 (zero)

I had a presenter-view code like this one:

class MyPresenter
  def render_content
    view_header = true
    yield(self, view_header)
  end

  def tlt(name)
    "fetching translation for #{name}"
  end
end

- language_presenter = MyPresenter.new
- language_presenter.render_content do |presenter, view_header|
  = content_tag :h2, presenter.tlt('top_header') if view_header
  = presenter.tlt('main_body')
  bla bla bla 

but for some reason the value of yield(self, view_header) was 0. Not nil, not empty string but a zero.

It turns out it was caused by the way how Haml outputs to the template. Long story short, using capture_haml will capture Haml to sting, and if you render that string with =  that will make it work properly:

- language_presenter = MyPresenter.new
= language_presenter.render_content do |presenter, view_header|
  - capture_haml do
    = content_tag :h2, presenter.tlt('top_header') if view_header
    = presenter.tlt('main_body')
    bla bla bla

source:

• http://stackoverflow.com/questions/3619699/haml-block-returning-0-on-yield
• http://haml.info/docs/yardoc/Haml/Helpers.html#capture_haml-instance_method
• https://github.com/equivalent/scrapbook2/blob/master/archive/mini-blogs/2014-10-23-yield-returning-zero-0.md