How to print directly to HAML buffer?

I'm trying this (haml 6.3.0):

def my_helper  Haml::Helpers.html_concat('Hello, world!')end

Then, in my.haml I do this:

- my_helper

However, I'm getting:

NoMethodError: undefined method `html_concat' for Haml::Helpers:Module

What is the right method? Please, don't suggest to use = my_helper. My real case is much more complicated and I do need to print to the buffer directly instead of returning a string.