Normally when using haml_tag you can add attributes by typing something like; haml_tag :a, class: 'my-class'
Unfortunately because of the hyphen in data attributes the same pattern doesn’t work if you try and add them, so haml_tag :a, data-id: '5' will crap itself.
What you can do though is call something like; haml_tag :a, data: {id: '5'} which will output the following HTML; <a data-id='5'></a>
You can also add multiple data attributes like so; haml_tag :a, data: {id: '5', type: 'arbitrary code'} which is pretty handy.