Difference between nil and null in YAML

Question

I'm working on a Liquid template in Jekyll, and I have 2 places where I'm not sure what to use: nil, null, or just empty.

1. In the front matter:

   ---
   layout: nil|null|<empty>
   ---
   

   • When I use nil, I get `Build Warning: Layout 'nil' requested in sitemap.xml does not exist, as this thread demonstrates.
   • null and blank both have no error.

2. In Liquid templating:

   {% if post.date != nil %}
   

   • null, nil, and blank all work, but "" makes the statement true, when the variable is not set.

So which should I use? Why? (Does this have anything to do with Ruby? I didn't add the tag.)

Answer 1

For the front matter, use null (as you've already seen). The front matter is interpreted as YAML which defines a null value as null (and possibly also with a tilde ~ though I've never tried that).

In the liquid templates, for the most part you'd be better off with leaving the comparison out:

{% if post.date %}

As that typically achieves what you're trying to do.