I’ve spent some time screwing around with creating custom Liquid tags.

The main motivation was to save some keystrokes because a lot of the time, I post links to articles in the following format:

<a href="http://url-to-article" title="article title • date • site">article title</a> • date • site

To cut down on the reduplication, I created the following Liquid tag:

{% link url:"http://url-to-article", title:"article title", date:"date", site:"site" %}

Initially, I had it only handle plain text strings, but then I got the bright idea to try and pass variables and Liquid templates as parameters, and this soon became a time-consuming endeavor.

For starters, there isn’t a lot of guidance on how to parse parameters of custom Liquid tags. A lot of them just have positional parameters, but this becomes problematic because (1) I don’t like having to remember the order parameters have to come in and (2) this also makes adding additional parameters cumbersome.

For example, when I realized that I also wanted to support this format:

<a href="http://url-to-article" title="article title • subtitle • date • author • site">article title</a> • subtitle • date • author • site

…I started with if/then/elsif/else statements. This got old real fast.

I figured out that the more recent versions of Liquid make it easier to parse parameters using this bit of code which creates a hash:

module Jekyll

  class LinkTag < Liquid::Tag

    def initialize(tag_name, markup, tokens)
      super
      @parameters = {}
      
      markup.scan(Liquid::TagAttributes) do |key, value|
        @parameters[key] = value
      end
    end

    …

But the harder part was figuring out how to lookup variables.

A lot of plugins (including some of the plugins built into Jekyll) use hairy regexes to figure out whether a parameter is a string or a variable but I just decided to copy-and-paste the way Liquid does it, where quoted strings are passed literally while unquoted strings are interpreted as variables.

{% link url:"http://url-to-article", date:"date", site:site.title %}

In this case, site.title would be replaced with “mahiwaga”.

There is a Liquid class defined called Expression, but I couldn’t figure out how to properly incorporate it into the code of custom Liquid tag and it doesn’t seem to be able to look up variables in context.registers[:site] so I just adapted it thus:

def expression_parse(context, markup)
  case markup
  when /\A'(.*)'\z/m # Single quoted strings
    $1
  when /\A"(.*)"\z/m # Double quoted strings
    $1
  else
    lookup_variable(context, markup)
  end
end

The lookup_variable method is from Jekyll::LiquidExtensions.

Incorporating Liquid variables into a parameter was actually easier.

{% link url:"http://url-to-article", date: "date", site:"originally posted on {{ site.title }}" %}

You just have to send the parameter back through the Liquid parser this way:

parameter['key'] = Liquid::Template.parse(expression_parse(context, parameter['key'])).render(context)

The entirety of the code is here:

linktag.rb • 2015 Oct 21 • mahiwaga.net

Not sure how useful it will be for anyone else, but it might serve as an example for how to write your own custom tags.