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:
To cut down on the reduplication, I created the following Liquid tag:
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:
…I started with
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:
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.
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:
lookup_variable method is from Jekyll::LiquidExtensions.
Incorporating Liquid variables into a parameter was actually easier.
You just have to send the parameter back through the Liquid parser this way:
The entirety of the code is here:
linktag.rb • 2015 Oct 21 • mahiwaga.pw
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.