> “Any sufficiently complicated C or Fortran program contains an ad-hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.” – Philip Greenspun
`liquid` ports [Shopify Liquid templates](https://shopify.github.io/liquid) to Go. It was developed for use in [gojekyll](https://github.com/osteele/gojekyll).
`liquid` provides a functional API for defining tags and filters. See examples [here](https://github.com/osteele/liquid/blob/master/filters/filters.go), [here](https://github.com/osteele/gojekyll/blob/master/filters/filters.go), and [here](https://github.com/osteele/gojekyll/blob/master/tags/tags.go). On the one hand, this isn't idiomatic Go. On the other hand, this made it possible to implement a boatload of Liquid and Jekyll filters without an onerous amount of boilerplate – in some cases, simply by passing a reference to a function from the standard library.
This library is in its early days. IMO it's not sufficiently mature to be worth snapping off a [versioned URL](http://labix.org/gopkg.in). In particular, the tag and filter extension API is likely to change.
The cyclomatic complexity checks on generated functions, hand-written parsers, and some of the generic interpreter functions, have been disabled (via `nolint: gocyclo`). IMO this check isn't appropriate for those classes of functions. This isn't a license to disable cyclomatic complexity or lint in general willy nilly.
* [Shopify.github.io/liquid](https://shopify.github.io/liquid) is the definitive reference.
* [Help.shopify.com](https://help.shopify.com/themes/liquid) goes into more detail, but includes features that aren't present in core Liquid as used by Jekyll.
* Shopify's [Liquid for Designers](https://github.com/Shopify/liquid/wiki/Liquid-for-Designers) is another take.
Michael Hamrah's [Lexing with Ragel and Parsing with Yacc using Go](https://medium.com/@mhamrah/lexing-with-ragel-and-parsing-with-yacc-using-go-81e50475f88f) was essential to understanding `go yacc`.
The [original Liquid engine](https://shopify.github.io/liquid), of course, for the design and documentation of the Liquid template language. Many of the tag and filter test cases are taken directly from the Liquid documentation.
* [karlseguin/liquid](https://github.com/karlseguin/liquid) is a dormant implementation that inspired a lot of forks.
* [acstech/liquid](https://github.com/acstech/liquid) is a more active fork of Karl Seguin's implementation. I submitted a couple of pull requests there.
* [hownowstephen/go-liquid](https://github.com/hownowstephen/go-liquid) is a more recent entry.
After trying each of these, and looking at how to extend them, I concluded that I wasn't going to get very far without a parser generator. I also wanted an easy API for writing filters.