[request] make arrays in liquid / concat filter

Problem

• Not being able to make arrays in liquid.
• Simple tasks end up requiring multiple server requests for information.
• Memory limit is often exceeded when using workarounds.

Context

• This is often seen when nesting for loops in template files (i.e. looping through articles then looping through article tag).
• Because Shopify doesn't offer a way to build arrays, we can't optimize to one for loop to create arrays.

Possible Solution

• The concat filter seems to be a possible solution. We can see that concat is concatenating hashes. If we could create arrays with this it would save a lot of requests from hitting the server.

Example of concat hashes

• Works great in specific situations: http://freakdesign.com.au/blogs/news/105090695-show-the-latest-shopify-articles-from-all-blogs.

Ideal fix

• We would like to be able use concat to build an array.
• We're not married to concat as the solution but it seems like the closest available solution.
• https://github.com/Shopify/liquid/blob/6434b8d2bb1cfabdd861bfc6f5be31c019171b82/lib/liquid/standardfilters.rb#L218

Current situation:

{% for article in blogs[settings.blog_widget_select].articles %}
   {% for tag in article.tags %}
      {% assign current_tag = tag | downcase %}
      {% if current_tag == 'tag 1' %}
         // print article
      {% endif %}
   {% endfor %}                     
{% endfor %}


{% for article in blogs[settings.blog_widget_select].articles %}
   {% for tag in article.tags %}
      {% assign current_tag = tag | downcase %}
      {% if current_tag == 'tag 2' %}
         // print article
      {% endif %}
   {% endfor %}                     
{% endfor %}

{% for article in blogs[settings.blog_widget_select].articles %}
   {% for tag in article.tags %}
      {% assign current_tag = tag | downcase %}
      {% if current_tag == 'tag 3' %}
         // print article
      {% endif %}
   {% endfor %}                     
{% endfor %}

{% for article in blogs[settings.blog_widget_select].articles %}
   {% for tag in article.tags %}
      {% assign current_tag = tag | downcase %}
      {% if current_tag == 'tag 4' %}
         // print article
      {% endif %}
   {% endfor %}                     
{% endfor %}

Ideal situation:

{% assign tag_1_articles = false %}
{% assign tag_2_articles = false %}
{% assign tag_3_articles = false %}
{% assign tag_4_articles = false %}

{% for article in blogs[settings.blog_widget_select].articles %}
   {% for tag in article.tags %}
      {% assign current_tag = tag | downcase %}
      {% if current_tag == 'tag 1' %}
         {% assign tag_1_articles = tag_1_articles | concat:article %}
      {% endif %}

      {% if current_tag == 'tag 2' %}
         {% assign tag_2_articles = tag_2_articles | concat:article %}
      {% endif %}

      {% if current_tag == 'tag 3' %}
         {% assign tag_3_articles = tag_3_articles | concat:article %}
      {% endif %}

      {% if current_tag == 'tag 4' %}
         {% assign tag_4_articles = tag_4_articles | concat:article %}
      {% endif %}
   {% endfor %}                     
{% endfor %}

// print tag_1_articles

// print tag_2_articles

// print tag_3_articles

// print tag_4_articles

CC: @NathanPJF

Read lib/liquid/standardfilters.rb around line 218 to understand the existing concat behavior, then examine how Liquid represents arrays and hashes in filters. The work is done when a documented, tested approach lets concat build arrays as shown in the issue, or the supported alternative is clearly established.