Allow `empty` to behave like an empty array (change needed in liquid-c)

[er1]

This allows the empty literal to act as an empty array when being used outside of comparisons. It would make empty array constructions such as {% assign x = '' | split: '' %} more readable as {% assign x = empty %} This should not impact existing uses since empty array will get coerced into empty string if and where it is being used.

Some tests are failing and depend on: https://github.com/Shopify/liquid-c/pull/131

[ashmaroli]

@er1 This looks like a breaking-change to me. Can you test if the empty array can be filled later on..?

{% assign dropship = empty %}
{% if product.price < store.sale_price %}
  {{ dropship | push: product }}
{% endif %}

[er1]

@ashmaroli It would depend on how that filter works. I am assuming that values are immutable and that the standard filters coerce to arrays when they need arrays before performing operations. This would be in line with how enumerable drops behave.

[ashmaroli]

@er1 IMO, {% assign x = empty %} doesn't improve readability. It isn't clear if x is an empty string or an empty array. It will take time for existing users to adjust to the fact that empty is a flexible entity now. Would it be possible to introduce [] or empty_array instead..?

[er1]

Would it be possible to introduce [] or empty_array instead..? @ashmaroli I agree that it looks a little unclear at first glance however I don't feel comfortable suggesting new literal names into the language. Existing code might have empty_array set via assigns and introducing a new literal this way will break that code since literals cannot be assigned. Using [] doesn't doesn't feel right unless we flat out introduce array literals into the language as I think it would suggest that something like [2,3,5] or [product1, product2] could be done and that is a much large discussion to have.

[ashmaroli]

Using [] doesn't doesn't feel right unless we flat out introduce array literals into the language

I see. Thanks for the feedback.

[er1]

failing tests in df49715 depend on Shopify/liquid-c#131