Request for suggestions: Support negative integers in the slice filter

[karreiro]

Author: Shopify
Expected end date: December 5, 2024

Background

Today, the slice filter is limited to counting items from the start index, with no option to count backwards from the end. This creates unnecessary toil, affecting the developer experience, as seen in the following examples.

It's straightforward counting from the index:

{% assign fruits = 'apple, banana, cherry, tomato, fig' | split: ", " %}

{{ fruits | slice: 1, 3 }}
{% # => [ "banana", "cherry", "tomato" ] %}

However, operations such as getting all array elements except the last two can become verbose:

{% assign end_index = fruits.size | minus: 2 %}

{{ fruits | slice: 0, end_index }}
{% # => [ "apple", "banana", "cherry" ] %}

Proposal

Inspired by slice operations in other languages, we propose supporting negative numbers in the second argument, making operations more intuitive by controlling the direction of elements being taken.

With this enhancement, that example becomes simpler and clearer:

{{ fruits | slice: 0, -2 }}
{% # => [ "apple", "banana", "cherry" ] %}

Here's how the negative number influences the direction of the second argument:

{{ fruits | slice: 1, 3 }}
{{ fruits | slice: 1, -3 }}

Limitations

This proposal is not currently backward compatible:

{{ fruits | slice: 0, -2 }}
{% # => [] %}

Therefore, we will conduct an impact assessment to evaluate the feasibility and ensure a smooth transition if this change is implemented.

Call for suggestions

We welcome any feedback or opinions on this proposal. Please share your thoughts by December 5, 2024. Your input is valuable as we prepare to begin active development on this initiative.

[galenking]

This seems low-risk since old themes wouldn’t use negatives, right?

[TeamDijon]

Do not forget that the first arguments accepts negative values:

{% liquid
  assign fruit_list = 'apple,banana,cherry,tomato,fig' | split: ','
  
  # Similar to | slice: 1, -3
  assign sliced_fruit_list = fruit_list | slice: -4, 1

  echo sliced_fruit_list | append: '<br>'
  # => ["banana"]
%}

As it can reasonably be expected that you will calculate either the -3 or the -4, I'm not sure if it is really necessary ?

Nice use-case though, I don't think I ever used the slice filter on arrays ! Thanks for sharing

[andershagbard]

This seems low-risk since old themes wouldn’t use negatives, right?

Could possibly impact some custom codes, if they treat negative values as 0. Although I think the impact is minimal, as slice is rarely used from my experience.

{%- liquid 
  # Count get assigned to -2
  assign count = array.size | minus: 5
  assign sliced_list = array | slice: 0, count

[jg-rp]

RFC 9535 - JSONPath has an excellent description of slice semantics for its "slice selector". As well as negative start and stop, it supports a step, which can also be negative.

I thought it would be interesting to see what a Liquid filter following the same rules looked like. In this example I've called the filter range so as not to break slice, and unlike slice, using range on a string returns an array of single character strings, which might not be useful.

(This could be simplified if start and stop were not optional, but how else could we say "from index x to the end" without knowing the length?)

def range(input, start=nil, stop=nil, step=nil)
  input = Utils.to_s(input) unless input.is_a?(Array)

  # Working around awkward keyword argument behavior so start, stop and
  # step are all optional. :(
  if start.is_a?(Hash)
    stop = start['stop']
    step = start['step']
    start = start['start']
  elsif stop.is_a?(Hash)
    start ||= stop['start']
    step ||= stop['step']
    stop = stop['stop']
  elsif step.is_a?(Hash)
    start ||= step['start']
    stop ||= step['stop']
    step = step['step']
  end

  step = Utils.to_integer(step || 1)
  length = input.length
  return [] if length.zero? || step.zero?

  start = Utils.to_integer(start) unless start.nil?
  stop = Utils.to_integer(stop)  unless stop.nil?

  normalized_start = if start.nil?
                        step.negative? ? length - 1 : 0
                      elsif start&.negative?
                        [length + start, 0].max
                      else
                        [start, length - 1].min
                      end

  normalized_stop = if stop.nil?
                      step.negative? ? -1 : length
                    elsif stop&.negative?
                      [length + stop, -1].max
                    else
                      [stop, length].min
                    end
  
  # This does not work with Ruby 3.1
  # input[(normalized_start...normalized_stop).step(step)]
  # 
  # But this does.
  (normalized_start...normalized_stop).step(step).map { |i| input[i] }
end

Usage examples

# frozen_string_literal: true

require "liquid"

source = <<~LIQUID
{% assign a = '0,1,2,3,4,5,6,7,8,9' | split: ',' -%}
{{ a | range: 1, 3 | join: ', '}}
{{ a | range: 1, -3 | join: ', '}}
{{ a | range: 1, 6, 2 | join: ', '}}
{{ a | range: step: 2 | join: ', '}}
{{ a | range: step: -1 | join: ', '}}
{{ a | range: 2, step: -1 | join: ', '}}
{{ a | range: -1, -6, -2 | join: ', '}}
{{ a | range: stop: 0, step: -1 | join: ', '}}
LIQUID

template = Liquid::Template.parse(source, error_mode: :strict)
puts template.render!

Output

1, 2
1, 2, 3, 4, 5, 6
1, 3, 5
0, 2, 4, 6, 8
9, 8, 7, 6, 5, 4, 3, 2, 1, 0
2, 1, 0
9, 7, 5
9, 8, 7, 6, 5, 4, 3, 2, 1