Liquid2 - Variables and Paths

[jg-rp]

Here I describe syntax and semantics for variables in Liquid2.

For context, and until such time that expressions have been defined for Liquid2, variables are used in output statements

{{ some_identifier }}

And as arguments to filters

{{ some_identifier | join: another_identifier }}

Or as arguments to tags

{% for item in some_identifier %}
    {% if item == other_identifier %}
        {{ item }}
    {% endif %}
{% endfor %}

Variables can be paths, with segments chained together to retrieve values from arbitrarily nested template context data.

{{ some_identifier.foo.bar }}

And bracket notation is used when a variable's name is not a valid identifier.

{{ some_identifier['foo bar'] }}

Variables and paths

Every variable is a JSONPath expression with syntax following RFC 9535, plus the following extensions/modifications.

• The root identifier is optional
• The singular query selector must be implemented (currently discussed here)

With these extensions to the JSONPath standard, most legacy Liquid variables are valid Liquid2 paths.

Syntax

TODO: modified ABNF from RFC 9535

Semantics

JSONPath expressions are evaluated with template contextual data as their query argument, and the results used as variables in the enclosing Liquid expression, after applying the following rules:

• If the query result is an empty node list, return the special value undefined.
• If the query result is a node list containing a single node, return the single node’s value.
• Otherwise return a sequence of values, one value for each node in the node list.

Examples

With the optional root identifier ($), these paths are equivalent:

$.some_identifier

some_identifier

Embedded queries are coerced to name or index selectors:

some_identifier[foo.bar]

$.some_identifier[$.foo.bar]

[jg-rp]

An implication of making RFC 9535 a dependency is that, for consistency, template string literals should follow the same rules for escape sequences. Such as Unicode escape sequences.

Liquid2 - strings defines Python style Unicode escape sequences, whereas RFC 9535 uses JSON style.

[jg-rp]

Discussions and development of a specification for Liquid2 have been moved to https://github.com/liquid2-org.

See liquid2-org/draft-liquid2-spec#1.