Mustache Extensions

Thor's mustache engine implements the Mustache spec plus the following non-standard extensions. Extensions are opt-in via template syntax — vanilla mustache templates render identically to the spec.

Pipes

A pipe expression appears inside a section tag or an interpolation tag and transforms the resolved value before rendering. Pipes let templates request different views of the same data without privileged Go-side support.

Syntax

{{#<key> | <op> <arg> <arg>... | <op> <arg>... }} … {{/<key>}}
{{<key> | <op> <arg> <arg>... | <op> <arg>...}}

The first whitespace-separated token after | is the op name; remaining tokens are its arguments.
Whitespace around | is optional: posts|group_by year and posts | group_by year are equivalent.
Multiple filters compose left-to-right.
At most 8 filters may appear in a single tag (compile-time constant MAX_PIPES in pipes.odin). Exceeding it is a parse error.
At most 2 args per filter (compile-time constant MAX_PIPE_ARGS). Exceeding it is a parse error.
The close tag is the bare key only. Pipe expressions are not allowed in close tags — {{/posts | group_by year}} is a parse error.
Pipes work in both section tags ({{#…}} and {{^…}}) and interpolation tags ({{…}} and {{&…}}). In sections, the transformed value becomes the section's context. In interpolations, the transformed value is what gets rendered.

Example

{{#posts | group_by year}}
  <section>
    <h2>{{key}}</h2>
    <ul>{{#items}}<li>{{title}}</li>{{/items}}</ul>
  </section>
{{/posts}}

<!-- Interpolation pipe: format a date string for display -->
<time datetime="{{date}}">{{date | format}}</time>

Available ops

`group_by <field>`

Buckets each element of a list by the value of <field>. Returns a list of Group values, each shaped as:

Group :: struct {
    key:   string,   // the distinct field value
    items: [dynamic]any,  // elements sharing that value
}

Group order preserves first-appearance order in the input list (not key-sorted).

Errors (returned as Data_Error at render time):

Argument count is not 1.
Input is not a list.
Any element is missing the named field.
Any element has an empty value for the named field.

`format` (no args yet)

Formats an ISO 8601 date string as a display string. Takes a string, returns a string (e.g. "2026-03-15T08:49:54-04:00" → "15 Mar 2026"). Invalid input (empty, too-short, non-string, or unparseable) returns a Data_Error. Templates that need to skip dateless pages should gate with a section — {{#date}}<time datetime="{{.}}">{{. | format}}</time>{{/date}} — so the section's truthiness check catches empty before the filter runs. Commonly used inline as {{date | format}} to render a display string while keeping the raw ISO available via {{date}} for the datetime= attribute.

Internally: parses the invariant YYYY-MM-DD prefix by char offset, stringifies time.Month(month_num) and slices [:3] for the abbreviation. Accepts any of these ISO 8601 forms (the date prefix is what matters): 2023-10-15T13:18:50-07:00, 2023-10-15T13:18:50-0700, 2023-10-15T13:18:50Z, 2023-10-15T13:18:50, 2023-10-15.

Future: will accept Go reference-date format strings (e.g. {{date | format "Mon Jan 2 2006"}}) and pull default format/timezone from site configuration.

Memory ownership

Parsed pipe filters (Pipe_Filter values) are stored inline on each Node via [dynamic; MAX_PIPES]Pipe_Filter, and args is inline on each Pipe_Filter via [dynamic; MAX_PIPE_ARGS]string. Both use Odin's fixed-capacity dynamic array type, so no per-tag heap allocations occur at parse time. The storage dies with the Template when delete_template is called.
Filter results (e.g. the [dynamic]Group returned by group_by, or the display string returned by format) are render-scoped allocations in context.temp_allocator. They die with the render call. No caller-side cleanup is needed.
The string data inside Pipe_Filter (op names, args) is borrowed from the template source — no cloning.

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