thor/mustache/EXTENSIONS.md

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

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 unparsable) 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.

Takes an optional arg for the Go reference-date layout to use:

• A double-quoted literal, spaces allowed: {{date | format "Mon Jan 2 2006"}}.
• A bare key, resolved from context like any other field: {{date | format long}} uses the value of long (e.g. a site-config field) as the layout.
• No arg: falls back to the date_format context key (typically date.format from thor.json).

Timezone conversion

When the timezone context key is set (typically date.timezone from thor.json, an IANA name like "America/New_York"), the format pipe converts dates to that timezone before formatting:

• Date has offset + target tz: adjusts to true UTC, then converts to the target timezone (DST-aware).
• Date has no offset + target tz: assumes the date is already in the target timezone — no conversion, only resolves the abbreviation.
• Date has offset + no target tz: displays in the source offset.
• Date has no offset + no target tz: displays as-is, assumes UTC.

The MST token reflects the active timezone:

Config tz	ISO has offset	MST output
"America/New_York"	yes	"EST" or "EDT" (DST-aware)
"America/New_York"	no	"EST" or "EDT"
not set	yes	"UTC-04:00" (from source offset)
not set	no	"UTC"

Timezone data is loaded once by init_site via core:time/timezone.region_load, using the site arena allocator. The resolved ^datetime.TZ_Region pointer is stored on Site.tz and passed to templates through Base_Data.timezone. The arena frees it automatically on destroy_site.

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.