LZR language reference

This page completes the LZR language surface documented across LZR templating and expressions and scope: how the parser really treats the two syntax layers, the numeric <for> loop, the <frame> element, the data-hm-* runtime binding vocabulary, and the diagnostics the compiler and renderer can emit. Every example and every diagnostic message on this page was produced by the real engine (lzr render-page / lzr compile-template), not transcribed from intent.

Two syntax layers: tags author, markers lower

LZR control flow has two textual forms. The structural tag form (<if>, <elseif>, <else>, <foreach>, <for>, <set>, <frame>) is the authoring surface. Before compilation the parser rewrites each structural tag into an internal marker region in mustache syntax — {{#if expr}}, {{else}}, {{/if}}, {{#foreach item,index in collection}}, {{#set name = expr}}, {{#for key=n;until=3;…}}, {{#frame name64=…}} — and those markers are what the compiler turns into region kinds (if_open, else, if_close, foreach_open, for_open, set, frame, expr, static). An <elseif> has no marker of its own: it lowers to {{else}} plus a nested {{#if}}, closed by as many {{/if}} markers as the chain nested.

The numeric range loop: <for>

Where <foreach> iterates a collection you already have, <for> generates its collection from numeric bounds. All bounds are expressions and must evaluate to numbers.

Verified example — this source:

<ol>
  <for until="{{ 3 }}" key="n">
    <li>n={{ n }} index={{ index }}</li>
  </for>
</ol>
<p><for from="{{ 2 }}" through="{{ 10 }}" step="{{ 4 }}" key="v" index="slot">{{ slot }}:{{ v }} </for></p>
		

renders through lzr render-page to:

<ol>
  <li>n=0 index=0</li>
  <li>n=1 index=1</li>
  <li>n=2 index=2</li>
</ol>
<p>0:2 1:6 2:10 </p>
		

A negative step counts down (from=3 until=0 step=-1 renders 3 2 1), and fractional steps keep their fractions while whole numbers render as integers (from=0 through=1 step=0.5 renders 0 0.5 1). Item identity inside the loop is the zero-based index over a synthetic range collection named after the loop key, so editing a bound refreshes the loop frame in place. The live scenario fixture is controls/for-range.

Bounded route-render jumps: <frame>

A <frame> declares a named region whose content is rendered from another route. The server render emits a host element; the browser runtime fills, refreshes, and navigates it. name and src are required; src may interpolate expressions (src="/items/{{ 40 + 2 }}" resolves to /items/42 — engine-verified).

A frame written with a body is a container: the server-authored body stays in the host as placeholder content and the host is stamped data-hm-frame-state="placeholder". That body is mandatory for loading="visible", loading="on-activate", and error="retain" — a self-closing frame with those values fails compile with frame.placeholder_missing. Host attributes on the tag pass through a whitelist (class, id, role, aria-hidden, data-*); everything else is dropped. Verified render:

<frame name="docs-stage" src="/runtime/frame/root-1/stage.html"
       loading="visible" history="replace" error="retain"
       class="stage-shell" data-stage="docs">
  <p>Placeholder body shown until the frame activates.</p>
</frame>
		

<div class="stage-shell" data-stage="docs" data-hm-frame="docs-stage"
  data-hm-frame-src="/runtime/frame/root-1/stage.html"
  data-hm-frame-target="runtime-frame-root-1-stage.html"
  data-hm-frame-route="/runtime/frame/root-1/stage.html"
  data-hm-frame-source-template="..." data-hm-frame-source-span="..."
  data-hm-frame-loading="visible" data-hm-frame-refresh="manual"
  data-hm-frame-history="replace" data-hm-frame-error="retain"
  data-hm-frame-state="placeholder">
  <p>Placeholder body shown until the frame activates.</p>
</div>
		

Live frame fixtures with runtime canaries live under src://runtime/frame/eager.html and src://runtime/frame/visible.html.

Runtime binding attributes: data-hm-*

Templates opt elements into browser-runtime behavior with data-hm-* attributes. The binding compiler (lzr compile-template-metadata) classifies every such attribute into a binding kind; the table below is the verified classification of a real report.

Frames join this vocabulary from the other side: a <frame> tag is projected into the same metadata report as data-hm-frame, data-hm-frame-src, and its option attributes, and the rendered host carries them (see above). Expressions inside attribute values additionally produce expression-binding rows that record the expression, its root, and its resolved data target. The expansion-provenance attribute data-hm-from is documented with components.

Diagnostics vocabulary

Compile and render diagnostics carry a severity, a stable code, a message, and a source span. Three families cover template structure; each code below was reproduced against the real engine. The malformed-repair scenario shows how the Inspector surfaces them with repair affordances.

lzr render-page -in src/controls/for-range.html -out /tmp/for-range.html
lzr compile-template-metadata -in src/docs/lzr-language-reference.html -out /tmp/meta.json
hypermake explain rendered/public/controls/for-range.html
hypermake affected-by src/docs/lzr-language-reference.html