@warn / @debug / @error

Diagnostics report from inside a rule; the selector is the condition. Nothing is written to the document.

Writing diagnostics

img:not([alt]) { @warn "img needs alt"; }
form button:not([type]) { @warn "button defaults to type=submit"; }
provider-fetch[is-error] { @error "fetch failed", prop("provision").error; }
todo-list {
  $todos: prop("provision").body;
  @debug "todos", $todos.length;                    /* again when $todos changes */
  @on todo-remove { @debug "remove", event.detail.id; }
}

Each statement evaluates its expression on the matched element — bindings, attr(), prop(), event inside an @on block, module functions — and reports it. A comma list reports one value per item; a single expression is one value, even when it is a list.

When it speaks

• @warn and @error speak once per element and rule. A warning repeated on every re-run is noise; a new offending element is its own warning. A run that resolves to preserve says nothing and does not count: @warn if(attr("data-bad"): "bad"; else: preserve); speaks once data-bad is set.
• @debug speaks on every application, so it re-logs when a binding or prop() it reads changes — a trace of the value over time.
• A failing expression reports a quark/error (like any declaration) instead of a diagnostic; under @warn / @error, once per element.
• Outside a rule there is no element to evaluate against: a top-level statement is rejected when the sheet is built.

Where it goes

• The console, at the matching logger level: Quark @warn (img:not([alt])): img needs alt. Published builds log errors only; raise the level in app code with import { QuarkLogger } from "@excom/quark" (or @excom/nucleus-kit) and QuarkLogger.level = 2 for @warn, 3 for @debug.
• The DevTools hook, as ["quark", "diagnostic"] — level, values, message, expression, the selector, rule and element — so the Nucleus DevTools extension and the agent tools can list them next to the rule. See JS API.

Use @warn for invariants a sheet can state better than a test (img:not([alt]), button:not([type])), @error for States that should never occur, and @debug while developing a rule — then remove it. The log() and debug() built-ins remain for tracing inside an expression.