Expressions

A value is an expression: names resolve through keywords, modules, built-ins and $bindings; operators follow JS; accessors never throw.

Evaluation

• Names resolve in order: value keywords → @use exports (bare as * exports, then namespaces) → built-in functions → $bindings. A $binding is read by walking up from the matched element to the nearest ancestor-or-self that holds it; an unbound $name is undefined (a wipe; while a prerendered page hydrates, what is painted is kept instead), while an unknown bare identifier is an error (a no-op). A module export shadows a built-in of the same name.
• Literals: unitless numbers are numbers; a number with a unit (10px, 50%) and a color (#ccc) evaluate to strings. Strings unescape \n, \t, \r, and \x → x. Interpolated strings and url(…) join their parts, with null / undefined parts rendered empty.
• & in an expression is the matched element's tag name.
• Accessors never throw: .field and [index] on null / undefined yield undefined. object.$name reads the property literally named $name (namespaced variables: math.$pi).
• Calls: a bare callee resolves through the name order above and must be a function. A method call on a value (item.name.trim()) is allowed for own-property functions (a @use namespace, a provided object) and for the allowed prototype methods; anything else is an error. Calling a method on null / undefined yields undefined. Named arguments are passed positionally (the name is ignored); $list... spreads an array.
• Operators: and / or short-circuit and return an operand (JS semantics, so $title or preserve works); not returns a boolean; == / != are loose; + concatenates when either side is a string (prefer #{$x} interpolation for building strings); - * / % and comparisons follow JS.
• if() returns the value of the first arm whose condition is truthy; with no match and no else it is undefined (a wipe). ternary() is the two-arm function form.
• Lists (space or comma separated) evaluate to arrays; maps to objects, with bare identifier keys taken literally ((name: "Ada") → { name: "Ada" }) and other keys stringified.
• Results: listeners and $variables apply synchronously; attributes, CSS variables, and content are painted in a batch. Module functions are called synchronously and what they return is not awaited: content refuses a promise (see Asynchronous work).

Example

[data-user] {
  $user: prop("provision");
  label: $user.profile.name or "Anonymous";
  title: if($user.role == "admin": "Administrator"; else: $user.role);
  data-tags: $user.tags.join(", ");
  --progress: "#{($user.done / $user.total * 100).toFixed(0)}%";
}

Facts over if()

Several rules branching on the same condition with if() is a smell: derive the condition once as an attribute (data-is-admin: $user.role == "admin"; — a boolean writes "" or removes the attribute) and select on it from Quark and CSS.