Spec §9 — Const evaluation

const expr evaluates expr during compilation and replaces it with its result: the emitted program carries the value as a literal, never the computation. Const evaluation and macro expansion (§10) run in the same fueled compile-time interpreter; they are the two phases that execute vilan code at build time.

9.1 The const expression

const is a prefix operator over an expression. It captures greedily: everything to the end of the surrounding expression folds (const 1 + 2 * 3 folds 7); parenthesize to narrow the extent — in (const square(4)) + square(2) the second call runs at runtime.

A const expression may appear anywhere an expression may, including module-level initializers — where it is also the way to run logic at load position without runtime cost: a const initializer ships as a plain value, participates in no platform coloring (§11.2), and cannot violate the initializer rules of §7.4 (nothing of it remains to run at load time).

9.2 The const environment

The evaluated expression may use exactly what the compiler can know:

• literals, and the pure operations of the language (§7.2’s evaluation order applies unchanged);
• functions whose bodies are themselves const-evaluable, transitively;
• imports, and immutable module bindings whose own initializers are const-evaluable.

Host capabilities do not exist at compile time. A call that requires the host — the clock, the filesystem, network, randomness, any external function without a compile-time definition — is a compile error inside const (“now() is not const-evaluable”), not a deferred runtime call: the answer would not be a constant.

The one deliberate exception is std::asset::emit(kind, content), callable only during const evaluation: it declares a build asset (the styling system’s CSS, for example) that the build writes beside the output. Asset emission is deterministic — same inputs, same files.

9.3 Failure and resource limits

Const evaluation is total by construction of the budget: each run is bounded by the interpreter’s fuel (steps) and depth (nesting), shared with macro expansion and configured by the manifest’s [macro] section (§11.4). Exhausting either, or panicking during evaluation, is a compile error carrying the const expression’s span — a runaway const fails the build; it cannot hang it.

9.4 Results

The result of a const expression is spliced as a literal of its type: numbers, strings, booleans, lists, tuples, structs and enums of const-evaluable contents. The expression’s type is checked exactly as if it ran at runtime; const never changes typing, only when the computation happens.