import type { Combinator } from '../types.ts';
/**
 * Positive lookahead. Succeeds (consuming nothing) when `combinator` matches at
 * this position; fails when it doesn't. The mirror of `not()`.
 *
 *   // "a mixin reference, not a class selector": require the punctuation ahead,
 *   // then let the real production consume it.
 *   const mixinRef = sequence(peek(regex(/[.#]/)), mixinCall)
 *
 * WHY THIS EXISTS AS A PRIMITIVE (not `not(not(X))`):
 *
 * `not(not(X))` is behaviourally a positive lookahead, but `not()` reports
 * `firstSet: any()` — it cannot know what it forbids — so `not(not(X))` reports
 * `any()` too. An arm leading with it therefore POISONS its choice's first-char
 * dispatch (`analyzeGating` flags it as the `double-not` anti-pattern), and among
 * sibling arms that share a first char the hand-rolled gate miscompiles.
 *
 * `peek(X)` carries X's first-set instead, so a leading `peek()` GATES its arm:
 * `choice(sequence(peek(regex(/[.#]/)), …), …)` still emits O(1) dispatch.
 *
 * The first-set is exact only when X is NON-NULLABLE. A nullable `peek(X)`
 * succeeds on the empty string, so it constrains no first character at all — it
 * reports `any()`, the sound over-approximation. (Same rule the sequence
 * first-set uses; see `isPositiveLookahead` in `first-set.ts`.)
 */
export declare function peek(combinator: Combinator<unknown>): Combinator<null>;
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