Function flattenSequence

src/compute-engine/boxed-expression/flatten.ts:71–93

(
  xs: ReadonlyArray<Expression>
)

}

/**
 * Like {@link flatten}, but ellipsis-fold barriers are held back.
 *
 * Lifts `Sequence`/`Nothing` and nested `operator` applications recursively,
 * exactly like `flatten`, EXCEPT that an operand which is a fold-barrier
 * product (a `Multiply` carrying a `ContinuationPlaceholder` at any depth) is
 * pushed back WHOLE: splicing it would smuggle the placeholder into the
 * enclosing fold. Because the lift is recursive, a barrier wrapped in a
 * `Sequence` — e.g. `Multiply(Sequence(Multiply(a, ContinuationPlaceholder)), b)`
 * — is caught too, which a top-level-only barrier check would miss.
 *
 * The third argument mirrors `flatten`'s: when `true` (the default) the
 * operands are made canonical before being examined.
 */
export function flattenHoldingBarriers<
  T extends ReadonlyArray<Expression> | Expression[],
>(ops: T, operator: string, canonicalize = true): T {
  const xs: ReadonlyArray<Expression> =
    !canonicalize || ops.every((x) => x.isCanonical)
      ? ops
      : ops.map((x) => x.canonical);

  const ys: Expression[] = [];
  for (const x of xs) {
    // Hold barrier-bearing products back, whole
    if (isFoldBarrierProduct(x)) {