Function holdMapAsync

src/compute-engine/boxed-expression/hold.ts:48–81

(
  expr: Expression,
  f: (x: Expression) => Promise<Expression | null>
)

        if (y !== null) result.push(y);
      }
    }
  }
  return def?.associative && !hasContinuation
    ? flatten(result, expr.operator, false)
    : result;
}

export async function holdMapAsync(
  expr: Expression,
  f: (x: Expression) => Promise<Expression | null>
): Promise<ReadonlyArray<Expression>> {
  if (!isFunction(expr)) return [];

  let xs = expr.ops;

  const def = expr.operatorDefinition;

  if (!def || xs.length === 0) return xs;

  // f(a, f(b, c), d) -> f(a, b, c, d)
  // Ellipsis fold barrier (see `holdMap`): a `ContinuationPlaceholder` operand
  // marks a notational sum/product; keep nested associative anchors intact.
  const hasContinuation = xs.some((x) => isContinuationOperand(x));
  if (def?.associative && !hasContinuation)
    xs = flatten(xs, expr.operator, false);

  //
  // Apply the hold as necessary
  //
  if (def.lazy) return xs;

  const result: Expression[] = [];
  for (const x of xs) {
    const h = x.operator;
    if (h === 'Hold') result.push(x);
    else {
      const op = h === 'ReleaseHold' && isFunction(x) ? x.op1 : x;