modestack.ts

type Mode = "safe" | "faster";
const modeStack: Mode[] = ["safe"];
const getMode = () => modeStack[0];
const pushMode = (mode: Mode) => modeStack.unshift(mode);
const popMode = () => modeStack.shift();

type HasLength = { length: number };

type WithModeRequest<Result> = {
  mode: Mode;
  action: () => Result;
};

function reallyPerform<Task extends HasLength>(task: Task): number {
  console.log(`${getMode()}: ${task}`);
  return task.length;
}

function perform<Task extends HasLength>(task: Task): number {
  return reallyPerform(task);
}

function withMode<Result>(request: WithModeRequest<Result>): Result {
  try {
    pushMode(request.mode);
    return request.action();
  } finally {
    popMode();
  }
}

export function main() {
  console.log(
    perform("something"),
    withMode({ mode: "faster", action: () => perform("reliable") }),
    perform(["again"])
  );
}

main();