Appendix: the same loop, in TypeScript

// The same loop as chapter 2, in TypeScript. Four parts, no new ideas:
// a model, tools, state, and a stopping rule.

interface Turn { role: "thought" | "action" | "observation"; text: string }
type ToolResult = { ok: true; value: number } | { ok: false; error: string };
type Tool = (arg: string) => ToolResult;

const tools: Record<string, Tool> = {
  weather: (city) => city.toLowerCase().includes("paris")
    ? { ok: true, value: 22 }
    : { ok: false, error: `no weather for ${city}` },
  calculator: (expr) => {
    const m = expr.match(/(-?\d+)\s*\+\s*(-?\d+)/);
    return m ? { ok: true, value: +m[1]! + +m[2]! } : { ok: false, error: `cannot parse ${expr}` };
  },
};

// The stand-in "model": it reads the ask and names a tool, exactly like
// academy_llm.tool_route. Deterministic so the loop is reproducible.
function toolRoute(prompt: string): string | null {
  const p = prompt.toLowerCase();
  if (/weather|temperature/.test(p)) return "weather";
  if (/\+|plus|compute/.test(p)) return "calculator";
  return null;
}

function runAgent(goal: string, plan: string[], maxSteps = 5): { answer: number | null; transcript: Turn[] } {
  const transcript: Turn[] = [{ role: "thought", text: `goal: ${goal}` }];
  let answer: number | null = null;

  for (const [i, subgoal] of plan.entries()) {
    if (i >= maxSteps) break;                              // the stopping rule
    const thought = answer === null ? subgoal : subgoal.replace("{prev}", String(answer));
    const tool = toolRoute(thought);                       // observe, decide
    if (!tool) { transcript.push({ role: "thought", text: `no tool for: ${thought}` }); break; }
    const result = tools[tool]!(thought);                  // act
    transcript.push({ role: "action", text: `${tool}(${thought})` });
    if (!result.ok) { transcript.push({ role: "observation", text: result.error }); break; }
    transcript.push({ role: "observation", text: String(result.value) });  // append
    answer = result.value;
  }
  return { answer, transcript };
}

const { answer, transcript } = runAgent(
  "the temperature in Paris plus 10",
  ["look up the weather temperature in Paris", "compute {prev} + 10"],
);
for (const t of transcript) console.log(`${t.role.padEnd(12)} ${t.text}`);
console.log(`\nanswer: ${answer}`);
console.log(answer === 32 ? "SAME LOOP, DIFFERENT SYNTAX: YES" : "SAME LOOP, DIFFERENT SYNTAX: NO");