nerve/packages/khala/src/moderator.ts

import jsonata from "jsonata";
import type { Result } from "./result.js";
import { err, ok } from "./result.js";

export type ModeratorDecision = { role: string };

export async function evaluateModerator(
  expression: string,
  context: Readonly<Record<string, unknown>>,
): Promise<Result<ModeratorDecision, string>> {
  let expr: ReturnType<typeof jsonata>;
  try {
    expr = jsonata(expression);
  } catch (e) {
    const msg = e instanceof Error ? e.message : String(e);
    return err(`jsonata compile: ${msg}`);
  }
  let out: unknown;
  try {
    out = await expr.evaluate(context);
  } catch (e) {
    const msg = e instanceof Error ? e.message : String(e);
    return err(`jsonata eval: ${msg}`);
  }
  if (out === null || out === undefined) {
    return err("moderator returned empty");
  }
  if (typeof out !== "object" || out === null || Array.isArray(out)) {
    return err("moderator must return an object");
  }
  const rec = out as Readonly<Record<string, unknown>>;
  const role = rec.role;
  if (typeof role === "string") {
    return ok({ role });
  }
  return err("moderator result missing string role");
}