pattern-matching.js

// @flow

function match<I: [*] | [*, *], O>(
  cb: (test: (predicate?: (...a: I) => any) => string) => { [string]: O | ((...a: I) => O) }
): (...a: I) => ?O {
  const predicates = {};
  function test(predicate: (...a: I) => any = () => true): string {
    const predicateName = Math.random().toFixed(5);
    predicates[predicateName] = predicate;
    return predicateName;
  }
  const tests: { [string]: O | ((...a: I) => O) } = cb(test);
  return function(...a: I): ?O {
    for (const predicateName in predicates) {
      if (predicates[predicateName](...a)) {
        const handler = tests[predicateName];
        return typeof handler === 'function' ? handler(...a) : handler;
      }
    }
  };
}

const matcher = match<[string, string], string>(test => ({
  [test(name => !name)]: 'User',
  [test(name => name.length < 10)]: (firstName, lastName) => `${firstName} ${lastName}`,
  [test(name => name.length >= 10)]: firstName => firstName,
  [test()]: () => {
    throw new Error('Unexpected case');
  },
  [test(number => number.toFixed())]: () => 123,
}));

const result = matcher('a', 'b');