[Functional Programming] 2. Function modelling -- Validation

We want to write a Monoid, which able to take function called 'run', therefore we able to run a serial of functions together to get a combined result.

Prototype:

/**
 * Take a `run` function, return a object which contain `run & concat`
 * methods.
 *
 * concat take other Monoid. we able to concat the reuslt inside concat function.
 * @param {run: function}
 */
const Fn = run => ({
  run,
  concat(otherM) {
    return Fn(x => run(x).concat(otherM.run(x)));
  }
});

Example:

const { List } = require("immutable-ext");

const Success = x => ({
  x,
  isFail: false,
  concat(otherM) {
    return otherM.isFail ? otherM : Success(x);
  }
});

const Fail = x => ({
  x,
  isFail: true,
  concat(otherM) {
    return otherM.isFail ? Fail(x.concat(otherM.x)) : Fail(x);
  }
});

const Validation = run => ({
  run,
  concat(otherM) {
    return Validation((key, x) => run(key, x).concat(otherM.run(key, x)));
  }
});

/////////////////

const isPresent = Validation((key, x) =>
  !!x ? Success(x) : Fail([`${key} is required`])
);
const isEmail = Validation((key, x) =>
  /@/.test(x) ? Success(x) : Fail([`${key} is not a valid email`])
);

const validate = (schema, obj) =>
  List(Object.keys(schema)).foldMap(
    key => schema[key].run(key, obj[key]),
    Success(obj)
  );

////////////////////

const validationSchema = {
  name: isPresent,
  email: isPresent.concat(isEmail)
};
const obj = {
  name: "Zhentian",
  email: "wan@wan.one"
};
const res = validate(validationSchema, obj);

console.log(res);

'Validtion' is the monid we create which enable to run a serial of validation functions such as 'isPresent' & 'isEmail'.

Output result is if 'Success': then return the object itself.

If 'Fail': Then return all the error message.