Example of monads to manage validations

monads.js

// base

const Valid = x => ({
  map: f => Valid(f(x)),
  chain: f => f(x),
  bimap: ({valid}) => Valid(valid(x)),
  cata: ({valid}) => valid(x)
})

const Wrong = x => ({
  map: f => Wrong(x),
  chain: f => Wrong(x),
  bimap: ({wrong}) => Wrong(wrong(x)),
  cata: ({wrong}) => wrong(x)
})

const validate = (pred, msgFn) => data =>
  pred(data)
    ? Valid(data)
    : Wrong(msgFn(data))

const isString = x =>
  x.constructor === String

const hasKey = key => x =>
  x && x.hasOwnProperty(key)

const validateKey = name =>
  validate(hasKey(name), () => `${name} not found`)

const field = (validation, msgFn, name) => data =>
  validateKey(name)(data)
    .map(data => data[name])
    .chain(validation)
    .bimap({
      wrong: err => msgFn(err),
      valid: x => data
    })

const chain = (a, b) =>
  a.chain(b)

const validations = all => data =>
  all.reduce(chain, Valid(data))

const stringField = name =>
  field(
    validate(isString, () => "isn't string"),
    msg => `${name} ${msg}`,
    name
  )

// validators

const validateUser = validations([
  stringField("name"),
  stringField("team")
]);

const validateProfile = validations([
  field(validateUser, msg => `user.${msg}`, "user"),
  stringField("color")
]);

const printValidation = {
  wrong: x => console.error("ERROR ", x),
  valid: x => console.log(x)
};

// runtime

validateProfile({
  user: { name: "jao", team: "A" },
  color: "red"
}).cata(printValidation);

validateProfile({
  user: { name: "jao", team: 1 },
  color: "red"
}).cata(printValidation);

validateProfile({
  color: "red"
}).cata(printValidation);

validateProfile(null)
  .cata(printValidation);