Functional TypeScript: Either vs Validation

functional-typescript-either-vs-validation-02.ts

import { HKT, URIS2, Type2 } from 'fp-ts/lib/HKT'
import { Applicative, Applicative2, Applicative2C } from 'fp-ts/lib/Applicative'

type Errors = Array<string>

function checkAge<M extends URIS2>(
  applicative: Applicative2<M>,
  fail: (errors: Errors) => Type2<M, Errors, Person>
): (p: Person) => Type2<M, Errors, Person>
function checkAge<M extends URIS2>(
  applicative: Applicative2C<M, Errors>,
  fail: (errors: Errors) => Type2<M, Errors, Person>
): (p: Person) => Type2<M, Errors, Person>
function checkAge<M>(
  applicative: Applicative<M>,
  fail: (errors: Errors) => HKT<M, Person>
): (p: Person) => HKT<M, Person> {
  return p => {
    if (p.age < 18) {
      return fail(['Too Young!'])
    } else if (p.age > 40) {
      return fail(['Too Old!'])
    }
    return applicative.of(p)
  }
}

function checkClothes<M extends URIS2>(
  applicative: Applicative2<M>,
  fail: (errors: Errors) => Type2<M, Errors, Person>
): (p: Person) => Type2<M, Errors, Person>
function checkClothes<M extends URIS2>(
  applicative: Applicative2C<M, Errors>,
  fail: (errors: Errors) => Type2<M, Errors, Person>
): (p: Person) => Type2<M, Errors, Person>
function checkClothes<M>(
  applicative: Applicative<M>,
  fail: (errors: Errors) => HKT<M, Person>
): (p: Person) => HKT<M, Person> {
  return p => {
    if (p.gender === 'Male' && p.clothes.indexOf('Tie') === -1) {
      return fail(['Smarten Up!'])
    } else if (p.gender === 'Female' && p.clothes.indexOf('Trainers') !== -1) {
      return fail(['Wear high heels'])
    }
    return applicative.of(p)
  }
}

function checkSobriety<M extends URIS2>(
  applicative: Applicative2<M>,
  fail: (errors: Errors) => Type2<M, Errors, Person>
): (p: Person) => Type2<M, Errors, Person>
function checkSobriety<M extends URIS2>(
  applicative: Applicative2C<M, Errors>,
  fail: (errors: Errors) => Type2<M, Errors, Person>
): (p: Person) => Type2<M, Errors, Person>
function checkSobriety<M>(
  applicative: Applicative<M>,
  fail: (errors: Errors) => HKT<M, Person>
): (p: Person) => HKT<M, Person> {
  return p => {
    if (['Drunk', 'Paralytic', 'Unconscious'].indexOf(p.sobriety) !== -1) {
      return fail(['Sober Up!'])
    }
    return applicative.of(p)
  }
}