contravariant functor in javascript

contravariant_functor.js

//https://medium.com/@drboolean/monoidal-contravariant-functors-are-actually-useful-1032211045c4


const log = require('./lib/log')

log(
  'testing'
)

const daggy = require('daggy')
const {
  foldMap
} = require('pointfree-fantasy')
const {
  concat,
  toUpper,
  prop,
  identity,
  range,
  compose
} = require('ramda')

const Comparison = daggy.tagged('Comparison', ['f'])

const GT = 1
const LT = -1
const EQ = 0

const concatOrd = (o1, o2) => (o1 === EQ) ? o2 : o1
const emptyOrd = EQ

Comparison.prototype.concat = function (c) {
  return Comparison((x, y) => concatOrd(this.f(x, y), c.f(x, y)))
}

Comparison.empty = () => Comparison((x, y) => EQ)
Comparison.prototype.empty = Comparison.empty

Comparison.prototype.contramap = function (g) {
  return Comparison((x, y) => this.f(g(x), g(y)))
}


const eq21 = (a, b) => {
  if (a === 21) {
    return GT
  }
  if (b === 21) {
    return LT
  }
  return EQ
}

const over21 = (a, b) => {
  if (a > 21) {
    return LT
  }
  if (b > 21) {
    return GT
  }
  return EQ
}

const greater = (a, b) => {
  if (a > b) {
    return GT
  }
  if (b > a) {
    return LT
  }
  return EQ
}

const blackjack = foldMap(Comparison, [eq21, over21, greater])

const cardToInt = prop('val')
const cards = [{
  suit: 'hearts',
  val: 8
}, {
  suit: 'diamonds',
  val: 4
}, {
  suit: 'clubs',
  val: 7
}]


log(
  cards.sort(blackjack.contramap(cardToInt).f)
)