Fantas, Eel, and Specification

semigroup.js

const { tagged } = require('daggy')

const First = tagged('val')

First.prototype.concat = function (that) {
  return this
}

const Min = tagged('val')

Min.prototype.concat = function (that) {
  return Min(Math.min(this.val, that.val))
}

const Any = tagged('val')

Any.prototype.concat = function (that) {
  return Any(this.val || that.val)
}

const Tuple4 = tagged('a', 'b', 'c', 'd')

Tuple4.prototype.concat = function (that) {
  return Tuple4(this.a.concat(that.a),
                this.b.concat(that.b),
                this.c.concat(that.c),
                this.d.concat(that.d))
}

const Customer = tagged(
  'name',
  'favouriteThings',
  'registrationDate',
  'hasMadePurchase'
)

const myStrategy = {
  to: customer => Tuple4(
    First(customer.name),

    customer.favouriteThings,

    Min(customer.registrationDate),

    Any(customer.hasMadePurchase)
  ),

  from: ({ a, b, c, d }) =>
    Customer(a.val, b, c.val, d.val)
}

const merge = strategy => x => y =>
  strategy.from(
    strategy.to(x)
      .concat(strategy.to(y))
  )

const Tom1 = Customer('Tom', ['socks'], 100000, false)
const Tom2 = Customer('TomH', ['gloves'], 90000, true)

// { name: 'Tom'
// , favouriteThings: ['socks', 'gloves']
// , registrationDate: 90000
// , hasMadePurchase: true
// }
merge(myStrategy)(Tom1)(Tom2)