ice-cream.js

const builder = require('../');
const { expect } = require('chai');

describe('ice cream', () => {
  it('builds an ice cream cone', () => {
    const serving = builder
      .steps({
        scoop: ({ scoops }, flavor) => ({ scoops: scoops.concat(flavor) }),
      })
      .endings({
        topping: (cone, topping) => ({ ...cone, topping })
      })
      .unwrap({ scoops: [] })

    const basic = serving
      .scoop('chocolate')

    const holygrail = basic
      .scoop('coconut')
      .scoop('banana')

    const perfection = holygrail
      .topping('cherry');

    const ruined = holygrail
      .topping('sprinkles');

    const enthusiast = basic
      .scoop('chocolate')
      .scoop('chocolate')
      .topping('chocolate chip');

    const disgrace = basic
      .scoop('rum & raisins')
      .topping('peanuts');

    expect(perfection).to.eql({
      scoops: ['chocolate', 'coconut', 'banana'],
      topping: 'cherry'
    });
    expect(ruined).to.eql({
      scoops: ['chocolate', 'coconut', 'banana'],
      topping: 'sprinkles'
    });
    expect(enthusiast).to.eql({
      scoops: ['chocolate', 'chocolate', 'chocolate'],
      topping: 'chocolate chip'
    });
    expect(disgrace).to.eql({
      scoops: ['chocolate', 'rum & raisins'],
      topping: 'peanuts'
    });
  })
})

const map = (source, mapper) =>
  Object.keys(source).reduce(
    (target, key) => ({ ...target, [key]: mapper(source[key], key) }),
    {}
  );

const update = (source, key, value) => Object.assign(
  {},
  source,
  { [key]: Object.assign({}, source[key], value) }
);

const echelon = (steps, endings, traverse) => Object.assign(
  map(steps, method => (...params) =>
    echelon(steps, endings, () => method(traverse(), ...params))
  ),
  map(endings, unwrap => (...params) =>
    unwrap(traverse(), ...params)
  )
);

const unwrap = (steps, endings, initialValue) => {
  return echelon(steps, endings, () => initialValue);
};

const builder = unwrap({
  steps(builder, steps) {
    return update(builder, 'steps', steps);
  },
  endings(builder, endings) {
    return update(builder, 'endings', endings);
  }
}, {
  unwrap(builder, initialValue) {
    return unwrap(builder.steps, builder.endings, initialValue);
  }
}, { steps: {}, endings: {} });

module.exports = builder;