Returns a value of Maybe object after applying set of transformers

compose-maybe-with-default.js

const R = require('ramda');

const composeMaybe = require('./compose-maybe').composeMaybe;

/**
 * Returns a value of Maybe object after applying set of transformers
 * default → [(y → z), (x → y), …, (o → p), ((a, b, …, n) → o)] → ((a, b, …, n) → z | default)
 */
const composeMaybeWithDefault = R.curry((defaultVal, transformers) =>
  R.memoize(R.o(R.invoker(1, 'withDefault')(defaultVal), composeMaybe(...transformers)))
);

module.exports = { composeMaybeWithDefault };

compose-maybe-with-default.spec.js

const R = require('ramda');

const composeMaybeWithDefault = require('./compose-maybe-with-default').composeMaybeWithDefault;

describe('composeMaybeWithDefault', () => {
  test('returns result of transformations', () => {
    const fn = composeMaybeWithDefault(0, [R.inc, R.add(3), R.divide(8)]);
    const result = fn(4);

    expect(result).toBe(6);
  });

  test('returns default value for null', () => {
    const fn = composeMaybeWithDefault(0, [R.inc, R.always(null), R.divide(8)]);
    const result = fn(4);

    expect(result).toBe(0);
  });

  test('returns default value for undefined', () => {
    const fn = composeMaybeWithDefault(0, [R.inc, () => {}, R.divide(8)]);
    const result = fn(4);

    expect(result).toBe(0);
  });

  test('returns NaN if result of calculations is NaN', () => {
    const fn = composeMaybeWithDefault(0, [R.inc, R.always(NaN), R.divide(8)]);
    const result = fn(4);

    expect(isNaN(result)).toBeTruthy();
  });

  test('returns 0 if result of calculations is 0', () => {
    const fn = composeMaybeWithDefault(5, [R.subtract(4)]);
    const result = fn(4);

    expect(result).toBe(0);
  });
});

compose-maybe.js

const R = require('ramda');

const mapToMaybe = require('./map-to-maybe').mapToMaybe;

/**
 * Returns version of compose for Maybe object
 * ((y → z), (x → y), …, (o → p), ((a, b, …, n) → o)) → ((a, b, …, n) → Maybe z)
 */
const composeMaybe = R.compose(
  R.memoize,
  R.apply(R.composeK),
  R.map(mapToMaybe),
  R.append(R.identity), // because R.composeK accepts at least 2 functions
  R.unapply(R.identity)
);

module.exports = { composeMaybe };

compose-maybe.spec.js

const R = require('ramda');
const { Maybe } = require('zoomjs');

const composeMaybe = require('./compose-maybe').composeMaybe;

describe('composeMaybe', () => {
  test('returns Maybe object', () => {
    const testObj = {
      first: 5
    };
    const resultObj = composeMaybe(R.prop('some'), R.inc, R.prop('first'))(testObj);

    expect(resultObj).toBeInstanceOf(Maybe);
  });

  test('resulting Maybe should contain result of transformations', () => {
    const testObj = {
      first: 5
    };
    const resultObj = composeMaybe(R.inc, R.prop('first'))(testObj);
    const value = resultObj.withDefault(0);

    expect(value).toBe(6);
  });

  test('returns Nothing for undefined value', () => {
    const testObj = {
      first: 5
    };
    const resultObj = composeMaybe(R.add(12), R.prop('some'), R.inc, R.prop('first'))(testObj);

    expect(resultObj.isNothing()).toBeTruthy();
  });

  test('returns Nothing for null value', () => {
    const resultObj = composeMaybe(R.always(null), R.inc)(5);

    expect(resultObj.isNothing()).toBeTruthy();
  });

  test('returns Maybe.Just object with a value of NaN', () => {
    const resultObj = composeMaybe(R.add(3), R.inc)('str');
    const value = resultObj.withDefault(0);

    expect(isNaN(value)).toBeTruthy();
  });

  test('should accept 1 transformer function too', () => {
    const resultObj = composeMaybe(R.inc)(3);
    const value = resultObj.withDefault(0);

    expect(value).toBe(4);
  });

  test('is memoized', () => {
    // eslint-disable-next-line fp/no-let
    let counter = 0;

    const fn = composeMaybe(
      R.add(3),
      val => {
        // eslint-disable-next-line fp/no-mutation
        counter += 1;
        return R.add(5, val);
      },
      R.inc
    );

    fn(3);
    fn(3);
    fn(7);
    fn(3);

    expect(counter).toBe(2);
  });
});

map-to-maybe.js

const R = require('ramda');

const { Maybe: { fromNullable } } = require('zoomjs');

/**
 * Returns Maybe object with result of transformation of applied value
 * (a -> b) -> a -> Maybe a
 */
const mapToMaybe = R.curry((transformer, val) => fromNullable(transformer(val)));

module.exports = { mapToMaybe };

map-to-maybe.spec.js

const R = require('ramda');
const { Maybe } = require('zoomjs');

const mapToMaybe = require('./map-to-maybe').mapToMaybe;

describe('mapToMaybe', () => {
  test('returns Maybe object', () => {
    const result = mapToMaybe(R.add(5))(3);

    expect(result).toBeInstanceOf(Maybe);
  });

  test('returns Maybe object with a value of transformation result', () => {
    const result = mapToMaybe(R.add(5))(3);
    const value = result.withDefault(0);

    expect(value).toBe(8);
  });

  test('returns Nothing for null value', () => {
    const result = mapToMaybe(R.always(null))(3);

    expect(result.isNothing()).toBeTruthy();
  });

  test('returns Nothing for undefined value', () => {
    const result = mapToMaybe(() => {})(3);

    expect(result.isNothing()).toBeTruthy();
  });

  test('returns Maybe.Just object with a value of NaN', () => {
    const result = mapToMaybe(R.add(5))('str');
    const value = result.withDefault(0);

    expect(isNaN(value)).toBeTruthy();
  });
});