Splitting/nesting _.flow with lodash (or ramda)

Question

I have two objects, one describes the features of a location, the other describes the prices of those features.

features = {
  improvements: [...] // any array of many id's
  building: {} // only one id, may be undefined
}
prices = {
  id_1: 10,
  ...
}

I want to iterate over features and collate all the prices. Sometimes features.building will be undefined, sometimes features.improvements will be empty.

Additional code/workbench on repl.it

I can do this with lodash in this manner:

result = _(features.improvements)
  .map(feature => prices[feature.id])
  .concat(_.cond([
    [_.isObject, () => prices[features.building.id]]
  ])(features.building)) // would like to clean this up
  .compact()
  .value();

I'm interested in writing this in a more functional manner, and I ended up with:

result = _.flow([
  _.partialRight(_.map, feature => prices[feature.id]),
  _.partialRight(_.concat, _.cond([
    [_.isObject, () => prices[features.building.id]]
  ])(features.building)),
  _.compact,
])(features.improvements)

I still have to almost secretly call features.building mid stream, which feels awkward to me.

What I'd like to get to is (pseudocoded):

flow([
  // maybe need some kind of "split([[val, funcs], [val, funcs]])?
  // the below doesn't work because the first
  // flow's result ends up in the second

  // do the improvement getting
  flow([
    _.map(feature => prices[feature.id])
  ])(_.get('improvements')),

  // do the building getting
  flow([
    _.cond([
      [_.isObject, () => [argument.id]]
    ])
  ])(_.get('building')),

  // concat and compact the results of both gets
  _.concat,
  _.compact,
])(features); // just passing the root object in

Is it possible? How would a more seasoned FP-Programmer approach this?

I'm open to solutions written with lodash-fp or rambda (or anything with good docs I can try to understand) since those probably give cleaner code because they're more functionally-orientated/curried than standard lodash.

Answer 1

Lodash

Here is a solution that uses _.flow():

1. Convert the features to an array using _.values(), _.flatten(), and _.compact() (to ignore building when undefined).
2. Convert to an array of ids with _.map().
3. Get the values with _.at().

const { values, flatten, compact, partialRight: pr, map, partial, at } = _;

const fn = prices => _.flow([
  values,
  flatten,
  compact,
  pr(map, 'id'),
  partial(at, prices)
])

const prices = {
  i_1: 'cost_i_1',
  i_2: 'cost_i_2',
  i_3: 'cost_i_3',
  i_4: 'cost_i_4',
  b_1: 'cost_b_1',
};

const features = {
  improvements: [
    {id: 'i_1'},
    {id: 'i_2'},
    {id: 'i_3'},
    {id: 'i_4'},
  ],
  building: {
    id: 'b_1'
  },
};

const result = fn(prices)(features);

console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>

lodash/fp

const { values, flatten, compact, map, propertyOf } = _;

const fn = prices => _.flow([
  values,
  flatten,
  compact,
  map('id'),
  map(propertyOf(prices))
])

const prices = {"i_1":"cost_i_1","i_2":"cost_i_2","i_3":"cost_i_3","i_4":"cost_i_4","b_1":"cost_b_1"};
const features = {"improvements":[{"id":"i_1"},{"id":"i_2"},{"id":"i_3"},{"id":"i_4"}],"building":{"id":"b_1"}};

const result = fn(prices)(features);

console.log(result);

<script src='https://cdn.jsdelivr.net/g/lodash@4(lodash.min.js+lodash.fp.min.js)'></script>

Ramda

1. Get the values, flatten, and filter undefineds using R.identity.
2. Get the id props with R.map.
3. Use a fliped R.props to get the ids values from prices

const { pipe, values, flatten, filter, identity, map, prop, flip, props } = R;

const propsOf = flip(props);

const fn = prices => pipe(
  values,
  flatten,
  filter(identity),
  map(prop('id')),
  propsOf(prices)
);

const prices = {"i_1":"cost_i_1","i_2":"cost_i_2","i_3":"cost_i_3","i_4":"cost_i_4","b_1":"cost_b_1"};
const features = {"improvements":[{"id":"i_1"},{"id":"i_2"},{"id":"i_3"},{"id":"i_4"}],"building":{"id":"b_1"}};

const result = fn(prices)(features);

console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.26.1/ramda.min.js"></script>