[Transducer] Create a Sequence Helper to Transduce Without Changing Collection Types

A frequent use case when transducing is to apply a transformation to items without changing the type of the collection.

In this lesson, we'll create a helper to do just that. seq will be similar to into, except the target type will be inferred from the source collection. For example, if transducing from an array, seq will create an array as the output collection as well.

seq is thus more restrictive and easier to consume than into.

The whole for 'seq' is we don't need to worry about collection type will different from target type. In short, input and output are the same type.

import {isPlainObject} from 'lodash';
import {compose, map, arrayReducer, objectReducer, transduce} from '../utils.js';

const into = (to, xf, collection) => {
  if (Array.isArray(to)) return transduce(xf, arrayReducer, to, collection);
  else if (isPlainObject(to)) return transduce(xf, objectReducer, to, collection);
  throw new Error('into only supports arrays and objects as `to`');
};

const seq = (xf, collection) => {
  if (Array.isArray(collection)) return transduce(xf, arrayReducer, [], collection);
  else if (isPlainObject(collection)) return transduce(xf, objectReducer, {}, collection);
  throw new Error('unsupported collection type');
};

seq(map(x => x*2), [1,2,3]);

const flip = compose(
  map(([k,v]) => ({[v*10]:k})),
);

seq(flip, {one: 1, two: 2, three: 3});