predicate-dispatch

A nice little function for creating predicate dispatching with the added bonus of supported promises for every parameter, return value, and object that they work with!

Install

npm install predicate-dispatch

Usage

const B = require('bluebird');
const {dispatch, ramdap: R} = require('predicate-dispatch');
// `dispatch` is the main function
// `ramdap` is the Ramda library with some convenience functions included for working with promises

const exampleObject = [{
    type: 'good',
    value: 'hello'
  }, {
    type: 'bad',
    value: 'goodbye'
  }];

const worldly = dispatch(
  // Property whose value to use as the `dispatch value`
  R.prop('type'),
  // Map the `dispatch value` to a function
  R.prop(R.__, {
    good: (s) => s + ' world',
    bad: (s) => s + ' cruel world'
  }),
  // Lens to focus on the property `value` and pass that to the dispatchee
  R.lensProp('value'));

B.all(exampleObject.map(worldly)).then(console.log);

/* Logs:
  [{
    type: 'good',
    value: 'hello world'
  }, {
    type: 'bad',
    value: 'goodbye cruel world'
  }]
*/

So there is a lot going on here under the covers. The key thing above is that the lens insures that the return value of the dipatched function is isolated to the part of the original input that the lens focused onto. Another thing to keep in mind is that the function will always return a promise. The function returned from dispatch is pervasively asynchronous.

So this type of dispatch is nice but the power of lenses and predicate-dispatching comes when you chain dispatchers together. We can see this with a very similar setup to above but with a nested object:

const B = require('bluebird');
const {dispatch, ramdap: R} = require('predicate-dispatch');
// `dispatch` is the main function
// `ramdap` is the Ramda library with some convenience functions included for working with promises

const exampleObject = {
  type: 'foo',
  child: {
    type: 'good',
    value: 'hello'
  }
};

const worldly = dispatch(
  // Property whose value to use as the `dispatch value`
  R.prop('type'),
  // Map the `dispatch value` to a function
  R.prop(R.__, {
    good: (s) => s + ' world',
    bad: (s) => s + ' cruel world'
  }),
  // Lens to focus on the property `value` and pass that to the dispatchee
  R.lensProp('value'));

const router = dispatch(
  R.prop('type'),
  // Map the `dispatch value` to a function
  R.prop(R.__, {
    foo: worldly
  }),
  // Lens to focus on the property `child` and pass the value of `child` to the dispatchee
  R.lensProp('child'));

router(exampleObject).then(console.log);

/* Logs:
  {
    type: 'foo',
    child: {
      type: 'good',
      value: 'hello world'
    }
  }
*/