strew(...reducer)(initial);
// Each `reducer` is expected to be a function which will be invoked with the current object
// returned by the previous `reducer` as first argument and get/set functions as second argument.
// The `strew(...reducer)` returns a new function that you're able to invoke with the initial
// object and that returns the new reduced object.

strew.get(source, ...path);
// Returns the value described by `path`, if it doesn't exists it returns `undefined`

strew.set(source, ...path, value); // or
strew.set(source, ...[...path, value]);
// Returns a new object with the value(s) described by `path`. If `value` is a function, it will be
// invoked with the existing value or `undefined` as argument and use the returned value instead.
// To have a function as a value you can create a function which returns your function.

const strew = require('strew');

const initial = {dog: 'bad'};

function cat() {
  // The clean way
  return current => Object.assign({}, current, {cat: 'hairy'});
  // { dog: 'bad', cat: 'hairy' }
}

function fish() {
  // Using the `set`-function
  return (current, {set}) => set(current, 'fish', 'stonefish', 'pretty', false);
  // { dog: 'bad',
  //   cat: 'hairy',
  //   fish: { stonefish: { pretty: false } } }
}

function bird() {
  // Using the `set`-function with multiple values
  return (current, {set}) => set(current,
    ['bird', 'alive', 'deafening'],
    ['bird', 'dead', 'silent']
  );
  // { dog: 'bad',
  //   cat: 'hairy',
  //   fish: { stonefish: { pretty: false } },
  //   bird: { alive: 'deafening', dead: 'silent' } }
}

function dog() {
  // Using `set`-function to modify a existing value
  return (current, {set}) => set(current, 'dog', value => value === 'bad' ? 'rude' : 'polite');
  // { dog: 'rude',
  //   cat: 'hairy',
  //   fish: { stonefish: { pretty: false } },
  //   bird: { alive: 'deafening', dead: 'silent' } }
}

function reptile(animal, species) {
  // The advanced way
  return (current, {set, get}) => {
    current = set(current, species, animal);

    let list = get(current, 'reptile', animal);

    if (typeof list === 'undefined') {
      list = [];
    }

    current = set(current, 'reptile', animal, [...list, species]);

    return current;
  }
}

const strewed = strew(
  cat(),
  fish(),
  bird(),
  dog(),
  reptile('snake', 'blank mamba'),
  reptile('snake', 'king kobra'),
  reptile('lizard', 'komodo dragon'),
  reptile('lizard', 'iguana')
)(initial);

console.log(strewed);
// { dog: 'rude',
//   cat: 'hairy',
//   fish: { stonefish: { pretty: false } },
//   bird: { alive: 'deafening', dead: 'silent' },
//   'blank mamba': 'snake',
//   reptile:
//    { snake: [ 'blank mamba', 'king kobra' ],
//    lizard: [ 'komodo dragon', 'iguana' ] },
//   'king kobra': 'snake',
//   'komodo dragon': 'lizard',
//   iguana: 'lizard' }

console.log(initial);
// { dog: 'bad' }