[Functional Programming] Compose Simple State ADT Transitions into One Complex Transaction

Posted 2021-02-02 answer1215

State is a lazy datatype and as such we can combine many simple transitions into one very complex one. This gives us a lot of control over how our state changes over time. In this lesson we will use this to our advantage and combine many transactions into one complex action. In the end only the final state will be reported which can be reasoned about as one transition.

 

const state = {
    cards: [
        {id: ‘green-square‘, color: ‘green‘, shape: ‘square‘},
        {id: ‘orange-square‘, color: ‘orange‘, shape: ‘square‘},
        {id: ‘blue-square‘, color: ‘blue‘, shape: ‘triangle‘}
    ],
    left: 8,
    moves: 0
}

const {State, when, assign, map, mapProps, propEq, curry, compose} = require(‘crocks‘);
const {modify} = State;

const inc = x => x + 1;
const dec = x => x - 1;

const clamp = (min, max) => x => Math.min(Math.max(min, x), max);
const clampAfter = curry((min, max, fn) =>
  compose(
    clamp(min, max),
    fn
  )
);

const limitMoves = clampAfter(0, 8);

const decLeft = () => over("left", limitMoves(dec));
const incMoves = () => over("moves", limitMoves(inc));



const markSelected = id => assignBy(propEq(‘id‘, id), {selected: true})
const assignBy = (pred, obj) => when(pred, assign(obj));
const over = (key, fn) => modify(mapProps({ [key]: fn }));

const selectCard = id => over(‘cards‘, map(markSelected(id)))

const answer = (id) => State.of(id)
    .chain(incMoves)
    .chain(incMoves)
    .chain(decLeft)
    .chain(selectCard);


console.log(
    JSON.stringify(
        answer(‘green-square‘).execWith(state),
        null,
        2
    )
); 
 /*
{
  "cards": [
    {
      "id": "green-square",
      "color": "green",
      "shape": "square"
    },
    {
      "id": "orange-square",
      "color": "orange",
      "shape": "square"
    },
    {
      "id": "blue-square",
      "color": "blue",
      "shape": "triangle"
    }
  ],
  "left": 7,
  "moves": 2
}
*/