[Functional Programming] Randomly Pull an Item from an Array with the State ADT (Pair)

Posted 2021-02-04 answer1215

Functor composition is a powerful concept that arises when we have one Functor nested in another Functor. It becomes even more powerful when both of those are Chains, allowing us to apply each Functor’s special properties and effects for a given computation.

We witness this power first hand by combining our beloved State with a Pair type mixing the ability to randomly pick an index from an Array using State and use it to draw an element from the Array. We will build up to an easy to use interface that allows us to pull as many elements as we need, by chaining on single State transaction.

// getDeck :: () -> State AppState Deck
const getDeck = () => generateCards()
    .map(xs => Pair([], xs));
/**
 Pair( 
     [ ],
     [ { id: "orange-square", color: "orange", shape: "square" }, { id: "orange-triangle", color: "orange", shape: "triangle" }, { id: "orange-circle", color: "orange", shape:
"circle" }, { id: "green-square", color: "green", shape: "square" }, { id: "green-triangle", color: "green", shape: "triangle" }, { id: "green-circle", color: "green", shape: "circle" }, { id: "blue-square", color: "blue", shape: "square" }, { id: "blue-triangle", color: "blue", shape: "triangle" }, { id: "blue-circle", color: "blue", shape: "circle" }, { id: "yellow-square", color: "yellow", shape: "square" }, { id: "yellow-triangle", color: "yellow", shape: "triangle" }, { id: "yellow-circle", color: "yellow", shape: "circle" } ] 
) 
 */
// draw ::  Integer -> Deck -> Deck    
const draw = compose(
    chain,
    drawCardAt
);
// const draw = index => deck => deck.chain(drawCardAt(index))

// drawRandom :: Deck -> State AppState Deck
// From the right side pair, get a random index, then draw the card by index
const drawRandom = converge(
    liftA2(draw),
    compose(
        randomIndex,
        snd
    ),
    liftState(identity)
)

console.log(
    getDeck()
        .chain(drawRandom)
        .chain(drawRandom)
        .chain(drawRandom)
        .evalWith(state).fst()
)
/**
 * [ { id: ‘orange-square‘, color: ‘orange‘, shape: ‘square‘ },
  { id: ‘blue-triangle‘, color: ‘blue‘, shape: ‘triangle‘ },
  { id: ‘blue-square‘, color: ‘blue‘, shape: ‘square‘ } ]
 */
console.log(
    getDeck()
        .chain(drawRandom)
        .chain(drawRandom)
        .chain(drawRandom)
        .evalWith(state).snd()
)
/**
 [ { id: ‘orange-triangle‘, color: ‘orange‘, shape: ‘triangle‘ },
  { id: ‘orange-circle‘, color: ‘orange‘, shape: ‘circle‘ },
  { id: ‘green-square‘, color: ‘green‘, shape: ‘square‘ },
  { id: ‘green-triangle‘, color: ‘green‘, shape: ‘triangle‘ },
  { id: ‘green-circle‘, color: ‘green‘, shape: ‘circle‘ },
  { id: ‘blue-circle‘, color: ‘blue‘, shape: ‘circle‘ },
  { id: ‘yellow-square‘, color: ‘yellow‘, shape: ‘square‘ },
  { id: ‘yellow-triangle‘, color: ‘yellow‘, shape: ‘triangle‘ },
  { id: ‘yellow-circle‘, color: ‘yellow‘, shape: ‘circle‘ } ]
 */

 

------

const {prop,assoc, Pair, pick, bimap, State, snd, identity, omit, curry, filter, fanout, converge,map, composeK, liftA2, equals, constant,option, chain, mapProps, find, propEq, isNumber, compose, safe} = require(‘crocks‘);
const  {get, modify, of} = State; 

const state = {
    colors: [ ‘orange‘, ‘green‘, ‘blue‘, ‘yellow‘ ],
    shapes: [ ‘square‘, ‘triangle‘, ‘circle‘ ],
    seed: Date.now()
};

const liftState = (fn) => compose(
    of, fn
);
const getState = key => get(prop(key))

// #region random 
// nextSeed :: Integer -> Integer
const nextSeed = seed =>
  (seed * 1103515245 + 12345) & 0x7fffffff

// value :: Integer -> Number
const value = seed =>
  (seed >>> 16) / 0x7fff

// normalize :: (Integer, Integer) -> Number -> Integer
const normalize = (min, max) =>
  x => Math.floor(x * (max - min)) + min

// getNextSeed :: () -> State AppState Integer
const getNextSeed = () =>
  get(({ seed }) => nextSeed(seed))

// updateSeed :: Integer -> State AppState ()
const updateSeed = seed =>
  modify(assoc(‘seed‘, seed))

// nextValue :: Integer -> State AppState Number
const nextValue = converge(
  liftA2(constant),
  liftState(value),
  updateSeed
)

// random :: () -> State AppState Number
const random =
  composeK(nextValue, getNextSeed)

// between :: (Integer, Integer) -> State AppState Integer
const between = (min, max) => 
    random()
        .map(normalize(min, max));

const randomIndex = xs => between(0, xs.length);        
// #endregion

// #region generate

const getColors = () => getState(‘colors‘).map(option([]));
const getShapes = () => getState(‘shapes‘).map(option([]));
const buildCard = curry((color, shape) => ({
    id: `${color}-${shape}`,
    color,
    shape
}));
const buildCards = liftA2(buildCard);
const generateCards = converge(
    liftA2(buildCards),
    getColors,
    getShapes
);

// #endregion

// #region draw 
const getAt = index => array => Array.of(array[index]);
const unsetAt = index => array => [
    ...array.slice(0, index),
    ...array.slice(index + 1)
];
const drawCardAt = index => fanout(getAt(index), unsetAt(index));
// #endregion

// getDeck :: () -> State AppState Deck
const getDeck = () => generateCards()
    .map(xs => Pair([], xs));

// draw ::  Integer -> Deck -> Deck    
const draw = compose(
    chain,
    drawCardAt
);
// const draw = index => deck => deck.chain(drawCardAt(index))

// drawRandom :: Deck -> State AppState Deck
const drawRandom = converge(
    liftA2(draw),
    compose(
        randomIndex,
        snd
    ),
    liftState(identity)
)

console.log(
    getDeck()
        .chain(drawRandom)
        .chain(drawRandom)
        .chain(drawRandom)
        .chain(drawRandom)
        .evalWith(state).fst()
)