[Javascript] Use a Pure RNG with the State ADT to Select an Element from State

The resultant in a State ADT instance can be used as a means of communication between different stateful transactions. It can be used to read and transform a portion of our state into a form that another transaction is dependent on. This allows us to only keep what is needed state, without filling it with calculations that are only needed for one or few transactions.

We take can take advantage of this by pulling not only a random number using the seed from our state, but also pulling a list of card and filtering them, to randomly select one from our calculated state. Then we use the resultant to store the calculation before we save the result to state.

Code for Random generator is here, which is not so important to this blog.

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

const state = {
    cards: [
        {id: 'green-square', color: 'green', selected: true, shape: 'square'},
        {id: 'orange-square', color: 'orange', shape: 'square'},
        {id: 'blue-triangle', color: 'blue', shape: 'triangle'}
    ],
    hint: null,
    seed: Date.now()
}

// #region helper

const liftState = fn => compose(
    of,
    fn
)

// 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)
// #endregion

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

The only important piece is 'bewteen' function, which can generate the Number between a min & max range.

The  idea for what we are going to achieve is that:

• Select all the unselected cards
• Randomly choose one unselected card by using random function
• Set the hint according to the selected card.

First, select all the unselected cards:

const selectState = (key, fn) => get(
    compose(
        map(fn),
        prop(key)
    )
)
const unselected = ({selected}) => !selected;
const getUnselectedCards = () => selectState('cards', filter(unselected)).map(option([]))

Second: Rnadomly choose one unselected card by using random function:

// randomIndex :: [a] -> State AppState a
const randomIndex = arr => between(0, arr.length)
const getAt = index => arr => arr[index];
const pickCard = converge(
    liftA2(getAt),
    randomIndex,
    liftState(identity)
)

Last, Set the hint according to the selected card:

const over = (key, fn) => modify(
    mapProps({[key]: fn})
)
const toHint = pick(['color', 'shape'])
// setHint :: Card -> State AppState()
const setHint = card => over('hint', constant(toHint(card)))
const nextHint = composeK(
    setHint,
    pickCard,
    getUnselectedCards
)

-----------------

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

const state = {
    cards: [
        {id: 'green-square', color: 'green', selected: true, shape: 'square'},
        {id: 'orange-square', color: 'orange', shape: 'square'},
        {id: 'blue-triangle', color: 'blue', shape: 'triangle'}
    ],
    hint: null,
    seed: Date.now()
}

// #region helper

const liftState = fn => compose(
    of,
    fn
)

// 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)
// #endregion

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

// Get all unselected card
// Randomly choose an unselected Card
// Set hint

const selectState = (key, fn) => get(
    compose(
        map(fn),
        prop(key)
    )
)
const unselected = ({selected}) => !selected;
const getUnselectedCards = () => selectState('cards', filter(unselected)).map(option([]))

// randomIndex :: [a] -> State AppState a
const randomIndex = arr => between(0, arr.length)
const getAt = index => arr => arr[index];
const pickCard = converge(
    liftA2(getAt),
    randomIndex,
    liftState(identity)
)
const over = (key, fn) => modify(
    mapProps({[key]: fn})
)
const toHint = pick(['color', 'shape'])
// setHint :: Card -> State AppState()
const setHint = card => over('hint', constant(toHint(card)))
const nextHint = composeK(
    setHint,
    pickCard,
    getUnselectedCards
)

console.log(
    nextHint()
        .execWith(state)
)