[Functional Programming] Transition State based on Existing State using the State ADT (liftState, composeK)

While sometimes outside input can have influence on how a given stateful transaction transitions, there are many times where the current state at the time of a transaction. We can see the power of this type of transaction by seeing what it would take to read from two different locations in state in parallel and then pass on the result of combining them under a comparison operation to another transition that will set a different location based on the result.

 

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

// getState :: String -> State Object (Maybe a)
const getState = key => get(prop(key));

// liftState :: ( a -> b) -> a -> State s b
const liftState = fn => compose(
    of,
    fn // getfn return value and pass into State.of
);

// getHint :: () -> State AppState Hint 
const getHint = () => getState('hint')
    .map(option({color: 'yay', shape: 'uwu'}));

// getCard :: String -> State AppState Card
const getCard = (id) => getState('cards')
    .map(chain(find(propEq('id', id)))) // find() return a Maybe, so need to use chain to unfold the value
    .map(option({id: null, color: 'unk', shape: 'unk'}));

// cardToHint :: State AppState Hint     
const cardToHint = composeK(
    liftState(omit(['id'])),
    getCard
)

// setIsCorrect :: Boolean -> State AppState ()    
const setIsCorrect = (b) => modify(mapProps({'isCorrect': constant(b)}));

const validateAnswer = converge(
    liftA2(equals),
    cardToHint,
    getHint
)

const feedback = composeK(
    setIsCorrect,
    validateAnswer
)


/////////////////////////////////////////////////////

const state = {
    cards: [
        {id: 'green-square', color: 'green', shape: 'square'},
        {id: 'orange-square', color: 'orange', shape: 'square'},
        {id: 'blue-triangle', color: 'blue', shape: 'triangle'}
    ],
    hint: {
        color: 'green',
        shape: 'square'
    },
    isCorrect: null
}

console.log(
    feedback('green-square')
        .execWith(state)
)