[Functional Programming Moand] Update The State Of A State Monad (put)

Stateful computations require the ability for their state to change overtime. We take a look on one way to replace the state portion of our stateful datatype with a value. We explore the mechanics of how this can be accomplished and introduce the put construction helper. We also see how we can run our instances with execWith to extract the state portion and discard the resultant.

 

Put state is replace the old state with new state:

//putState :: s -> State s ()
const putState = state => State(() => Pair(Unit(), state));
console.log(
    putState('new')
        .runWith('OLD') //Pair( (), "put" )
)

 

This is useful when we want to create 'reset':

//putState :: s -> State s ()
const putState = state => State(() => Pair(Unit(), state));
//reset :: () -> State String ()
const reset = () => putState('default');
console.log(
    reset()
        .execWith('OLD') // default
)

 

This opreation is commonly used, therefore there is a well made function 'put':

const { Pair, Unit, curry, objOf, compose, State, mapProps, prop, option } = require("crocks");

const { put, get } = State;

//reset :: () -> State String ()
const reset = () => put('default');
console.log(
    reset()
        .execWith('OLD') // default
)
console.log(
    put('new')
        .runWith('OLD') //Pair( (), "new" )
)