[Functional Programming Monad] Substitute State Using Functions With A State Monad (get, evalWith)

We take a closer look at the get construction helper and see how we can use it to lift a function that maps the state portion and updates the resultant with the result. Using get in this fashion, we then demonstrate how we can make accessors that can then be extended to create more complex interactions.

As there are times that we only want to pull the resultant for a given computation, we take a look at running our instances with the evalWith method. evalWith will run our computations, throwing away the state, returning the resultant.

get() function return a Pair(a, s). 'a' is the return value (variable) and 's' is the orginal state.

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

const { modify, get } = State;

const burgers = {
    burgers: 4
}

const taocs = {
    taocs: 10
}

const res = get()
    .map(prop('burgers'))
    .runWith(burgers)

console.log(res); // Pair( Just 4, { burgers: 4 } )

One thing we can improve from the code above is combine

// From
get()
    .map(prop('burgers'))

// To
get(prop('burgers'))

Second the return value for the 'a' is wrapped in Maybe, we want to provided some default value for Nothing case:

const res = get(prop('taocs'))
    .runWith(burgers)

console.log(res); // Pair( Nothing, { burgers: 4 } )

To do that we can create 'defaultProp':

const defaultProp = (key, def) => get(prop(key)).map(option(def));
const res = defaultProp('taocs', 0)
    .runWith(burgers)

console.log(res); // Pair( 0, { burgers: 4 } )

Thrid, we can  'compose' to simply the code further:

const defaultProp = (key, def) => compose(
    option(def),
    prop(key)
)
const getBurgers = get(defaultProp('taocs', 0))
const res = getBurgers
    .runWith(burgers)

// Pair( 0, { burgers: 4 } )

Last, we don't really care about the Pair(0, {burgers: 4}), we only interest the value: we can replace 'runWith' with 'evalWith':

const defaultProp = (key, def) => compose(
    option(def),
    prop(key)
)
const getBurgers = get(defaultProp('burgers', 0))
const res = getBurgers
    .evalWith(burgers) // 4

 ----

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

const { modify, get } = State;

const burgers = {
    burgers: 4
}

const taocs = {
    taocs: 10
}
const defaultProp = (key, def) => compose(
    option(def),
    prop(key)
)
const getBurgers = get(defaultProp('burgers', 0))
const burgerToTacos = getBurgers.map(objOf('tacos'));
const res = burgerToTacos
    .evalWith(burgers) // 4

console.log(res); // { tacos: 4 }