[Functional Programming Monad] Map And Evaluate State With A Stateful Monad

We explore our first stateful transaction, by devising a means to echo our state value into the resultant for independent modification. With our state value in the resultant, we explore using mapto lift functions into our type as a means to modify the resultant.

In our exploration we find that not only can we change values of the resultant, but we can change the type as needed. We also find that we can combine subsequent map calls into function compositions, allowing us to clean up our code and combine our mappings to avoid excessive interactions with our State datatype.

To wrap it all up, we take a look at a handy State construction helper called get that we can use to query the state into resultant. This allows to read from our state for computations that are based on the value of a given state.

 

const { curry, compose, Pair, State, mapProps, composeK } = require("crocks");

const { get } = State;

// State s a
// We define State as a fixed type of state 's' or the left and a variable 'a' on the right
// (s -> (a, s))
// State Unit defined as Pair(a, s) with 'a' variable on the left and 's' on the right

// add :: Number -> Number -> Number 
const add = x => y => x + y;

// pluralize :: (String, String) -> Number -> String
const pluralize = (single, plural) => num => `${num} ${Math.abs(num) === 1 ? single : plural}`;

// getState :: () -> State s
const getState = () => State(s => Pair(s, s))

// makeAwesome :: Number -> String
const makeAwesome = pluralize('Awesome', 'Awesomes')

// flow :: Number -> String
const flow = compose(
    makeAwesome,
    add(10)
)
console.log(
    getState()
        .map(flow)
        .runWith(2)
        .fst()
)

 

Get state constructor is so common, there is well made function call 'get' to replace our 'getState':

const { curry, compose, Pair, State, mapProps, composeK } = require("crocks");

const { get } = State;

// State s a
// We define State as a fixed type of state 's' or the left and a variable 'a' on the right
// (s -> (a, s))
// State Unit defined as Pair(a, s) with 'a' variable on the left and 's' on the right

// add :: Number -> Number -> Number 
const add = x => y => x + y;

// pluralize :: (String, String) -> Number -> String
const pluralize = (single, plural) => num => `${num} ${Math.abs(num) === 1 ? single : plural}`;

// makeAwesome :: Number -> String
const makeAwesome = pluralize('Awesome', 'Awesomes')

// flow :: Number -> String
const flow = compose(
    makeAwesome,
    add(10)
)
console.log(
    get()
        .map(flow)
        .runWith(2)
        .fst()
)