[Functional Programming] Use a Javascript Array to Construct a Maybe

Much of our code in real life is full of if statements that check the validity of a given input to see if a given computation should be performed. Using the ever popular Maybe construction allows us capture this disjunction in one place, keeping our functions free of similar if statements that can pollute the intention of the function. You do not even need a fancy library to get this benefit, as Javascript ships with everything you need with it's build in Array.

 

const compose = (f, g) => x => f(g(x));

const maybe = pred => x => pred(x) ? [x]: [];
const mapMaybe = fn => m => m.map(fn);
const optionMaybe = def => m => m.length > 0 ? m[0]: def;

///////////
const add = a => b => a + b;
const pow = a => a * a;
const isValid = x => typeof x === 'number' && !Number.isNaN(x);

///////////
const doMath = compose(
  pow,
  add(10)
)
const safeDoMath = compose(
  mapMaybe(doMath),
  maybe(isValid)
);
const option = (def) => compose(
  optionMaybe(def),
  safeDoMath
)
const app = option('Not a number')

//////////
const result = app();

console.log(result)