[Compose] 9. Delay Evaluation with LazyBox

We rewrite the Box example using lazy evaulation.

Here is Box example:

const Box = (x) => ({
  map: f => Box(f(x)),
  fold: f => f(x)
});

const res = Box(' 64 ')
         .map(abba => abba.trim())
         .map(trimmed => new Number(trimmed))
         .map(number => number + 1)
         .map(x => String.fromCharCode(x))
         .fold(x => x.toLowerCase());

console.log(res); // 'a'

So how to make it as Lazy Box? The Answer is instead of passing a value to the Box, we pass and function into it.

const LazyBox = (fn) => ({
  map: g => LazyBox(() => g(fn())),
  fold: g => g(fn()) // call the g()
});

const res = LazyBox(() => ' 64 ')
         .map(abba => abba.trim())
         .map(trimmed => new Number(trimmed))
         .map(number => number + 1)
         .map(x => String.fromCharCode(x))
         .fold(x => x.toLowerCase());

console.log(res); // 'a'

inside map function, we use function defination:

() => g(fn())

Just defined, but not call. Using g() is to make it composeable.

When actually 'fold', we call fn():

fold: g => g(fn()) // call the g()

One important thing to take away, to make it lazy, wrap into a function