[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 + )

         .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 + )

         .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

巴特西

[Compose] 9. Delay Evaluation with LazyBox

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