Functor函子:带有改变值的map方法的值容器

FP defines its own types, and functors are among the most basic. Think of it as a container that holds any value. It can be a string, date, boolean, array, object, etc.
This container must have a map method to be considered a functor.
Here’s a basic example

const Identity = (x) => ({
  value: x,
  map: (fn) => Identity(fn(x))
});

This function, Identity, takes a value and returns a functor.

const name = Identity('Bobo');
console.log(name);

Logging name shows us a functor of this shape

{ value: 'Bobo', map: [Function: map] }

How would you change this functor’s inner value, ‘Bobo’?
This is JavaScript, so we could mutate it.

const name = Identity('Bobo');
name.value = name.value.toUpperCase();
console.log(name);

But we know FP actively discourages this practice, so what if we used map instead?

const name = Identity('Bobo');
const newName = name
  .map(value => value.toUpperCase());
console.log({ name, newName });

This way is immutable, easier to read, and composes better.
And when you need to extract it, just access its .value property.

Summary #

• Functors are containers you can map over.
• Functors can hold any value
• map is a composable, immutable way to change a functor’s value.

  Who Cares? #

  Lenses use functors. We’ll be diving into them very soon.