What Are They?

A brief introduction to functors, and how they'll tie into this course. (4 min. read)

We'll cover the following
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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

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

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

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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.

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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?

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

Without affecting the previous functor, we’ve created a brand new functor with our desired value!

And since map must return the same type, we can chain it similar to array’s map.

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const name = Identity('Bobo');
const newName = name
  .map(value => value.slice(0, 3))
  .map(value => `My name is not ${value}!`)
  .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.