How Ramda Uses Them

Dive into Ramda's map function and how to override it in your functors. (5 min. read)

We'll cover the following

Ramda’s map function is great for arrays.

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import { map, multiply } from 'ramda';
const functor = [1, 2, 3];
const result = map(multiply(2), functor);
console.log({ result });

But did you know it also works for objects?

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import { map, multiply } from 'ramda';
const functor = { x: 1, y: 2, z: 3 };
const result = map(multiply(2), functor);
console.log({ result });

And strings!

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import { concat, map } from 'ramda';
const functor = 'Hello';
const result = map(concat('yay'), functor);
console.log({ result });

They can all be looped over and transformed, so map supports them!

You can even override map by defining a special method.

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import { map, multiply } from 'ramda';
const functor = {
  value: 10,
  'fantasy-land/map': () => 'You have been overridden!'
};
const result = map(multiply(2), functor);
console.log({ result });
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Notice map ignored multiply(2) and went straight to the override function. This is because it looks out for functors with the fantasy-land/map method.

Defining this method means “Give me full control of the mapping function”. Whatever it returns will be used.

Fantasy Land…?

Yep. This is a specification for using functors and their friends in JavaScript. Definitely beyond the scope of this course, but useful background info.

Summary

  • Ramda’s map works with arrays, objects, and strings.
  • Functors with a fantasy-land/map method can override map and return whatever.

Who Cares?

The next topic, lenses, employ this strategy to a great extent. Without map's override capabilities, lenses would need a big refactoring.