Functional Programming Patterns With RamdaJS!/

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Longhand Lenses

Longhand Lenses

Ramda's lensProp and lensIndex are convenience functions. Understanding the longhand version primes us to dive even deeper. (3 min. read)

We'll cover the following...

lensProp and lensIndex are actually built on top of the lower-level lens function.

This

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Javascript (babel-node)
import { lensProp, view } from 'ramda';
const name = lensProp('name');
const result = view(name, { name: 'Bobo' });
console.log({ result });

Is a convenient shorthand for this

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Javascript (babel-node)
import { assoc, lens, prop, view } from 'ramda';
const name = lens(prop('name'), assoc('name'));
const result = view(name, { name: 'Bobo' });
console.log({ result });

See the difference?

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// shorthand
lensProp('name');
// longhand
lens(prop('name'), assoc('name'));

Why prop() and assoc()?

Ramda’s prop function lets you get object properties.

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Javascript (babel-node)
import { prop } from 'ramda';
const person = { name: 'Bobo' };
const name = prop('name', person);
console.log({ name });

And assoc lets you set them without mutating the original object.

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Javascript (babel-node)
import { assoc } from 'ramda';
const person = { name: 'Bobo' };
const personWithAge = assoc('age', 25, person);
console.log({ person, personWithAge });

And since lenses need a getter and a setter to perform their duties, prop and assoc make the perfect combination to immutably handle and change lenses.

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Javascript (babel-node)
import { assoc, lens, prop, set, view } from 'ramda';
const data = { name: 'Bobo' };
const name = lens(prop('name'), assoc('name'));
const original = view(name, data);
const newObj = set(name, 'Another Name', data);
console.log({
  original,
  newObj
});