Thinking in Ramda: Lenses

Thinking in Ramda: Lenses

This post is Part 8 of a series about functional programming called Thinking in Ramda.

In Part 6 and Part 7, we learned how to read, update, and transform object properties and array elements in a declarative, immutable way.

Ramda provides a more general tool for performing these operations, the lens.

What is a Lens?

A lens combines a “getter” function and a “setter” function into a single unit. Ramda provides a set of functions for working with lenses.

We can think of a lens as something that focuses on a specific part of a larger data structure.

How Do I Create a Lens?

The most generic way to create a lens in Ramda is with the lens function. lens takes a getter function and a setter function and returns the new lens.

Creating a Lens

const person = {
  name: 'Randy',
  socialMedia: {
    github: 'randycoulman',
    twitter: '@randycoulman'
  }
}
const nameLens = lens(prop('name'), assoc('name'))
const twitterLens = lens(
  path(['socialMedia', 'twitter']),
  assocPath(['socialMedia', 'twitter'])
)

Here we’re using prop and path as our getter functions and assoc and assocPath as our setter functions.

Note that we had to duplicate the property and path arguments to these functions. Fortunately, Ramda provides nice shortcuts for the most common uses of lenses: lensProp, lensPath, and lensIndex.

• lensProp creates a lens that focuses on a property of an object.
• lensPath creates a lens that focuses on a nested property of an object.
• lensIndex creates a lens that focuses on an element of an array.

We could rewrite our lenses above with lensProp and lensPath:

Using lensProp and lensPath

const nameLens = lensProp('name')
const twitterLens = lensPath(['socialMedia', 'twitter'])

That’s a lot simpler and gets rid of the duplication. In practice, I find that I almost never need to use the generic lens function.

What Can I Do With It?

OK, great, we’ve created some lenses. What can we do with them?

Ramda provides three functions for working with lenses.

• view reads the value of the lens.
• set updates the value of the lens.
• over applies a transformation function to the lens.

Using lenses

view(nameLens, person) // => 'Randy'
set(twitterLens, '@randy', person)
// => {
//   name: 'Randy',
//   socialMedia: {
//     github: 'randycoulman',
//     twitter: '@randy'
//   }
// }
over(nameLens, toUpper, person)
// => {
//   name: 'RANDY',
//   socialMedia: {
//     github: 'randycoulman',
//     twitter: '@randycoulman'
//   }
// }

Notice that set and over return the entire object with the lens’ focused property modified as specified.

Conclusion

Lenses can be handy if we have a somewhat complex data structure that we want to abstract away from calling code. Rather than exposing the structure or providing a getter, setter, and transformer for every accessible property, we can instead expose lenses.

Client code can then work with our data structure using view, set, and over without being coupled to the exact shape of the structure.

Next

We’ve now learned about a lot of what Ramda provides; certainly enough to do most of what we need to do in our programs. Wrap-up reviews the series and mentions some other topics we might want to explore on our own.