Virtual Record Fields Using Lenses

Nov 26, 2020

The following blog post is a short excerpt from by book on optics: "Optics By Example". If you learn something from the post you'll likely enjoy the rest of the book too!

Optics by Example provides a comprehensive example-driven guide to manipulating data with optics, covering Lenses, Traversals, Prisms, Isos, as well as many design patterns and extension libraries.

As thanks for checking out the blog you can grab a copy on sale with this link until the end of 2020.

Lenses are commonly used for getting and setting fields on records, but they're actually much more adaptable than that! This post dives into the idea of "virtual fields" using optics.

Virtual fields can can provide many benefits:

Let's dive in!

What is a virtual field

To establish terms, I'll define a virtual field as any piece of data we might be interested in which doesn't exist as a concrete field in a given record definition. In languages like Java or Python these are sometimes called "computed properties" or "managed attributes".

Oftentimes we'll use virtual fields to present data from concrete fields in a more convenient way, or to maintain certain invariants on the concrete fields. Sometimes virtual fields combine several concrete fields together, other times they're used to avoid introducing breaking changes when refactoring the structure of the record.

No matter what you use them for, at the end of the day they're really just normal lenses! Let's look at a concrete example.

Writing a virtual field

Let's look at the following type:

data Temperature =
    Temperature { _location :: String
                , _celsius  :: Float
    deriving (Show)
makeLenses ''Temperature

This generates the field lenses:

location :: Lens' Temperature String
celsius  :: Lens' Temperature Float

Which we can use to get, set, or modify the temperature in Celsius like so:

>>> let temp = Temperature "Berlin" 7.0
>>> view celsius temp

>>> set celsius 13.5 temp
Temperature {_location = "Berlin", _celsius = 13.5}

-- Bump the temperature up by 10 degrees Celsius
>>> over celsius (+10) temp
Temperature {_location = "Berlin", _celsius = 17.0}

Now what about our American colleagues who'd prefer Fahrenheit? It's easy enough to write a function which converts Celsius to Fahrenheit and call that on the result of celsius, but we'd still need to set new temperatures in Celsius! How can we avoid this dissonance between units?

First we'll define our conversion functions back and forth, nothing too interesting there, if I'm honest I just stole the formulas from wikipedia:

celsiusToFahrenheit :: Float -> Float
celsiusToFahrenheit c = (c * (9/5)) + 32

fahrenheitToCelsius :: Float -> Float
fahrenheitToCelsius f = (f - 32) * (5/9)

Here's one way we could get and set using Fahrenheit:

>>> let temp = Temperature "Berlin" 7.0
-- View temp in Berlin in Fahrenheit 
>>> celsiusToFahrenheit . view celsius temp

-- Set temperature to 56.3 Fahrenheit
>>> set celsius (fahrenheitToCelsius 56.3) temp
Temperature {_location = "Berlin", _celsius = 13.5}

-- Bump the temp up by 18 degrees Fahrenheit
>>> over celsius (fahrenheitToCelsius . (+18) . celsiusToFahrenheit) temp
Temperature {_location = "Berlin", _celsius = 17.0}

The first two aren't too bad, but the over example is getting a bit clunky and error prone! It's hard to see what's going on, and since every type is Float it'd be easy to forget or misplace one of our conversions.

If we instead encode the Fahrenheit version of the temperature as a virtual field using optics we gain improved usability, cleaner code, and avoid a lot of possible mistakes.

Let's see what that looks like.

We can write a fahrenheit lens in terms of the existing celsius lens! We embed the back-and-forth conversions into the lens itself.

fahrenheit :: Lens' Temperature Float
fahrenheit = lens getter setter
    getter = celsiusToFahrenheit . view celsius
    setter temp f = set celsius (fahrenheitToCelsius f) temp

Look how much it cleans up the call sites:

>>> let temp = Temperature "Berlin" 7.0
>>> view fahrenheit temp

>>> set fahrenheit 56.3 temp
Temperature {_location = "Berlin", _celsius = 13.5}

>>> over fahrenheit (+18) temp
Temperature {_location = "Berlin", _celsius = 17.0}

Much nicer, easier to read, and less error prone! Even though our Temperature record doesn't actually have a concrete field for the temperature in Fahrenheit we managed to fake it by using lenses to create a virtual field! If we export a smart constructor for our Temperature type and only export the lenses from our Temperature module then the two field lenses are completely indistinguishable.

Breakage-free refactoring

In addition to providing more functionality in a really clean way, another benefit of using lenses instead of field accessors for interacting with our data is that we gain more freedom when refactoring.

To continue with the Temperature example, let's say as we've developed our wonderful weather app further we've discovered that Kelvin is a much better canonical representation for temperature data. We'd love to swap our _celsius field for a _kelvin field instead and base all our measurements on that.

We'll consider two possible alternate universes, in the first, this post was never written, so we didn't use lenses to access our fields 😱

In the second (the one you're living in) I published this post and of course knew well enough to use lenses as the external interface instead.

A world without lenses

In the sad universe without any lenses we had the following code scattered throughout our app:

updateTempReading :: Temperature -> IO Temperature
updateTempReading temp = do
  newTempInCelsius <- readOutdoorTemp
  return temp{_celsius=newTempInCelsius}

Then we refactored our Temperature object to the following:

data Temperature =
    Temperature { _location :: String
                , _kelvin  :: Float
    deriving (Show)
makeLenses ''Temperature

Now, unfortunately, every file that used record update syntax now fails to compile. This is because the _celsius field we are depending on with our record-update-syntax no longer exists. If we had instead used positional pattern matching the situation would be even worse:

updateTempReading :: Temperature -> IO Temperature
updateTempReading (Temperature location _) = do
  newTempInCelsius <- readOutdoorTemp
  return (Temperature location newTempInCelsius)

In this case the code will still happily compile, but we've switched measurement units this code is now completely incorrect!

The glorious utopian lenses universe

Come with me now to the happy universe. In this world we've decided to use lenses as our interface for interacting with Temperatures, meaning we didn't expose the field accessors and thus disallowed fragile record-update syntax. We used the celsius lens to perform the update instead:

updateTempReading :: Temperature -> IO Temperature
updateTempReading temp = do
  newTempInCelsius <- readTemp
  return $ set celsius newTempInCelsius temp

Now when we refactor, we can export a replacement celsius lens in place of the old generated one, and nobody need be aware of our refactoring!

data Temperature =
    Temperature { _location :: String
                , _kelvin  :: Float
    deriving (Show)
makeLenses ''Temperature

celsius :: Lens' Temperature Float
celsius = lens getter setter
    getter = (subtract 273.15) . view kelvin
    setter temp c = set kelvin (c + 273.15) temp

By adding the replacement lens we avoid breaking any external users of the type! Even our fahrenheit lens was defined in terms of celsius, so it will continue to work perfectly.

This is a simple example, but this principle holds for more complex refactorings too. When adopting this style it's important to avoid exporting the data type constructor or field accessors and instead export a "smart constructor" function and the lenses for each field.

When you're writing more complex virtual fields it's relatively easy to write lenses that don't abide by the lens laws. In practice, this is usually perfectly fine. In many cases it's completely fine to break these laws for the sake of pragmatism (especially in application code). In fact the lens library itself exports many law-breaking optics. The important thing is to think about whether the lenses for your type behave in a way that's intuitive to the caller or not, and whether it maintains any invariants your type may have.


In Optics By Example I include exercises after each section to help readers sharpen their skills. The book has answers too, but for this blog post you're on your own. Give these a try!

Consider this data type for the following exercises:

data User =
  User { _firstName :: String
       , _lastName :: String
       , _username :: String
       , _email :: String
       } deriving (Show)
makeLenses ''User
  1. We've decided we're no longer going to have separate usernames and emails; now the email will be used in place of a username. Your task is to delete the _username field and write a replacement username lens which reads and writes from/to the _email field instead. The change should be unnoticed by those importing the module. Assume we haven't exported the constructor, or any of our field accessors, only the generated lenses

  2. Write a lens for the user's fullName. It should append the first and last names when "getting". When "setting" treat everything up to the first space as the first name, and everything following it as the last name.

It should behave something like this:

>>> let user = User "John" "Cena" ""
>>> view fullName user
"John Cena"
>>> set fullName "Doctor of Thuganomics" user
    { _firstName = "Doctor"
    , _lastName = "of Thuganomics"
    , _email = ""

Hopefully you learned something 🤞! If you did, please consider checking out my book: It teaches the principles of using optics in Haskell and other functional programming languages and takes you all the way from an beginner to wizard in all types of optics! You can get it here. Every sale helps me justify more time writing blog posts like this one and helps me to continue writing educational functional programming content. Cheers!