Algebraic lenses


Dec 18, 2019

This is a blog post about optics, if you're at all interested in optics I suggest you go check out my book: Optics By Example. It covers everything you need to go from a beginner to master in all things optics! Check it out and tell your friends, now onwards to the post you're here for.

In this post we're going to dig into an exciting new type of optics, the theory of which is described in this abstract by Mario Román, Bryce Clarke, Derek Elkins, Jeremy Gibbons, Bartosz Milewski, Fosco Loregian, and Emily Pillmore. Thanks go out to these awesome folk for researching optics at a high level! The more that we realize the Category Theory representations of optics the more we can convince ourselves that they're a true and beautiful abstraction rather than just a useful tool we stumbled across.

I'm not really a "Mathy" sort of guy, I did very little formal math in university, and while I've become comfortable in some of the absolute basics of Category Theory through my travels in Haskell, I certainly wouldn't consider myself well-versed. I AM however well versed in the practical uses of optics, and so of course I need to keep myself up to speed on new developments, so when this abstract became available I set to work trying to understand it!

Most of the symbols and Category Theory went straight over my head, but I managed to pick out a few bits and pieces that we'll look at today. I'll be translating what little I understand into a language which I DO understand: Haskell!

If the above wasn't enough of a disclaimer I'll repeat: I don't really understand most of the math behind this stuff, so it's very possible I've made a few (or a lot) of errors though to be honest I think the result I've come to is interesting on its own, even if not a perfect representation of the ideas in the abstract. Please correct me if you know better :)

There are several new types of optics presented in the paper, we'll start by looking at one of them in particular, but will set the groundwork for the others which I'll hopefully get to in future posts. Today we'll be looking at "Algebraic lenses"!

Translating from Math

We'll start by taking a look at the formal characterization of algebraic lenses presented in the abstract. By the characterization of an optic I mean a set of values which completely describe the behaviour of that optic. For instance a Lens s t a b is characterized by a getter and a setter: (s -> a, s -> b -> t) and an Iso s t a b is characterized by its to and from functions: (s -> a, b -> t).

The paper presents the characterization of an algebraic lens like this: (my apologies for lack of proper LaTeX on my blog 😬)

My blog has kind of butchered the formatting, so feel free to check it out in the abstract instead.

I'm not hip to all these crazy symbols, but as best as I can tell, we can translate it roughly like this:

If you squint a bit, this looks really close to the characterization of a standard lens, the only difference being that instead of a single s we have some container f filled with them. The type of container further specifies what type of algebraic lens we're dealing with. For instance, the paper calls it a List Lens if f is chosen to be a list [], but we can really define optics for nearly any choice of f, though Traversable and Foldable types are a safe bet to start.

So, what can we actually do with this characterization? Well for starters it implies we can pass it more than one s at once, which is already different than a normal lens, but we can also use all of those s's alongside the result of the continuation (i.e. b) to choose our return value t. That probably sounds pretty overly generalized, and that's because it is! We're dealing with a mathematical definition, so it's intentionally as general as possible.

To put it into slightly more concrete terms, an Algebraic lens allows us to run some aggregation over a collection of substates of our input, then use the result of the aggregation to pick some result to return.

The example given in the paper (which we'll implement soon) uses an algebraic lens to do classification of flower measurements into particular species. It uses the "projection" function from the characterization (e.g. s -> a) to select the measurements from a Flower, and the "selection" function (f s -> b -> t) to take a list of Flowers, and a reference set of measurements, to classify those measurements into a species, returning a flower with the selected measurements and species.

We'll learn more about that as we implement it!

First guesses at an implementation

In the abstract we're given the prose for what the provided examples are intended to do, unfortunately we're only given a few very small code snippets without any source code or even type-signatures to help us out, so I'll mostly be guessing my way through this. As far as I can tell the paper is more concerned with proving the math first, since an implementation must exist if the math works out right? Let's see if we can take on the role of applied mathematician and get some code we can actually run 😃. I'll need to take a few creative liberties to get everything wired together.

Here are the examples given in the abstract:

-- Assume 'iris' is a data-set (e.g. list) of flower objects
>>> (iris !! 1) ^. measurements
(4.9 , 3.0 , 1.4 , 0.2)

>>> iris ?. measurements ( Measurements 4.8 3.1 1.5 0.1)
Iris Setosa (4.8 , 3.1 , 1.5 , 0.1)

>>> iris >- measurements . aggregateWith mean
Iris Versicolor (5.8, 3.0, 3.7, 1.1)

We're not provided with the implementation of ?., >-, Measurements, measurements, OR aggregateWith, nor do we have the data-set that builds up iris... Looks like we've got our work cut out for us here 😓

To start I'll make some assumptions to build up a dummy data-set of flowers to experiment with:

-- Some flower species
data Species = Setosa | Versicolor | Virginica
  deriving Show

-- Our measurements will just be a list of floats for now
data Measurements = Measurements {getMeasurements :: [Float]}
  deriving Show

-- A flower consists of a species and some measurements
data Flower = Flower { flowerSpecies :: Species
                     , flowerMeasurements :: Measurements}
  deriving Show

versicolor :: Flower
versicolor = Flower Versicolor (Measurements [2, 3, 4, 2])

setosa :: Flower
setosa = Flower Setosa (Measurements [5, 4, 3, 2.5])

flowers :: [Flower]
flowers = [versicolor, setosa]

That gives us something to fool around with at least, even if it's not exactly like the data-set used in the paper.

Now for the fun part, we need to figure out how we can somehow cram a classification algorithm into an optic! They loosely describe measurements as a list-lens which "encapsulates some learning algorithm which classifies measurements into a species", but the concrete programmatic definition of that will be up to my best judgement I suppose.

I'll be implementing these as Profunctor optics, they tend to work out a bit cleaner than the Van-Laarhoven approach, especially when working with "Grate-Like" optics which is where an algebraic-lens belongs. The sheer amount of guessing and filling in blanks I had to do means I stared at this for a good long while before I figured out a way to make this work. One of the tough parts is that the examples show the optic work for a single flower (like the (iris !! 1) ^. measurements example), but it somehow also runs a classifier over a list of flowers as in the iris ?. measurements ( Measurements 4.8 3.1 1.5 0.1) example. We need to find the minimal profunctor constraints which allow us to lift the characterization into an actual runnable optic!

I've been on a bit of a Corepresentable kick lately and it seemed like a good enough place to start. It also has the benefit of being easily translated into Van-Laarhoven optics if needed.

Here was my first crack at it:

import Data.Profunctor
import Data.Profunctor.Sieve
import Data.Profunctor.Rep
import Data.Foldable

type Optic p s t a b = p a b -> p s t

listLens :: forall p f s t a b
         . (Corepresentable p, Corep p ~ f, Foldable f)
         => (s -> a)
         -> ([s] -> b -> t)
         -> Optic p s t a b
listLens project flatten p = cotabulate run
  where
    run :: f s -> t
    run fs = flatten (toList fs) (cosieve p . fmap project $ fs)

This is a LOT to take in, let's address it in pieces.

First things first, a profunctor optic is simply a morphism over a profunctor, something like: p a b -> p s t.

Next, the Corepresentable constraint:

Corepresentable has Cosieve as a superclass, and so provides us with both of the following methods:

Cosieve p f       => cosieve    :: p a b -> f a -> b
Corepresentable p => cotabulate :: (Corep p d -> c) -> p d c

These two functions together allow us to round-trip our profunctor from p a b into some f a -> b and then back! In fact, this is the essence of what Corepresentable means, we can "represent" the profunctor as a function from a value in some context f to the result.

Profunctors in general can't simply be applied like functions can, these two functions allow us to reflect an opaque and mysterious generic profunctor into a real function that we can actually run! In our implementation we fmap project over the f s's to get f a, then run that through the provided continuation: f a -> b which we obtain by running cosieve on the profunctor argument, then we can flatten the whole thing using the user-provided classification-style function.

Don't worry if this doesn't make a ton of sense on its own, it took me a while to figure out. At the end of the day, we have a helper which allows us to write a list-lens which composes with any Corepresentable profunctor. This allows us to write our measurements classifier, but we'll need a few helper functions first.

First we'll write a helper to compute the Euclidean distance between two flowers' measurements (e.g. we'll compute the difference between each set of measurements, then sum the difference):

measurementDistance :: Measurements -> Measurements -> Float
measurementDistance (Measurements xs) (Measurements ys) =
    sqrt . sum $ zipWith diff xs ys
  where
    diff a b = (a - b) ** 2

This will tell us how similar two measurements are, the lower the result, the more similar they are.

Next we'll write a function which when given a reference set of flowers will detect the flower which is most similar to a given set of measurements. It will then build a flower by combining the closest species and the given measurements.

classify :: [Flower] -> Measurements -> Maybe Flower
classify flowers m
  | null flowers = Nothing
  | otherwise =
  let Flower species _ = minimumBy
                          (comparing (measurementDistance m . flowerMeasurements))
                          flowers
   in Just $ Flower species m

This function returns its result in Maybe, since we can't classify anything if we're given an empty data-set.

Now we have our pieces, we can build the measurements list-lens!

measurements :: (Corepresentable p, Corep p ~ f, Foldable f) 
             => Optic p Flower (Maybe Flower) Measurements Measurements
measurements = listLens flowerMeasurements classify

We specify that the container type used in the Corepresentable instance must be foldable so that we can convert it into a list to do our classification.

Okay! Now we have enough to try some things out! The first example given in the abstract is:

>>> (iris !! 1) ^. measurements

Which we'll translate into:

>>> (flowers !! 1) ^. measurements

But unfortunately we get an error:

No instance for (Corepresentable
                      (Data.Profunctor.Types.Forget Measurements))
    arising from a use of ‘measurements’

By the way, all the examples in this post are implemented using my highly experimental Haskell profunctor optics implementation proton. Feel free to play with it, but don't use it in anything important.

Hrmm, looks like (^.) uses Forget for its profunctor and it doesn't have a Corepresentable instance! We'll come back to that soon, let's see if we can get anything else working first.

The next example is:

iris ?. measurements (Measurements 4.8 3.1 1.5 0.1)

I'll admit I don't understand how this example could possibly work, optics necessarily have the type p a b -> p s t, so how are they passing a Measurements object directly into the optic? Perhaps it has some other signature, but we know that's not true from the previous example which uses it directly as a lens! Hrmm, I strongly suspect that this is a typo, mistake, or most likely this example is actually just short-hand pseudocode of what an implementation might look like and we're discovering a few rough edges. Perhaps the writers of the paper thought of something sneaky that I missed. Without the source code for the example we'll never know, but since I can't see how this version could work, let's modify it into something close which I can figure out.

It appears as though (?.) is an action which runs the optic. Actions in profunctor optics tend to specialize the optic to a specific profunctor, then pass the other arguments through it using that profunctor as a carrier. We know we need a profunctor that's Corepresentable, and the simplest instance for that is definitely Costar! Here's what it looks like:

newtype Costar f a b = Costar (f a -> b)

Costar is basically the "free" Corepresentable, it's just a new-type wrapper around a function from values in a container to a result. You might also know it by the name Cokleisli, they're the same type, but Costar is the one we typically use with Profunctors.

If we swap the arguments in the example around a bit, we can write an action which runs the optic using Costar like this:

(?.) :: (Foldable f) => f s -> Optic (Costar f) s t a b -> b -> t
(?.) xs f a = (runCostar $ f (Costar (const a))) xs

The example seems to use a static value for the comparison, so I use const to embed that value into the Costar profunctor, then run that through the provided profunctor morphism (i.e. optic).

This lets us write the example like this instead:

>>> flowers ?. measurements $ Measurements [5, 4, 3, 1]
Just (Flower Setosa (Measurements [5.0,4.0,3.0,1.0]))

Which is really close to the original, we just added a $ to make it work.

>>> iris ?. measurements (Measurements 4.8 3.1 1.5 0.1)

Let's see if this is actually working properly. We're passing a "fixed" measurement in as our aggregation function, meaning we're comparing every flower in our list to these specific measurements and will find the flower that's "closest". We then build a flower using the species closest to those measurements alongside the provided measurements. To test that this is actually working properly, let's try again with measurements that match our versicolor flower more closely:

>>> setosa
Flower Setosa (Measurements [5.0,4.0,3.0,2.5])
>>> versicolor
Flower Versicolor (Measurements [2.0,3.0,4.0,2.0])

-- By choosing measurements close to the `versicolor` in our data-set
-- we expect the measurements to be classified as Versicolor
>>> flowers ?. measurements $ Measurements [1.9, 3.2, 3.8, 2]
Just (Flower Versicolor (Measurements [1.9,3.2,3.8,2.0]))

We can see that indeed it now switches the classification to Versicolor! It appears to be working!

Even though this version looks a lot like the example in the abstract, it doesn't quite feel in line with style of existing optics libraries so I'll flip the arguments around a bit further: (I'll rename the combinator to ?- to avoid confusion with the original)

(?-) :: (Foldable f) => f s -> Optic (Costar f) s t a b -> b -> t
(?-) xs f a = (runCostar $ f (Costar (const a))) xs

The behaviour is the same, but flipping the arguments allows it to fit the "feel" of other optics combinators better (IMHO), we use it like this:

>>> flowers & measurements ?- Measurements [5, 4, 3, 1]
Just (Flower Setosa (Measurements [5.0,4.0,3.0,2.5]))

We pass in the data-set, and "assign" our comparison value to be the single Measurement we're considering.

Making measurements a proper lens

Before moving on any further, let's see if we can fix up measurements so we can use (^.) on a single flower like the first example does. Remember, (^.) uses Forget as the concrete profunctor instead of Costar, so whatever we do, it has to have a valid instance for the Forget profunctor which looks like this:

newtype Forget r a b = Forget (a -> r)

As an exercise for the reader, try to implement Corepresentable for Forget (or even Cosieve) and you'll see it's not possible, so we'll need to find a new tactic. Perhaps there's some other weaker abstraction we can invent which works for our purposes.

The end-goal here is to create an optic out of the characterization of an algebraic lens, so what if we just encode that exact idea into a typeclass? It's so simple it just might work! Probably should have started here, sticking with the optics metaphor: hindsight is 20/20.

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}

class Profunctor p => Algebraic f p | p -> f where
  algebraic :: (s -> a) -> (f s -> b -> t) -> p a b -> p s t

type AlgebraicLens f s t a b = forall p. Algebraic f p => p a b -> p s t
type AlgebraicLens' f s a = AlgebraicLens f s s a a

By keeping f general we can write list-lenses or any other type of algebraic lens. I added a functional dependency here to help with type-inference. This class represents exactly what we want an algebraic lens to do. It's entirely possible there's a more general profunctor class which has equivalent power, if I'm missing one please let me know!

Now that we have a typeclass we'll implement an instance for Costar so we can still use our (?.) and (?-) actions:

instance Functor f => Algebraic f (Costar f) where
  algebraic project flatten p = cotabulate run
    where
      run fs = flatten fs (cosieve (lmap project p) fs)

Technically this implementation works on any Corepresentable profunctor, not just Costar, so we could re-use this for a few other profunctors too!

Did we make any progress? We need to see if we can implement an instance of Algebraic for Forget, if we can manage that, then we can use view over our measurements optic just like the example does.

instance Algebraic Proxy (Forget r) where
  algebraic project _flatten (Forget f) = Forget (f . project)

Well that was pretty painless! This allows us to do what our Corepresentable requirement didn't.

I've arbitrarily chosen Proxy as the carrier type because it's empty and doesn't contain any values. The carrier itself isn't every used, but I needed to pick something and this seemed like a good a choice as any. Perhaps a higher-rank void type would be more appropriate, but we'll cross that bridge when we have to.

With that, we just need to re-implement our measurements optic using Algebraic:

measurements :: Foldable f 
             => AlgebraicLens f Flower (Maybe Flower) Measurements Measurements
measurements = algebraic flowerMeasurements classify

The name measurements is a bit of a misnomer, it does classification and selection, which is quite a bit more than just selecting the measurements! Perhaps a better name would be measurementsClassifier or something. I'll stick to the name used in the abstract for now.

Now we can view through our measurements optic directly! This fulfills the first example perfectly!

>>> (flowers !! 1) ^. measurements
Measurements [5.0,4.0,3.0,2.5]

Awesome! All that's left to have a proper lens is to be able to set as well. In profunctor optics, the set and modify actions simply use the (->) profunctor, so we'll need an instance for that. Technically (->) is isomorphic to Costar Identity, so we could use the exact same implementation we used for our Costar instance but there's a simpler implementation if we specialize. It turns out that Identity makes a good carrier type since it holds exactly one argument.

instance Algebraic Identity (->) where
  algebraic project flatten p = run
    where
      run s = flatten (Identity s) (p . project $ s)

Now we can modify or set measurements through our algebraic lens too:

>>> versicolor & measurements .~ Measurements [9, 8, 7, 6]
Flower Versicolor Measurements [9.0,8.0,7.0,6.0]

Since we can get and set, our algebraic lens is indeed a full-blown lens! This is surprisingly interesting interesting since we didn't make any use of Strong which is how most lenses are implemented, and in fact Costar isn't a Strong profunctor!

You might be curious how this actually works at all, behind the scenes the algebraic lens receives the new measurements as though it were the result of an aggregation, then uses those measurements with the Species of the single input flower (which of course hasn't changed), thus appearing to modify the flower's measurements! It's the "long way round" but it behaves exactly the same as a simpler lens would.

Here's one last interesting instance just for fun:

instance Algebraic Proxy Tagged where
  algebraic project flatten (Tagged b) = Tagged (flatten Proxy b)

Tagged is used for the review actions, which means we can try running our algebraic lens as a review:

>>> review measurements (Measurements [1, 2, 3, 4])
Nothing

I suppose that's what we can expect, we're effectively classifying measurements without any data-set, so our classify function 'fails' with it's Nothing value. It's very cool to know that we can (in general) run algebraic lenses in reverse like this!

Running custom aggregations

We have one more example left to look at:

>>> iris >- measurements . aggregateWith mean
Iris Versicolor (5.8 , 3.0 , 3.7 , 1.1)

In this example they compute the mean of each of the respective measurements across their whole data-set, then find the species of flower which best represents the "average flower" of the data-set.

In order to implement this we'd need to implement aggregateWith, which is a Kaleidoscope, and that's a whole different type of optic, so we'll continue this thread in a subsequent post but we can get most of the way there with what we've got already if we write a slightly smarter aggregation function.

To spoil kaleidoscopes just a little, aggregateWith allows running aggregations over lists of associated measurements. That is to say that it groups up each set of related measurements across all of the flowers, then takes the mean of each set of measurements (i.e. the mean all the first measurements, the mean of all the second measurements, etc.). If we don't mind the inconvenience, we can implement this exact same example by baking that logic into an aggregation function and thus avoid the need for a Kaleidoscope until the next blog post 😉

Right now our measurements function focuses the Measurements of a set of flowers, the only action we have right now ignores the data-set entirely and accepts a specific measurement as input, but we can easily modify it to take a custom aggregation function:

infixr 4 >-
(>-) :: Optic (Costar f) s t a b -> (f a -> b) -> f s -> t
(>-) opt aggregator xs = (runCostar $ opt (Costar aggregator)) xs

My version of the combinator re-arranges the arguments a bit (again) to make it read a bit more like %~ and friends. It takes an algebraic lens on the left and an aggregation function on the right. It'll run the custom aggregation and hand off the result to the algebraic lens.

This lets us write the above example like this:

>>> flowers & measurements >- avgMeasurement

But we'll need to define the avgMeasurement function first. It needs to take a Foldable container filled with measurements and compute the average value for each of the four measurements. If we're clever about it transpose can re-group all the measurements exactly how we want!

mean :: Fractional a => [a] -> a
mean [] =  0
mean xs = sum xs / fromIntegral (length xs)

avgMeasurement :: Foldable f => f Measurements -> Measurements
avgMeasurement ms = Measurements (mean <$> groupedMeasurements)
  where
    groupedMeasurements :: [[Float]]
    groupedMeasurements = transpose (getMeasurements <$> toList ms)

We manually pair all the associated elements, then construct a new set of measurements where each value is the average of that measurement across all the inputs.

Now we can finally find out what species the average flower is closest to!

>>> flowers & measurements >- avgMeasurement
Just (Flower Versicolor (Measurements [3.5,3.5,3.5,2.25]))

Looks like it's closest to the Versicolor species!

We can substitute avgMeasurement for any sort of aggregation function of type [Measurements] -> Measurements and this expression will run it on our data-set and return the species which is closest to those measurements. Pretty cool stuff!

Custom container types

We've stuck with a list so far since it's easy to think about, but algebraic lenses work over any container type so long as you can implement the aggregation functions you want on them. In this case we only require Foldable for our classifier, so we can hot-swap our list for a Map without any changes!

>>> M.fromList [(1.2, setosa), (0.6, versicolor)] 
      & measurements >- avgMeasurement
Just (Flower Versicolor (Measurements [3.5,3.5,3.5,2.25]))

This gives us the same answer of course since the foldable instance simply ignores the keys, but the container type is carried through any composition of algebraic lenses! That means our aggregation function now has type: Map Float Measurements -> Measurements, see how it still projects from Flower into Measurements even inside the map? Let's say we want to run a scaling factor over each of our measurements as part of aggregating them, we can bake it into the aggregation like this:

scaleBy :: Float -> Measurements -> Measurements
scaleBy w (Measurements m) = Measurements (fmap (*w) m)

>>> M.fromList [(1.2, setosa), (0.6, versicolor)] 
      & measurements >- avgMeasurement . fmap (uncurry scaleBy) . M.toList
Just (Flower Versicolor (Measurements [3.5,3.5,3.5,2.25]))

Running the aggregation with these scaling factors changed our result and shows us what the average flower would be if we scaled each flower by the amount provided in the input map.

This isn't a perfect example of what other containers could be used for, but I'm sure folks will be dreaming up clever ideas in no time!

Other aggregation types

Just as we can customize the container type and the aggregation function we pass in we can also build algebraic lenses from any manor of custom "classification" we want to perform. Let's write a new list-lens which partitions the input values based on the result of the aggregation. In essence classifying each point in our data-set as above or below the result of the aggregation.

partitioned :: forall f a. (Ord a, Foldable f) => AlgebraicLens f a ([a], [a]) a a
partitioned = algebraic id splitter
  where
    splitter :: f a -> a -> ([a], [a])
    splitter xs ref
      = (filter (< ref) (toList xs), filter (>= ref) (toList xs))

It's completely fine for our s and t to be completely disparate types like this.

This allows us to split a container of values into those which are less than the aggregation, or greater/equal to it. We can use it with a static value like this:

>>> [1..10] & partitioned ?- 5
([1,2,3,4],[5,6,7,8,9,10])

Or we can provide our own aggregation function; let's say we want to split it into values which are less than or greater than the mean of the data-set. We'll use our modified version of >- for this:

>>> mean [3, -2, 4, 1, 1.3]
1.46

>>> [3, -2, 4, 1, 1.3] & partitioned >- mean
([-2.0,1.0,1.3], [3.0,4.0])

Here's a list-lens which generalizes the idea behind minimumBy, maximumBy, etc. into an optic. We allow the user to provide a selection function for indicating the element they want, then the optic itself will pluck the appropriate element out of the collection.

-- Run an aggregation on the first elements of the tuples
-- Select the second tuple element which is paired with the value
-- equal to the aggregation result.
onFirst :: (Foldable f, Eq a) => AlgebraicLens f (a, b) (Maybe b) a a
onFirst = algebraic fst picker
  where
    picker xs a = lookup a $ toList xs

-- Get the character paired with the smallest number
>>> [(3, 'a'), (10, 'b'), (2, 'c')] & onFirst >- minimum
Just 'c'

-- Get the character paired with the largest number
>>> [(3, 'a'), (10, 'b'), (2, 'c')] & onFirst >- maximum
Just 'b'

-- Get the character paired with the first even number
>>> [(3, 'a'), (10, 'b'), (2, 'c')] & onFirst >- head . filter even
Just 'b'

If our structure is indexable we can do this much more generally and build a library of composable optics which dig deeply into structures and perform selection aggregations over anything we want. It may take a little work to figure out the cleanest set of combinators, but here's a simplified example of just how easy it is to start messing around with:

-- Pick some substate or projection from each value,
-- The aggregation selects the index of one of these projections and returns it
-- Return the 'original state' that lives at the chosen index
selectingOn :: (s -> a) -> AlgebraicLens [] s (Maybe s) a (Maybe Int)
selectingOn project = algebraic project picker
  where
    picker xs i = (xs !!) <$> i

-- Use the `Eq` class and return the index of the aggregation result in the original list
indexOf :: Eq s => AlgebraicLens [] s (Maybe Int) s s
indexOf = algebraic id (flip elemIndex)

-- Project each string into its length, 
-- then select the index of the string with length 11,
-- Then find and return the element at that index
>>> ["banana", "pomegranate", "watermelon"] 
      & selectingOn length . indexOf ?- 11
Just "pomegranate"

-- We can can still use a custom aggregation function,
-- This gets the string of the shortest length. 
-- Note we didn't need to change our chain of optics at all!
>>> ["banana", "pomegranate", "watermelon"] 
      & selectingOn length . indexOf >- minimum
Just "banana"

I'm sure you can already imagine all sorts of different applications for this sort of thing. It may seem more awkward than the straight-forward Haskell way of doing these things, but it's a brand new idea, it'll take time for the ecosystem to grow around it and for us to figure out the "best way".

Summarizing Algebraic Lenses

The examples we've looked at here are just a few of many possible ways we can use Algebraic lenses! Remember that we can generalize the f container into almost anything! We can use Maps, Lists, we could even use a function as the container! In addition we can use any sort of function in place of the classifier, there's no requirement that it has to return the same type as its input. Algebraic lenses allow us to compose lenses which focus on a specific portion of state, run a comparison or aggregation there (e.g. get the maximum or minimum element from the collection based on some property), then zoom back out and select the larger element which contains the minimum/maximum substate!

This means we can embed operations like minimumBy, findBy, elemIndex and friends as composable optics! There are many other interesting aggregations to be found in statistics, linear algebra, and normal day-to-day tasks. I'm very excited to see where this ends up going, there are a ton of possibilities which I haven't begun to think about yet.

Algebraic lenses also tend to compose better with Grate-like optics than traditional Strong Profunctor based lenses, they work well with getters and folds, and can be used with setters or traversals for setting or traversing (but not aggregating). They play a role in the ecosystem and are just one puzzle piece in the world of optics we're still discovering.

Thanks for reading! We'll dig into Kaleidoscopes soon, so stay tuned!

Updates & Edits

After releasing this some authors of the paper pointed out some helpful notes (thanks Bryce and Mario!)

It turns out that we can further generalize the Algebraic class further while maintaining its strength.

The suggested model for this is to specify profunctors which are Strong with respect to Monoids. To understand the meaning of this, let's take a look at the original Strong typeclass:

class Profunctor p => Strong p where
  first' :: p a b -> p (a, c) (b, c)
  second' :: p a b -> p (c, a) (c, b)

The idea is that a Strong profunctor can allow additional values to be passed through freely. We can restrict this idea slightly by requiring the value which we're passing through to be a Monoid:

class Profunctor p => MStrong p where
  mfirst' ::  Monoid m => p a b -> p (a, m) (b, m)
  mfirst' = dimap swap swap . msecond'
  msecond' ::  Monoid m => p a b -> p (m, a) (m, b)
  msecond' = dimap swap swap . mfirst'

  {-# MINIMAL mfirst' | msecond' #-}

This gives us more power when writing instances, we can "summon" a c from nowhere via mempty if needed, but can also combine multiple c's together via mappend if needed. Let's write all the needed instances of our new class:

instance MStrong (Forget r) where
  msecond' = second'

instance MStrong (->) where
  msecond' = second'

instance MStrong Tagged where
  msecond' (Tagged b) = Tagged (mempty, b)

instance Traversable f => MStrong (Costar f) where
  msecond' (Costar f) = Costar (go f)
    where
      go f fma = f <$> sequenceA fma

The first two instances simply rely on Strong, all Strong profunctors are trivially MStrong in this manner. To put it differently, MStrong is superclass of Strong (although this isn't reflected in libraries at the moment). I won't bother writing out all the other trivial instances, just know that all Strong profunctors have an instance.

Tagged and Costar are NOT Strong profunctors, but by taking advantage of the Monoid we can come up with suitable instances here! We use mempty to pull a value from thin air for Tagged, and Costar uses the Applicative instance of Monoid m => (m, a) to sequence its input into the right shape.

Indeed, this appears to be a more general construction, but at first glance it seems to be orthogonal; how can we regain our algebraic function using only the MStrong constraint?

import Control.Arrow ((&&&))

algebraic :: forall m p s t a b
           . (Monoid m,  MStrong p) 
           => (s -> m) 
           -> (s -> a) 
           -> (m -> b -> t) 
           -> Optic p s t a b
algebraic inject project flatten p
  = dimap (inject &&& id)  (uncurry flatten) $  strengthened
  where
    strengthened :: p (m, s) (m, b)
    strengthened = msecond' (lmap project p)

This is perhaps not the most elegant definition, but it matches the type without doing anything outright stupid, so I suppose it will do (type-hole driven development FTW)!

We require from the user a function which injects the state into a Monoid, then use MStrong to project that monoid through the profunctor's action. On the other side we use the result of the computation alongside the Monoidal summary of the input value(s) to compute the final aggregation.

We can recover our standard list-lens operations by simply choosing [s] to be our Monoid.

listLens :: MStrong p => (s -> a) -> ([s] -> b -> t) -> Optic p s t a b
listLens = algebraic pure

In fact, we can easily generalize over any Alternative container. Alternative's provide a Monoid over their Applicative structure, and we can use the Alt newtype wrapper from Data.Monoid to use an Alternative structure as a Monoid.

altLens :: (Alternative f, MStrong p) 
        => (s -> a) -> (f s -> b -> t) -> Optic p s t a b
altLens project flatten = malgebraic (Alt . pure)  project (flatten . getAlt)

So now we've got a fully general algebraic lens which allows aggregating over any monoidal projection of input, including helpers for doing this over Alternative structures, or lists in particular! This gives us a significant amount of flexibility and power.

I won't waste everyone's time by testing these new operations here, take heart that they do indeed work the same as the original definitions provided above.

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!