20-Dec

Haskell - A layman's deep-dive in monoids

This article will introduce an important and interesting concept in functional programming: Monoids. The focus will be on monoids in Haskell.

7 min read

By Morten Kolstad

December 20, 2020

This article will introduce an important and interesting concept in functional programming: Monoids. The focus will be on monoids in Haskell.

What are monoids?

Monoid is a concept that comes from abstract algebra, where it is defined as : a set equipped with an associative binary operation and an identity element.

A binary operation on a set is a function that takes two arguments from the set and produces another one. Being associative means that the grouping of parentheses does not matter. For a binary operation <>, that means that x <> (y <> z) === (x <> y) <> z.

An identity element is an element of the set where mempty <> x === x and x <> mempty === x. So, it is kind of an "empty" element, that is "ignored" by <>.

Translated into Haskell:

A Set ==> A Type a
binary operation ==> a function <> :: a -> a -> a
identity element ==> mempty :: a

So, implemented in Haskell it could become:

class Monoid a where
    (<>) :: a -> a -> a
    mempty :: a

This was how it was defined earlier, but now this class is split into two: Semigroup and Monoid. A semigroup is just a monoid without the identity element part, a monoid is then a semigroup with an identity. So now it is:

class Semigroup a where
    (<>) :: a -> a -> a

class Semigroup a => Monoid a where
    mempty :: a

(When I refer to a "monoid instance" later in the article, I mean both the semigroup and monoid instance for a type, not just the monoid instance).

Basic Monoids

Let us start with some basic, but common and useful monoids.

Sum and Product

When we want to get the sum of values, we can use Sum. Sum is a monoid for numbers where the binary operation is addition (+) and the identity element is 0.

newtype Sum a = Sum { getSum :: a }

instance Num a => Semigroup (Sum a) where
        Sum a <> Sum b = Sum (a+b)

instance Num a => Monoid (Sum a) where
        mempty = Sum 0

Sum 2 <> Sum 4
> Sum 6

mempty :: Sum Int
> Sum 0

If we instead want to multiply values we can use Product. Here the binary operation is multiplication (*) and the identity element is 1.

newtype Product a = Product { getProduct :: a }

instance Num a => Semigroup (Product a) where
        Product a <> Product b = Product (a*b)


instance Num a => Monoid (Product a) where
        mempty = Product 1

Product 4 <> Product 5 <> mempty
> Product 20

But how do we use this on multiple values, like combining all the elements in a list using <>?

Foldable

Haskell has a type class for data structures that can be folded, called Foldable. To be folded just means to combine the values inside the structure, which is exactly what we want to do when using monoids.

foldMap :: (Foldable t, Monoid m) => (a -> m) -> t a -> m

What this function does is to apply the a -> m function on every a in the t a combining all the resulting m values using <> and uses mempty when the foldable is empty.

This a bit abstract, so let us see some concrete examples. Let us first look at lists:

instance Foldable [] where
    foldMap _ []     = mempty
    foldMap f (x:xs) = f x <> foldMap f xs

We can see that foldMap f [x0,x1,x2,x3...,xn] === f x0 <> (f x1 <> (f x2 <> (f x3.... <> f xn))). But since <> needs to be associative, these parentheses does not matter, so it is the same as : f x0 <> f x1 <> f x2 <> f x3.... <> f xn

foldMap Sum [1,2,3]
> Sum {getSum = 6}

foldMap (Sum . length) ["hei","hopp"]
> Sum {getSum = 7}

foldMap (Product . fst) [(5,"abc"),(2,"12"),(3,"xx")]
> Product {getProduct = 30}

foldMap (foldMap (const (Sum 1))) ["abc","df"]
> Sum {getSum = 5}

Maybe is also a foldable container.

instance Foldable Maybe where
    foldMap _ Nothing = mempty
    foldMap f (Just x) = f x

foldMap reverse Nothing :: String
> ""

foldMap Sum (Just 5)
> Sum 5

foldMap (foldMap Product) [Just 2, Nothing, Just 4]
> Product  8

When we don't need to transform the values inside the Foldable, we can use the fold function.

fold :: (Foldable t , Monoid m) => t m -> m
fold = foldMap id

fold (Just "abc")
> "abc"

fold Nothing
> ""

fold [Sum 1, Sum 3, Sum 5]
> Sum 9

Now we have seen how to fold data structures using monoids, so let us take a look at some more monoid instances.

Any and All

All and Any are Bool newtypes for monoid instances for (&&) with mempty = True and (||) with mempty = False, respectively.

They can be use to check that any or all elements in a foldable container satisfies a predicate.

foldMap (Any . (==5)) [1,3,5,2]
> Any True


foldMap (All . (>2) . length) ["abc","xs"]
> All False

Bool can also be made into a monoid using the xor-operation.

Min and Max

When we want to find the maximal value, we can use the Max monoid.

newtype Max a = Max { getMax :: a }

instance Ord a => Semigroup (Max a) where
  Max a <> Max b = Max (max a b)

instance (Ord a, Bounded a) => Monoid (Max a) where
  mempty = minBound

mempty <> Max 2 <> Max 3
> Max 3

For miniumum, we use Min.

newtype Min a = Min { getMin :: a }


instance Ord a => Semigroup (Min a) where
  Min a <> Min b = Min (min a b)

instance (Ord a, Bounded a) => Monoid (Min a) where
  mempty = maxBound

mempty <> Min 2 <> Min 3
> Min 2

As we can see, the identity element for Min is maxBound and Max is minBound.

mempty :: Min Int
> Min {getMin = 9223372036854775807}
mempty :: Max Int
> Max {getMax = -9223372036854775808}

This means that if we fold an empty container, we get a value which may be confusing in some cases.

fold [] :: Min Int
> Min {getMin = 9223372036854775807}

Instead we sometimes want to represent this "failure" using Maybe.

Lifting a Semigroup into a Monoid

If we take a look at the Maybe data type: data Maybe a = Nothing | Just a, we can see that it can represent a set of values (Just a) with an additional element Nothing.

If the a's are monoids, we can combine them using <> and then chose Nothing as our identity element. In other words, we embeded a semigroup in a monoid by adjoining an identity.

The monoid instance for Maybe implements this idea:


instance Semigroup a => Semigroup (Maybe a) where
    Nothing <> b       = b
    a       <> Nothing = a
    Just a  <> Just b  = Just (a <> b)

instance Semigroup a => Monoid (Maybe a) where
    mempty = Nothing

fold [Just (Sum 1), Just (Sum 2),Nothing]
> Just (Sum 3)

We can use this to fold into a semigroup. An example of a semigroup that is not a monoid is First, which has the semigroup implementation a <> _ = a, meaning that it will always pick the left argument. There is understandably no identity element for this semigroup, but by using Maybe (First Int) we use Nothing as our identity. We have now created a monoid from the First semigroup, and can therefore use it in a fold.

foldMap (Just . First) [1,2,3]
> Just (First 1)

Back to our "problem" with the min/max identity elements: If we now wrap up our type inside a Maybe, the result is more sensible.

>fold [] :: Maybe (Min Int)
Nothing

Lists

Lists are monoids with (<>) = (++) and mempty = [].

[1,2,3] <> [4,5]
> [1,2,3,4,5]

foldMap (\x -> [x,x*2]) [1,2,3]
> [1,2,2,4,3,6]

In fact, the list is an important monoid, called the free monoid. This comes from the fact that if you take the requirements for a monoid and create the minimal structure needed to satisfy these requirements, you end up with list monoid instance.

Tuples

A very neat property about monoids is that they compose over products. Which in practice means, that if you have two monoids, you can combine them in a tuple, and then that tuple is also a monoid.

instance (Monoid a, Monoid b) => Monoid (a,b) where
    (a,b) <> (a',b') = (a<>a',b<>b')

instance (Monoid a, Monoid b) => Monoid (a,b) where
        mempty = (mempty, mempty)

("abc",Sum 2) <> ("abc",Sum 1)
> ("abcabc", Sum 3)

(This is also implemented for tuples of size 3,4 and 5)

This makes it easy to accumulate multiple monoid values in a single fold. We can use this to find the minimum, maximum, length, sum and product of a list, all in one pass in a neat way.

foldMap (\x -> (Min x, Max x, Sum 1, Sum x, Product x)) [5,1,3]
> (Min 1,Max 5,Sum 3,Sum 9,Product 15)

And for another example: Let us say that we have a list of x y coordinates and want to find, the bottom left and top right corner, i.e. (min x, min y) and (max x, max y).

> foldMap (\(x,y) -> ((Min x, Min y),(Max x, Max y))) [(0,5),(5,0),(5,10),(7::Int,3::Int)]
((Min 0,Min 0), (Max 7,Max 1))

Ap - Lifting a Monoid inta an Applicative

Applacatives have a strong connection to monoids. In categoory theory they are called lax monoidal functors. Every Applicative gives us a monoid, by defining (<>) = liftA2 (<>) and mempty = pure mempty. This is the instance for the Ap newtype.

[Sum 1,Sum 2] <> [Sum 10, Sum 11,Sum 12]
> [Sum 1,Sum 2,Sum 10,Sum 11,Sum 12]

Ap [Sum 1,Sum 2] <> Ap [Sum 10, Sum 11,Sum 12]
> Ap {getAp = [Sum 11,Sum 12,Sum 13,Sum 12,Sum 13,Sum 14]}

We see here how <> for regular lists, just appends the lists toghether, but <> for Ap [] combines every element in the first list with every element in the second list, which mirrors the applicative instance for lists.

> fold [Just "a", Nothing, Just "c"]
Just "ac"
> fold [Ap $ Just "a", Ap $ Nothing, Ap $ Just "c"]
Ap {getAp = Nothing}

As we have seen earlier, in the regular Maybe monoid, Nothing is the identity element. Moreover, the applicative instance for maybe short circuits when you encounter Nothing. Because the monoid instance for Ap Maybe inherits this behaviour, the second example ends up being Nothing, but in the first example the Nothing is "ignored".

There also exists another applicative instance for lists, namely ZipList, which combines elements pair-wise.

liftA2 (,) (ZipList [1,2,3]) (ZipList [4,5,6])
> ZipList {getZipList = [(1,4),(2,5),(3,6)]}

Ap (ZipList [Sum 1,Sum 2]) <> Ap (ZipList [Sum 10, Sum 11,Sum 12])
> Ap {getAp = ZipList [Sum 11,Sum 13]}

Set and Map

Both sets (Data.Set) and maps (Data.Map) have monoid instances, with their respective union as the binary operator and empty as the identity element.

foldMap Set.fromList [[1,2,3],[3,4,5],[4,5,6]]
> Set.fromList [1,2,3,4,5,6]

Map.fromList [(0,"abc"),(1,"xyz")] <> Map.fromList [(1,"www"),(2,"q")]
> Map.fromList [(0,"abc"),(1,"xyz"),(2,"q")]

As we can see, the Map monoid instance only keeps the values from the left map when encountering duplicate keys. Often we would like to combine the values instead. One alternative in this case is to use Map.unionsWith instead of <>

Map.unionsWith (<>) Map.fromList [(0,"abc"),(1,"xyz")] Map.fromList [(1,"www"),(2,"q")]
> Map.fromList [(0,"abc"),(1,"xyzwww"),(2,"q")]

But sometimes we actually need this to be a monoid instance. The appendmap package provides a newtype AppendMap for exactly this purpose.

AppendMap (Map.fromList [(0,"abc"),(1,"xyz")]) <>  AppendMap (Map.fromList [(1,"www"),(2,"q")])
> AppendMap Map.fromList [(0,"abc"),(1,"xyzwww"),(2,"q")]

foldMap fold [Just "Merry", Nothing, Just " Monoidal ", Nothing ,Just "Christmas"] <> "!"
> "Merry Monoidal Christmas!"`

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