Getting started with fp-ts: Monoid

In the last post we saw that a Semigroup captures the concept of "merging" values (via concat). A Monoid is any Semigroup that happens to have a special value which is "neutral" with respect to concat.

Type class definition

As usual in fp-ts the type class Monoid, contained in the fp-ts/Monoid module, is implemented as a TypeScript interface, where the neutral value is named empty

import { Semigroup } from 'fp-ts/Semigroup'

interface Monoid<A> extends Semigroup<A> {
  readonly empty: A
}

The following laws must hold

Right identity: concat(x, empty) = x, for all x in A
Left identity: concat(empty, x) = x, for all x in A

Whichever side of concat we put the value empty, it must make no difference to the value.

Note. If an empty value exists then is unique.

Instances

Most of the semigroups we saw in the previous post are actually monoids

/** number `Monoid` under addition */
const monoidSum: Monoid<number> = {
  concat: (x, y) => x + y,
  empty: 0
}

/** number `Monoid` under multiplication */
const monoidProduct: Monoid<number> = {
  concat: (x, y) => x * y,
  empty: 1
}

const monoidString: Monoid<string> = {
  concat: (x, y) => x + y,
  empty: ''
}

/** boolean monoid under conjunction */
const monoidAll: Monoid<boolean> = {
  concat: (x, y) => x && y,
  empty: true
}

/** boolean monoid under disjunction */
const monoidAny: Monoid<boolean> = {
  concat: (x, y) => x || y,
  empty: false
}

You may wonder if all semigroups are also monoids. That's not the case, as a counterexample consider the following semigroup

const semigroupSpace: Semigroup<string> = {
  concat: (x, y) => x + ' ' + y
}

We can't find an empty value such that concat(x, empty) = x.

Let's write some Monoid instances for more complex types. We can build a Monoid instance for a struct like Point

type Point = {
  x: number
  y: number
}

if we can provide a Monoid instance for each field

import { getStructMonoid } from 'fp-ts/Monoid'

const monoidPoint: Monoid<Point> = getStructMonoid({
  x: monoidSum,
  y: monoidSum
})

We can go on and feed getStructMonoid with the instance just defined

type Vector = {
  from: Point
  to: Point
}

const monoidVector: Monoid<Vector> = getStructMonoid({
  from: monoidPoint,
  to: monoidPoint
})

Folding

When using a monoid instead of a semigroup, folding is even simpler: we don't need to explicitly provide an initial value (the implementation can use the monoid's empty value for that)

import { fold } from 'fp-ts/Monoid'

fold(monoidSum)([1, 2, 3, 4]) // 10
fold(monoidProduct)([1, 2, 3, 4]) // 24
fold(monoidString)(['a', 'b', 'c']) // 'abc'
fold(monoidAll)([true, false, true]) // false
fold(monoidAny)([true, false, true]) // true

Monoids for type constructors

We already know that given a semigroup instance for A we can derive a semigroup instance for Option<A>.

If we can find a monoid instance for A then we can derive a monoid instance for Option<A> (via getApplyMonoid) which works like this

x	y	concat(x, y)
none	none	none
some(a)	none	none
none	some(a)	none
some(a)	some(b)	some(concat(a, b))

import { getApplyMonoid, some, none } from 'fp-ts/Option'

const M = getApplyMonoid(monoidSum)

M.concat(some(1), none) // none
M.concat(some(1), some(2)) // some(3)
M.concat(some(1), M.empty) // some(1)

We can derive two other monoids for Option<A> (for all A)

1) getFirstMonoid...

Monoid returning the left-most non-None value

x	y	concat(x, y)
none	none	none
some(a)	none	some(a)
none	some(a)	some(a)
some(a)	some(b)	some(a)

import { getFirstMonoid, some, none } from 'fp-ts/Option'

const M = getFirstMonoid<number>()

M.concat(some(1), none) // some(1)
M.concat(some(1), some(2)) // some(1)

2) ...and its dual: getLastMonoid

Monoid returning the right-most non-None value

x	y	concat(x, y)
none	none	none
some(a)	none	some(a)
none	some(a)	some(a)
some(a)	some(b)	some(b)

import { getLastMonoid, some, none } from 'fp-ts/Option'

const M = getLastMonoid<number>()

M.concat(some(1), none) // some(1)
M.concat(some(1), some(2)) // some(2)

As an example, getLastMonoid can be useful for managing optional values

import { Monoid, getStructMonoid } from 'fp-ts/Monoid'
import { Option, some, none, getLastMonoid } from 'fp-ts/Option'

/** VSCode settings */
interface Settings {
  /** Controls the font family */
  fontFamily: Option<string>
  /** Controls the font size in pixels */
  fontSize: Option<number>
  /** Limit the width of the minimap to render at most a certain number of columns. */
  maxColumn: Option<number>
}

const monoidSettings: Monoid<Settings> = getStructMonoid({
  fontFamily: getLastMonoid<string>(),
  fontSize: getLastMonoid<number>(),
  maxColumn: getLastMonoid<number>()
})

const workspaceSettings: Settings = {
  fontFamily: some('Courier'),
  fontSize: none,
  maxColumn: some(80)
}

const userSettings: Settings = {
  fontFamily: some('Fira Code'),
  fontSize: some(12),
  maxColumn: none
}

/** userSettings overrides workspaceSettings */
monoidSettings.concat(workspaceSettings, userSettings)
/*
{ fontFamily: some("Fira Code"),
  fontSize: some(12),
  maxColumn: some(80) }
*/

Next post Introduction to property based testing

Blog

Getting started with fp-ts: Monoid

Giulio Canti

Type class definition

Instances

Folding

Monoids for type constructors

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