Getting started with fp-ts: Semigroup

gcanti profile image Giulio Canti Updated on ・6 min read

Since semigroups are such a fundamental abstraction of functional programming, this blog post will be longer than usual.

General definition

A semigroup is a pair (A, *) in which A is a non-empty set and * is a binary associative operation on A, i.e. a function that takes two elements of A as input and returns an element of A as output...

*: (x: A, y: A) => A

... while associative means that the equation

(x * y) * z = x * (y * z)

holds for all x, y, z in A.

Associativity simply tells us that we do not have to worry about parenthesizing an expression and can write x * y * z.

Semigroups capture the essence of parallelizable operations

There are plenty of examples of semigroups:

  • (number, *) where * is the usual multiplication of numbers
  • (string, +) where + is the usual concatenation of strings
  • (boolean, &&) where && is the usual conjunction

and many more.

Type class definition

As usual in fp-ts the type class Semigroup, contained in the fp-ts/lib/Semigroup module, is implemented as a TypeScript interface, where the operation * is named concat

interface Semigroup<A> {
  concat: (x: A, y: A) => A

The following law must hold

  • Associativity: concat(concat(x, y), z) = concat(x, concat(y, z)), for all x, y, z in A

The name concat makes a particular sense for arrays (see later) but, based on the context and the type A for which we are implementing an instance, the semigroup operation can be interpreted with different meanings

  • "concatenation"
  • "merging"
  • "fusion"
  • "selection"
  • "addition"
  • "substitution"

and many more.


This is how we can implement the semigroup (number, *)

/** number `Semigroup` under multiplication */
const semigroupProduct: Semigroup<number> = {
  concat: (x, y) => x * y

Note that you can define different semigroup instances for the same type. Here's the implementation of the semigroup (number, +) where + is the usual addition of numbers

/** number `Semigroup` under addition */
const semigroupSum: Semigroup<number> = {
  concat: (x, y) => x + y

Another example, with strings this time

const semigroupString: Semigroup<string> = {
  concat: (x, y) => x + y

I can't find an instance!

What if, given a type A, you can't find an associative operation on A? You can create a (trivial) semigroup instance for every type just using the following constructions

/** Always return the first argument */
function getFirstSemigroup<A = never>(): Semigroup<A> {
  return { concat: (x, y) => x }

/** Always return the second argument */
function getLastSemigroup<A = never>(): Semigroup<A> {
  return { concat: (x, y) => y }

Another technique is to define a semigroup instance for Array<A> (*), called the free semigroup of A.

function getArraySemigroup<A = never>(): Semigroup<Array<A>> {
  return { concat: (x, y) => x.concat(y) }

and map the elements of A to the singleton elements of Array<A>

function of<A>(a: A): Array<A> {
  return [a]

(*) strictly speaking is a semigroup instance for non empty arrays of A

Note. Here concat is the native array method, which kind of explains the initial choice for the name of the Semigroup operation.

The free semigroup of A is the semigroup whose elements are all possible non-empty finite sequences of elements of A.

Deriving from Ord

There's another way to build a semigroup instance for a type A: if we already have an Ord instance for A, then we can "turn it" into a semigroup.

Actually two possible semigroups

import { ordNumber } from 'fp-ts/lib/Ord'
import { getMeetSemigroup, getJoinSemigroup } from 'fp-ts/lib/Semigroup'

/** Takes the minimum of two values */
const semigroupMin: Semigroup<number> = getMeetSemigroup(ordNumber)

/** Takes the maximum of two values  */
const semigroupMax: Semigroup<number> = getJoinSemigroup(ordNumber)

semigroupMin.concat(2, 1) // 1
semigroupMax.concat(2, 1) // 2

Let's write some Semigroup instances for more complex types

type Point = {
  x: number
  y: number

const semigroupPoint: Semigroup<Point> = {
  concat: (p1, p2) => ({
    x: semigroupSum.concat(p1.x, p2.x),
    y: semigroupSum.concat(p1.y, p2.y)

This is mostly boilerplate though. The good news is that we can build a Semigroup instance for a struct like Point if we can provide a Semigroup instance for each field.

Indeed the fp-ts/lib/Semigroup module exports a getStructSemigroup combinator:

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

const semigroupPoint: Semigroup<Point> = getStructSemigroup({
  x: semigroupSum,
  y: semigroupSum

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

type Vector = {
  from: Point
  to: Point

const semigroupVector: Semigroup<Vector> = getStructSemigroup({
  from: semigroupPoint,
  to: semigroupPoint

getStructSemigroup is not the only combinator provided by fp-ts, here's a combinator that allows to derive a Semigroup instance for functions: given an instance of Semigroup for S we can derive an instance of Semigroup for functions with signature (a: A) => S, for all A

import { getFunctionSemigroup, Semigroup, semigroupAll } from 'fp-ts/lib/Semigroup'

/** `semigroupAll` is the boolean semigroup under conjunction */
const semigroupPredicate: Semigroup<(p: Point) => boolean> = getFunctionSemigroup(

Now we can "merge" two predicates on Points

const isPositiveX = (p: Point): boolean => p.x >= 0
const isPositiveY = (p: Point): boolean => p.y >= 0

const isPositiveXY = semigroupPredicate.concat(isPositiveX, isPositiveY)

isPositiveXY({ x: 1, y: 1 }) // true
isPositiveXY({ x: 1, y: -1 }) // false
isPositiveXY({ x: -1, y: 1 }) // false
isPositiveXY({ x: -1, y: -1 }) // false


By definition concat works with only two elements of A, what if we want to concat more elements?

The fold function takes a semigroup instance, an initial value and an array of elements:

import { fold, semigroupSum, semigroupProduct } from 'fp-ts/lib/Semigroup'

const sum = fold(semigroupSum)

sum(0, [1, 2, 3, 4]) // 10

const product = fold(semigroupProduct)

product(1, [1, 2, 3, 4]) // 24

Semigroups for type constructors

What if we want to "merge" two Option<A>? There are four cases:

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

There's a problem with the last one, we'd need something to "merge" two As.

That's what Semigroup does! We can require a semigroup instance for A and then derive a semigroup instance for Option<A>. This is how the getApplySemigroup combinator works

import { semigroupSum } from 'fp-ts/lib/Semigroup'
import { getApplySemigroup, some, none } from 'fp-ts/lib/Option'

const S = getApplySemigroup(semigroupSum)

S.concat(some(1), none) // none
S.concat(some(1), some(2)) // some(3)


We've seen that semigroups help us any time we want to "concat", "merge", or "combine" (whatever word gives you the best intuition) several data into one.

Let's wrap all together with a final example (adapted from Fantas, Eel, and Specification 4: Semigroup)

Let's imagine you're building some system in which you store customer records that look like this:

interface Customer {
  name: string
  favouriteThings: Array<string>
  registeredAt: number // since epoch
  lastUpdatedAt: number // since epoch
  hasMadePurchase: boolean

For whatever reason you might end up with duplicate records for the same person. What we need is a merge strategy. That's what semigroups are all about

import {
} from 'fp-ts/lib/Semigroup'
import { getMonoid } from 'fp-ts/lib/Array'
import { ordNumber, contramap } from 'fp-ts/lib/Ord'

const semigroupCustomer: Semigroup<Customer> = getStructSemigroup({
  // keep the longer name
  name: getJoinSemigroup(contramap((s: string) => s.length)(ordNumber)),
  // accumulate things
  favouriteThings: getMonoid<string>(), // <= getMonoid returns a Semigroup for `Array<string>` see later
  // keep the least recent date
  registeredAt: getMeetSemigroup(ordNumber),
  // keep the most recent date
  lastUpdatedAt: getJoinSemigroup(ordNumber),
  // Boolean semigroup under disjunction
  hasMadePurchase: semigroupAny

    name: 'Giulio',
    favouriteThings: ['math', 'climbing'],
    registeredAt: new Date(2018, 1, 20).getTime(),
    lastUpdatedAt: new Date(2018, 2, 18).getTime(),
    hasMadePurchase: false
    name: 'Giulio Canti',
    favouriteThings: ['functional programming'],
    registeredAt: new Date(2018, 1, 22).getTime(),
    lastUpdatedAt: new Date(2018, 2, 9).getTime(),
    hasMadePurchase: true
{ name: 'Giulio Canti',
  favouriteThings: [ 'math', 'climbing', 'functional programming' ],
  registeredAt: 1519081200000, // new Date(2018, 1, 20).getTime()
  lastUpdatedAt: 1521327600000, // new Date(2018, 2, 18).getTime()
  hasMadePurchase: true }

The function getMonoid returns a Semigroup for Array<string>. Actually it returns something more than a semigroup: a monoid.

So what's a monoid? In the next post I'll talk about Monoids

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Hi Giulio, nice post.

I was just wondering why the semigroup for Option behaves the way you describe and not returning the non empty Option when combining an empty Option with a non empty one?



That's one of the possible Semigroup instances, which corresponds to the "computation failed" meaning of Option (so if you combine two computations and one of them fails the combination as a whole fails too). There are other possible instances though, see getFirstMonoid, getLastMonoid and getMonoid from the Option module


I see, thanks for the clarification!