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Functional design: combinators

gcanti profile image Giulio Canti Updated on ・3 min read

In this article the term "combinator" refers to the combinator pattern

A style of organizing libraries centered around the idea of combining things. Usually there is some type T, some "primitive" values of type T, and some "combinators" which can combine values of type T in various ways to build up more complex values of type T

So the general shape of a combinator is

combinator: Thing -> Thing

The goal of a combinator is to create new "things" from previously defined "things".

Since the result can be passed back as input, you get a combinatorial explosion of possibilities, which makes this pattern very powerful.

If you mix and match several combinators together, you get an even larger combinatorial explosion.

So a design that you may often find in a functional module is

  • a small set of very simple "primitives"
  • a set of "combinators" for combining them into more complicated structures

Let's see some examples.

Example 1: Eq

The getEq combinator: given an instance of Eq for A, we can derive an instance of Eq for Array<A>

import { Eq } from 'fp-ts/lib/Eq'

export function getEq<A>(E: Eq<A>): Eq<Array<A>> {
  return {
    equals: (xs, ys) => xs.length === ys.length && xs.every((x, i) => E.equals(x, ys[i]))
  }
}

Usage

/** a primitive `Eq` instance for `number` */
export const eqNumber: Eq<number> = {
  equals: (x, y) => x === y
}

// derived
export const eqArrayOfNumber: Eq<Array<number>> = getEq(eqNumber)

// derived
export const eqArrayOfArrayOfNumber: Eq<Array<Array<number>>> = getEq(eqArrayOfNumber)

// derived
export const eqArrayOfArrayOfArrayOfNumber: Eq<Array<Array<Array<number>>>> = getEq(
  eqArrayOfArrayOfNumber
)

// etc...

Another combinator, contramap: given an instance of Eq for A and a function from B to A, we can derive an instance of Eq for B

export const contramap = <A, B>(f: (b: B) => A) => (E: Eq<A>): Eq<B> => {
  return {
    equals: (x, y) => E.equals(f(x), f(y))
  }
}

Usage

export interface User {
  id: number
  name: string
}

export const eqUser: Eq<User> = contramap((user: User) => user.id)(eqNumber)

// mix with `getArraySetoid`
export const eqArrayOfUser: Eq<Array<User>> = getEq(eqUser)

Example 2: Monoid

The getIOMonoid combinator: given an instance of Monoid for A, we can derive an instance of Monoid for IO<A>

import { IO } from 'fp-ts/lib/IO'
import { Monoid } from 'fp-ts/lib/Monoid'

export function getIOMonoid<A>(M: Monoid<A>): Monoid<IO<A>> {
  return {
    concat: (x, y) => () => M.concat(x(), y()),
    empty: () => M.empty
  }
}

We can use getIOMonoid to derive another combinator, replicateIO: given a number n and an action mv of type IO<void>, we can derive an action that performs n times mv

import { fold } from 'fp-ts/lib/Monoid'
import { replicate } from 'fp-ts/lib/Array'

/** a primitive `Monoid` instance for `void` */
export const monoidVoid: Monoid<void> = {
  concat: () => undefined,
  empty: undefined
}

export function replicateIO(n: number, mv: IO<void>): IO<void> {
  return fold(getIOMonoid(monoidVoid))(replicate(n, mv))
}

Usage

//
// helpers
//

/** logs to the console */
export function log(message: unknown): IO<void> {
  return () => console.log(message)
}

/** returns a random integer between `low` and `high` */
export const randomInt = (low: number, high: number): IO<number> => {
  return () => Math.floor((high - low + 1) * Math.random() + low)
}

//
// program
//
import { chain } from 'fp-ts/lib/IO'
import { pipe } from 'fp-ts/lib/pipeable'

function fib(n: number): number {
  return n <= 1 ? 1 : fib(n - 1) + fib(n - 2)
}

/** calculates a random fibonacci and prints the result to the console */
const printFib: IO<void> = pipe(
  randomInt(30, 35),
  chain(n => log(fib(n)))
)

replicateIO(3, printFib)()
/*
1346269
9227465
3524578
*/

Example 3: IO

We can build many other combinators for IO, for example the time combinator mimics the analogous Unix command: given an action IO<A>, we can derive an action IO<A> that prints to the console the elapsed time

import { IO, io } from 'fp-ts/lib/IO'
import { now } from 'fp-ts/lib/Date'
import { log } from 'fp-ts/lib/Console'

export function time<A>(ma: IO<A>): IO<A> {
  return io.chain(now, start =>
    io.chain(ma, a => io.chain(now, end => io.map(log(`Elapsed: ${end - start}`), () => a)))
  )
}

Usage

time(replicateIO(3, printFib))()
/*
5702887
1346269
14930352
Elapsed: 193
*/

With partials...

time(replicateIO(3, time(printFib)))()
/*
3524578
Elapsed: 32
14930352
Elapsed: 125
3524578
Elapsed: 32
Elapsed: 189
*/

Can we make the time combinator more general? We'll see how in the next article.

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Discussion

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I recently wrote about the combinator pattern in my article on property testing via JSVerify. I blinked just now at reading the phrase "combinatorial explosion" which we both used in our articles – but it turns out you wrote it first (Feb. vs Mar.)! Um, great minds think alike?

Anyway, combinators to me represent almost the entire point of FP – composition. The ability to connect pieces of code together seamlessly, building up complexity without getting lost in the wiring. This article has some nice examples.

I recently became aware of FP-TS from a tweet; it looks good! Might be the lever that finally pushes me into adopting TS itself, as FP in JS works but I miss the typing of e.g. Haskell. Enjoying your article series, thanks for posting it.

 

Hey you might enjoy exploring how this term is used in a variety of contexts, it's quite useful!

google.com/search?q=combinatorial+...

 

About you search on functional JS typing, you may already heard of Sanctuary-def. You loose static analysis but gain runtime (optional) type system. It supports Hindley-Milner like type signature.

 

This seems to be a nice post, but typescript makes it so hard to read...

 

I don't see any "time" combinator here, and the reference to it in the next post isn't really coming from here. What am I missing?

 

Example 3 was missing, thanks for pointing out