@arrows/composition

Table of contents

1. Introduction
2. Installation
3. API reference
   • chain
   • chainRight
   • compose
   • curry
   • pipe
   • pipe.now
   • rail
   • rail.async
   • railRight
   • railRight.async
   • tap
4. License

Introduction

The library contains a collection of higher-order functions for composing your applications with reusable functions.

All chain-like functions are manually chunked (segmented). It is a deliberate decision (to not use automatic currying) to allow a function to accept a variadic number of arguments in a segment, which leads in my opinion to a nicer to use API (and is faster as a bonus).

The library has built-in type definitions, which provide an excellent IDE support.

Installation

Via NPM:

npm i @arrows/composition

Via Yarn:

yarn add @arrows/composition

All modules can be imported independently (to reduce bundle size), here are some import methods (you can use either CommonJS or ES modules):

import composition from '@arrows/composition'

import { pipe } from '@arrows/composition'

import pipe from '@arrows/composition/pipe'

API reference

chain

Allows you to build your own, chain-like functions

Executes provided functions from left to right, passing the result from one function to the other. Accepts a wrapping function which can execute additional instructions before executing each passed function.

Parameters

Arguments listed below have to be passed separately, as segments.

• wrappingFn - two-argument function thats is responsible for executing each function in a chain.

• ...fns - an arbitrary number of one-argument functions to be executed in a chain

• initialArg - initial argument, passed to the first function in a chain (through a wrapping function)

Arguments of a wrapping function:

• fn - function in a chain

• input - current value

• isLast - boolean flag indicating if a function is a last one in a chain

Returns

• Returns a final value.

Interface

(wrapping_function) => (fn, fn?, ..., fn?) => (initial_value) => result

Examples

const { chain } = require('@arrows/composition')

// Create custom wrapping function:
const wrappingFn = (fn, input) => {
  console.log(input)
  return fn(input)
}

// Create reusable chain-like function:
const pipeWithLog = chain(wrappingFn)

// Create concrete function
const calculate = pipeWithLog((x) => x + 1, (x) => x * 2)

console.log(calculate(0))

// -> 0
// -> 1
// -> 2 (final result)

---

chainRight

Works like chain, but executes functions from right to left.

Parameters

Arguments listed below have to be passed separately, as segments.

• wrappingFn - two-argument function thats is responsible for executing each function in a chain.

• ...fns - an arbitrary number of one-argument functions to be executed in a chain

• initialArg - initial argument, passed to the first function in a chain (through a wrapping function)

Arguments of a wrapping function:

• fn - function in a chain

• input - current value

• isLast - boolean flag indicating if a function is a last one in a chain

Returns

• Returns a final value.

Interface

(wrapping_function) => (fn, fn?, ..., fn?) => (initial_value) => result

Example

const { chainRight } = require('@arrows/composition')

// Create custom wrapping function:
const wrappingFn = (fn, input) => {
  console.log(input)
  return fn(input)
}

// Create reusable chain-like function:
const composeWithLog = chainRight(wrappingFn)

// Create concrete function
const calculateRight = composeWithLog((x) => x + 1, (x) => x * 2)

console.log(calculateRight(0))

// -> 0
// -> 0
// -> 1 (final result)

---

compose

Chains provided functions from right to left.

Parameters

Arguments listed below have to be passed separately, as segments.

• ...fns - an arbitrary number of one-argument functions to be executed in a chain

• initialArg - initial argument, passed to the first function in a chain

Returns

• Returns a final value.

Interface

(fn, fn?, ..., fn?) => (initial_value) => result

Example

const { compose } = require('@arrows/composition')

const addPrefixes = compose(
  (text) => `prefix1-${text}`,
  (text) => `prefix2-${text}`,
)

addPrefixes('arrows') // -> "prefix1-prefix2-arrows"

---

curry

Wraps function as an automatically curried one.

Parameters

• fn - an arbitrary function

Returns

• Returns a curried version of the function.

Interface

(fn) =>  curried_fn

Example

const { curry } = require('@arrows/composition')

const rawAdd = (a, b) => a + b

const add = curry(rawAdd)

add(1, 2) // -> 3
add(1)(2) // -> 3

---

pipe

Chains provided functions from left to right.

Parameters

Arguments listed below have to be passed separately, as segments.

• ...fns - an arbitrary number of one-argument functions to be executed in a chain

• initialArg - initial argument, passed to the first function in a chain

Returns

• Returns a final value.

Interface

(fn, fn?, ..., fn?) => (initial_value) => result

Example

const { pipe } = require('@arrows/composition')

const addSuffixes = pipe(
  (text) => `${text}-suffix1`,
  (text) => `${text}-suffix2`,
)

addSuffixes('arrows') // -> "arrows-suffix1-suffix2"

---

pipe.now

Chains provided functions from left to right, takes an initial value as a first argument.

Provides an alternative API with better type inference for immediately evaluated calculations.

Parameters

• initialArg - initial argument, passed to the first function in a chain
• ...fns - an arbitrary number of one-argument functions to be executed in a chain

Returns

• Returns a final value.

Interface

(initial_value, fn, fn?, ..., fn?) => result

Example

const { pipe } = require('@arrows/composition')

const result = pipe.now(
  'arrows',
  (text) => `${text}-suffix1`,
  (text) => `${text}-suffix2`,
) // -> "arrows-suffix1-suffix2"

---

rail

Works like pipe, but additionally:

• if one of the functions throws or returns an error - returns that error,
• if one of the functions returns undefined - passes previous argument to the next function.

Parameters

Arguments listed below have to be passed separately, as segments.

• ...fns - an arbitrary number of one-argument functions to be executed in a chain

• initialArg - initial argument, passed to the first function in a chain

Returns

• Returns a final value.

Interface

(fn, fn?, ..., fn?) => (initial_value) => result

Examples

Automatically passing down an error:

const { rail } = require('@arrows/composition')
const { filter, map, reduce } = require('@arrows/array')

const sumDogsAge = rail(
  filter((pet) => pet.specie === 'dog'),
  map((pet) => {
    if (pet.age < 0) {
      throw new Error('Wrong age!')
    }
    return pet.age
  }),
  reduce((a, b) => a + b, 0),
)

const pets = [
  { specie: 'dog', name: 'Charlie', age: 4 },
  { specie: 'cat', name: 'Luna', age: 6 },
  { specie: 'dog', name: 'Ollie', age: -10 },
]

const result = sumDogsAge(pets)

if (result instanceof Error) {
  console.log('Oops!')
} else {
  console.log(`Sum: ${result}`)
}

// -> "Oops!"

Automatically passing previous argument, if function returns undefined:

const { rail } = require('../../lib/index')

const addUser = rail(
  (user) => ({ type: 'user', data: user }),
  (entry) => {
    console.log(`Saving ${entry.data.name} to the database.`)
    // Returns undefined, argument will be automatically
    // passed to the next function.
  },
  (entry) => `Saved user: ${entry.data.name}`,
)

const user = { name: 'Joe' }

console.log(addUser(user))

/* Output:
Saving Joe to the database.
Saved user: Joe
*/

---

rail.async

Works similar to rail, but additionally, if argument is a promise - resolves that promise before passing to the next function.

Parameters

Arguments listed below have to be passed separately, as segments.

• ...fns - an arbitrary number of one-argument functions to be executed in a chain

• initialArg - initial argument, passed to the first function in a chain

Returns

• Returns a final value wrapped in a promise.

Interface

(fn, fn?, ..., fn?) => (initial_value) => result

Example

Automatically passing down an error:

const { railAsync } = require('../../lib/index')

const dbFake = {
  id: 0,
  save() {
    const result = Promise.resolve(this.id)
    this.id++
    return result
  },
}

const addArticle = railAsync(
  (article) => ({ type: 'article', data: article }),
  (entry) => dbFake.save(entry),
  (id) => `Article id: ${id}`,
)

const main = async () => {
  const article = {
    title: 'Railway oriented programming',
    content: 'Lorem ipsum...',
  }

  const result = await addArticle(article)
  console.log(result)
}

main() // -> "Article id: 0"

railRight

Works like rail, but executes functions from right to left.

railRight.async

Works like railAsync, but executes functions from right to left.

tap

Executes provided function with provided argument and returns an argument without changes.

Useful for executing void functions without breaking the chain.

Note: rail* functions handle this automatically, so you can use void functions directly.

Parameters

• fn - an arbitrary function

• arg - any value

Returns

• Returns argument as-is.

Interface

(fn, arg) => arg

Example:

const { pipe, tap } = require('../../lib/index')
const { sort, reduce, _rest, _butLast } = require('@arrows/array')

const sumNotes = pipe(
  sort((a, b) => a - b),
  tap(console.log),
  _rest,
  tap(console.log),
  _butLast,
  tap(console.log),
  reduce((a, b) => a + b, 0),
)

const notes = [16, 17.5, 19, 15, 18]

console.log(sumNotes(notes))

/* Output:
[ 15, 16, 17.5, 18, 19 ]
[ 16, 17.5, 18, 19 ]
[ 16, 17.5, 18 ]
51.5
*/

License

Project is under open, non-restrictive ISC license.