rubico

🏞 a shallow river in northeastern Italy, just south of Ravenna

a functional promise library

Installation

Examples

Documentation

Transducers

More Examples

Introduction

Asynchronous programming in JavaScript has evolved over the years

In the beginning, there were callbacks

function doAsyncThings(a, cb) {
  doAsyncThingA(a, function(errA, b) {
    if (errA) return cb(errA)
    doAsyncThingB(b, function(errB, c) {
      if (errB) return cb(errB)
      doAsyncThingC(c, function(errC, d) {
        if (errC) return cb(errC)
        doAsyncThingD(d, function(errD, e) {
          if (errD) return cb(errD)
          doAsyncThingE(e, function(errE, f) {
            if (errE) return cb(errE)
            cb(null, f)
          })
        })
      })
    })
  })
}

To stay within maximum line lengths, we created Promises

Then began the chains of then

const doAsyncThings = a => doAsyncThingA(a)
  .then(b => doAsyncThingB(b))
  .then(c => doAsyncThingC(c))
  .then(d => doAsyncThingD(d))
  .then(e => doAsyncThingE(e))

This was fine until we wanted JavaScript to look like a normal language

async/await, the latest in asynchrony

const doAsyncThings = async a => {
  const b = await doAsyncThingA(a)
  const c = await doAsyncThingB(b)
  const d = await doAsyncThingC(c)
  const e = await doAsyncThingD(d)
  const f = await doAsyncThingE(e)
  return f
}

And now, a functional way to do async things

import { pipe } from 'rubico'

const doAsyncThings = pipe([
  doAsyncThingA,
  doAsyncThingB,
  doAsyncThingC,
  doAsyncThingD,
  doAsyncThingE,
])

rubico is a functional (programming) promise library.

rubico resolves on two promises:

  1. simplify asynchronous programming in JavaScript
  2. enable functional programming in JavaScript

programs written with rubico follow a point-free style

rubico works in server and browser JavaScript environments

Installation

with deno

import {
  pipe, fork, assign, tap, tryCatch, switchCase,
  map, filter, reduce, transform,
  any, all, and, or, not,
  eq, gt, lt, gte, lte,
  get, pick, omit,
} from 'https://deno.land/x/rubico/rubico.js'

in the browser

<script type="module">
  import r, {
    pipe, fork, assign, tap, tryCatch, switchCase,
    map, filter, reduce, transform,
    any, all, and, or, not,
    eq, gt, lt, gte, lte,
    get, pick, omit,
  } from 'https://deno.land/x/rubico/rubico.js'
</script>

with npm

npm i rubico

Examples

The following examples compare promise chains, async/await, and rubico

Make a request

// promise chains
fetch('https://jsonplaceholder.typicode.com/todos/1')
  .then(res => res.json())
  .then(console.log) // > {...}

// async/await
void (async () => {
  const res = await fetch('https://jsonplaceholder.typicode.com/todos/1')
  const data = await res.json()
  console.log(data) // > {...}
})()

// rubico
import { pipe } from 'rubico.js'

pipe([
  fetch,
  res => res.json(),
  console.log, // > {...}
])('https://jsonplaceholder.typicode.com/todos/1')

Make multiple requests

const todoIDs = [1, 2, 3, 4, 5]

// promise chains
Promise.resolve(todoIDs.filter(id => id <= 3))
  .then(filtered => Promise.all(filtered.map(
    id => `https://jsonplaceholder.typicode.com/todos/${id}`
  )))
  .then(urls => Promise.all(urls.map(fetch)))
  .then(responses => Promise.all(responses.map(res => res.json())))
  .then(data => data.map(x => console.log(x))) // > {...} {...} {...}

// async/await
void (async () => {
  const filtered = todoIDs.filter(id => id <= 3)
  const urls = await Promise.all(filtered.map(id => `https://jsonplaceholder.typicode.com/todos/${id}`))
  const responses = await Promise.all(urls.map(fetch))
  const data = await Promise.all(responses.map(res => res.json()))
  data.map(x => console.log(x)) // > {...} {...} {...}
})()

// rubico
import { pipe, map, filter } from 'rubico.js'

pipe([
  filter(id => id <= 3),
  map(id => `https://jsonplaceholder.typicode.com/todos/${id}`),
  map(fetch),
  map(res => res.json()),
  map(console.log), // > {...} {...} {...}
])(todoIDs)

Documentation

rubico exports 23 functions

pipe, fork, assign tap, tryCatch, switchCase

map, filter, reduce, transform

any, all and, or, not

eq, gt, lt, gte, lte

get, pick, omit

pipe

chains functions from left to right; functionN(...(function2(function1(function0(x)))))

y = pipe(functions)(x)

functions is an array of functions

x is anything

if x is a function, pipe chains functions from right to left, see transducers

y is the output of running x through the chain of functions

y is wrapped in a Promise if any of the following are true:

  • any function of functions is asynchronous
pipe([
  x => x + ' ',
  x => x + 'world',
])('hello') // => 'hello world'

pipe([
  async x => x + ' ',
  x => x + 'world',
])('hello') // => Promise { 'hello world' }

fork

parallelizes functions with input, retaining functions' type and shape

y = fork(functions)(x)

functions is an array of functions or an object of functions

all functions of functions are run concurrently

x is anything

if functions is an array, y is functions.map(f => f(x))

if functions is an object, y is an object of entries key: f(x) for entry key: f of functions

y is wrapped in a Promise if any of the following are true:

  • any function of functions is asynchronous
fork([
  x => x + 'world',
  x => x + 'mom'
])('hello') // => ['hello world', 'hello mom']

fork([
  x => x + 'world',
  async x => x + 'mom'
])('hello') // => Promise { ['hello world', 'hello mom'] }

fork({
  a: x => x + 'world',
  b: x => x + 'mom',
})('hello') // => { a: 'hello world', b: 'hello mom' }

fork({
  a: x => x + 'world',
  b: async x => x + 'mom',
})('hello') // => Promise { { a: 'hello world', b: 'hello mom' } }

assign

parallelizes functions with input, merging output into input

y = assign(functions)(x)

functions is an object of functions

all functions of functions are run concurrently

x is an object

output is an object of entries key: f(x) for entry key: f of functions

y is output merged into x

y is wrapped in a Promise if any of the following are true:

  • any function of functions is asynchronous
assign({
  hi: x => 'hi ' + x,
  bye: x => 'bye ' + x,
})({ name: 'Ed' }) // => { name: 'Ed', hi: 'hi Ed', bye: 'bye Ed' }

assign({
  async hi: x => 'hi ' + x,
  bye: x => 'bye ' + x,
})({ name: 'Ed' }) // => Promise { { name: 'Ed', hi: 'hi Ed', bye: 'bye Ed' } }

assign({
  name: () => 'not Ed',
})({ name: 'Ed' }) // => { name: 'not Ed' }

tap

calls a function with input, returning input

y = tap(f)(x)

x is anything

f is a function that expects one argument x

y is x

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous

if x is a function, y is a transduced reducing function, see transducers

y = tap(f)(x); reduced = reduce(y)(z)

reduce is reduce,

z is an iterable, async iterable, or object

zi is an element of z

f is a function that expects one argument zi

reduced is equivalent to reduce(x)(z)

tap(
  console.log, // > 'hey'
)('hey') // => 'hey'

const asyncConsoleLog = async x => console.log(x)

tap(
  asyncConsoleLog, // > 'hey'
)('hey') // => Promise { 'hey' }

const concat = (y, xi) => y.concat([xi])

reduce(
  tap(console.log)(concat), // > 1 2 3 4 5
  [],
)([1, 2, 3, 4, 5]) // => [1, 2, 3, 4, 5]

tryCatch

tries a function with input, catches with another function

y = tryCatch(f, g)(x)

f is a function that expects one argument x

g is a function that expects two arguments err and x

x is anything

err is a value potentially thrown by f(x)

if f(x) throws err, y is g(err, x), else y is f(x)

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous
  • f is synchronous, g is asynchronous, and f(x) threw
const onError = (e, x) => `${x} is invalid: ${e.message}`

tryCatch(
  x => x,
  onError,
)('hello') // => 'hello'

const throwGoodbye = () => { throw new Error('goodbye') }

tryCatch(
  throwGoodbye,
  onError,
)('hello') // => 'hello is invalid: goodbye'

const rejectWithGoodbye = () => Promise.reject(new Error('goodbye'))

tryCatch(
  rejectWithGoodbye,
  onError,
)('hello') // => Promise { 'hello is invalid: goodbye' }

switchCase

an if, else if, else construct for functions

y = switchCase(functions)(x)

x is anything

functions is an array of functions

given

  • predicate if functions if1, if2, ..., ifN
  • corresponding do functions do1, do2, ..., doN
  • an else function elseDo

functions is an array of functions

[
  if1, do1,
  if2, do2,
  ..., ...,
  elseDo,
]

switchCase evaluates functions in functions from left to right

y is the first do(x) whose corresponding if(x) is truthy

y is wrapped in a Promise if any of the following are true:

  • any evaluated functions are asynchronous
const isOdd = x => x % 2 === 1

switchCase([
  isOdd, () => 'odd',
  () => 'even',
])(1) // => 'odd'

switchCase([
  async isOdd, () => 'odd',
  () => 'even',
])(1) // => Promise { 'odd' }

map

applies a function to each element of input, retaining input type and shape

y = map(f)(x)

x is an iterable, an async iterable, an object, or a function

xi is an element of x

f is a function that expects one argument xi

y is of type and shape x with f applied to each element, with some exceptions:

  • if x is an async iterable but not a built-in type, y is a generated async iterable
  • if x is an iterable but not a built-in type, y is a generated iterable
  • if x is an iterable but not a built-in type and f is asynchronous, y is an iterable of promises

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous and x is not an async iterable

if x is a function, y is a transduced reducing function, see transducers

y = map(f)(x); reduced = reduce(y)(z)

reduce is reduce,

z is an iterable, async iterable, or object

zi is an element of z

f is a function that expects one argument zi

x is a reducing function that expects two arguments y and f(zi)

reduced is equivalent to reduce(x)(map(f)(z))

const square = x => x ** 2

map(
  square,
)([1, 2, 3, 4, 5]) // => [1, 4, 9, 16, 25]

const asyncSquare = async x => x ** 2

map(
  asyncSquare,
)([1, 2, 3, 4, 5]) // => Promise { [1, 4, 9, 16, 25] }

map(
  Math.abs,
)(new Set([-2, -1, 0, 1, 2])) // => { Set { 0, 1, 2 } }

const double = ([k, v]) => [k + k, v + v]

map(
  double,
)(new Map([['a', 1], ['b', 2]])) // => Map { 'aa' => 2, 'bb' => 4 }

map(
  byte => byte + 1,
)(new Uint8Array([97, 98, 99])) // Uint8Array [ 98, 99, 100 ]

map(
  word => word + 'z',
)({ a: 'lol', b: 'cat' }) // => { a: 'lolz', b: 'catz' }

filter

filters elements out of input based on provided predicate

y = filter(f)(x)

x is an iterable, an async iterable, an object, or a function

xi is an element of x

f is a function that expects one argument xi

y is of type and shape x with elements xi where f(xi) is truthy, with some exceptions:

  • if x is an async iterable but not a built-in type, y is a generated async iterable
  • if x is an iterable but not a built-in type, y is a generated iterable
  • if x is an iterable but not a bulit-in type and f is asynchronous, filter will throw a TypeError

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous and x is not an async iterable

if x is a function, y is a transduced reducing function, see transducers

y = filter(f)(x); reduced = reduce(y)(z)

reduce is reduce,

z is an iterable, async iterable, or object

zi is an element of z

f is a function that expects one argument zi

x is a reducing function that expects two arguments y and zi

reduced is equivalent to reduce(x)(filter(f)(z))

const isOdd = x => x % 2 === 1
filter(
  isOdd,
)([1, 2, 3, 4, 5]) // => [1, 3, 5]

const asyncIsOdd = async x => x % 2 === 1
filter(
  asyncIsOdd,
)([1, 2, 3, 4, 5]) // => Promise { [1, 3, 5] }

filter(
  letter => letter !== 'y',
)('yoyoyo') // => 'ooo'

const abcSet = new Set(['a', 'b', 'c'])
filter(
  letter => abcSet.has(letter),
)(new Set(['a', 'z'])) // => Set { 'a' }

filter(
  ([key, value]) => key === value,
)(new Map([[0, 1], [1, 1], [2, 1]])) // => { Map { 1 => 1 } }

filter(
  bigint => bigint <= 3n,
)(new BigInt64Array([1n, 2n, 3n, 4n, 5n])) // => BigInt64Array [1n, 2n, 3n]

filter(
  value => value === 1,
)({ a: 1, b: 2, c: 3 }) // => { a: 1 }

reduce

transforms input according to provided reducing function and initial value

y = reduce(f, x0)(x)

x is an iterable, an async iterable, or an object

xi is an element of x

f is a reducing function that expects two arguments y and xi

x0 is optional, and if provided:

  • y starts as x0
  • iteration begins with the first element of x

if x0 is not provided:

  • y starts as the first element of x
  • iteration begins with the second element of x

y is f(y, xi) for each successive xi

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous
  • x is an async iterable
const add = (y, xi) => y + xi

reduce(
  add,
)([1, 2, 3, 4, 5]) // => 15

reduce(
  add, 100,
)([1, 2, 3, 4, 5]) // => 115

const asyncAdd = async (y, xi) => y + xi

reduce(
  asyncAdd,
)([1, 2, 3, 4, 5]) // => Promise { 15 }

const asyncNumbersGeneratedIterable = (async function*() {
  for (let i = 0; i < 5; i++) { yield i + 1 }
})() // generated async iterable that yields 1 2 3 4 5

const concat = (y, xi) => y.concat([xi])

reduce(
  concat, [],
)(asyncNumbersGeneratedIterable) // => Promise { [1, 2, 3, 4, 5] }

reduce(
  concat, [],
)({ a: 1, b: 1, c: 1, d: 1, e: 1 }) // => [1, 2, 3, 4, 5]

transform

transforms input according to provided transducer and initial value

y = transform(f, x0)(x)

x is an iterable, an async iterable, or an object

f is a transducer, see transducers

x0 is null, an array, a string, a set, a map, a typed array, or a writable

y is x transformed with f into x0

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous
  • x is an async iterable

in the following examples, map is map

const square = x => x ** 2

transform(map(
  square,
), null)([1, 2, 3, 4, 5]) // => null

const asyncSquare = async x => x ** 2

transform(map(
  asyncSquare,
), [])([1, 2, 3, 4, 5]) // => Promise { [1, 4, 9, 16, 25] }

transform(map(
  square,
), '')([1, 2, 3, 4, 5]) // => '1491625'

transform(map(
  square,
), new Set())([1, 2, 3, 4, 5]) // => Set { 1, 4, 9, 16, 25 }

transform(map(
  number => [number, square(number)],
), new Map())([1, 2, 3, 4, 5]) // => Map { 1 => 1, 2 => 4, 3 => 9, 4 => 16, 5 => 25 }

const charToByte = x => x.charCodeAt(0)

transform(map(
  square,
), new Uint8Array())([1, 2, 3, 4, 5]), // => Uint8Array [1, 4, 9, 16, 25]

const asyncNumbersGeneratedIterable = (async function*() {
  for (let i = 0; i < 5; i++) { yield i + 1 }
})() // generated async iterable that yields 1 2 3 4 5

transform(map(
  square,
), process.stdout)(asyncNumbersGeneratedIterable) // > 1 4 9 16 25
// => Promise { process.stdout }

any

applies a function to each element of input, returns true if any evaluations truthy

y = any(f)(x)

x is an iterable or an object

xi is an element of x

f is a function that expects one argument xi

y is true if all f(xi) are truthy, false otherwise

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous
const isOdd = x => x % 2 === 1

any(
  isOdd,
)([1, 2, 3, 4, 5]) // => true

const asyncIsOdd = async x => x % 2 === 1

any(
  asyncIsOdd,
)([1, 2, 3, 4, 5]) // => Promise { true }

any(
  isOdd,
)({ b: 2, d: 4 }) // => false

all

applies a function to each element of input, returns true if all evaluations truthy

y = all(f)(x)

x is an iterable or an object

xi is an element of x

f is a function that expects one argument xi

y is true if any f(xi) are truthy, false otherwise

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous
const isOdd = x => x % 2 === 1

all(
  isOdd,
)([1, 2, 3, 4, 5]) // => false

const asyncIsOdd = async x => x % 2 === 1

all(
  asyncIsOdd,
)([1, 2, 3, 4, 5]) // => Promise { false }

all(
  isOdd,
)({ a: 1, c: 3 }) // => true

and

applies each function of functions to input, returns true if all evaluations truthy

y = and(functions)(x)

x is anything

functions is an array of functions

f is a function of functions

y is true if all f(x) are truthy, false otherwise

y is wrapped in a Promise if any of the following are true:

  • any f is asynchronous
const isOdd = x => x % 2 === 1

const asyncIsOdd = async x => x % 2 === 1

const lessThan3 = x => x < 3

and([
  isOdd,
  lessThan3,
])(1) // => true

and([
  asyncIsOdd,
  lessThan3,
])(1) // => Promise { true }

and([
  isOdd,
  lessThan3,
])(2) // => false

or

applies each function of functions to input, returns true if any evaluations truthy

y = or(functions)(x)

x is anything

functions is an array of functions

f is a function of functions

y is true if any f(x) are truthy, false otherwise

y is wrapped in a Promise if any of the following are true:

  • any f is asynchronous
const isOdd = x => x % 2 === 1

const asyncIsOdd = async x => x % 2 === 1

const lessThan3 = x => x < 3

or([
  isOdd,
  lessThan3,
])(5) // => true

or([
  asyncIsOdd,
  lessThan3,
])(5) // => Promise { true }

or([
  isOdd,
  lessThan3,
])(6) // => false

not

applies a function to input, logically inverting the result

y = not(f)(x)

x is anything

f is a function that expects one argument x

y is true if f(x) is falsy, false otherwise

y is wrapped in a Promise if any of the following are true:

  • f is asynchronous
const isOdd = x => x % 2 === 1

const asyncIsOdd = async x => x % 2 === 1

not(
  isOdd,
)(2) // => true

not(
  asyncIsOdd,
)(2) // => Promise { true }

not(
  isOdd,
)(3) // => false

eq

tests left strictly equals right

y = eq(left, right)(x)

x is anything

left is a non-function value or a function that expects one argument x

right is a non-function value or a function that expects one argument x

leftCompare is left if left is a non-function value, else left(x)

rightCompare is right if right is a non-function value, else right(x)

y is true if leftCompare strictly equals rightCompare, false otherwise

y is wrapped in a Promise if any of the following are true:

  • left is asynchronous
  • right is asynchronous
const square = x => x ** 2

const asyncSquare = async x => x ** 2

eq(
  square,
  1,
)(1) // => true

eq(
  asyncSquare,
  1,
)(1) // => Promise { true }

eq(
  square,
  asyncSquare,
)(1) // => Promise { true }

eq(
  1,
  square,
)(2) // => false

eq(1, 1)() // => true
eq(0, 1)() // => false

gt

tests left greater than right

y = gt(left, right)(x)

x is anything

left is a non-function value or a function that expects one argument x

right is a non-function value or a function that expects one argument x

leftCompare is left if left is a non-function value, else left(x)

rightCompare is right if right is a non-function value, else right(x)

y is true if leftCompare is greater than rightCompare

y is wrapped in a Promise if any of the following are true:

  • left is asynchronous
  • right is asynchronous
gt(
  x => x,
  10,
)(11) // => true

gt(
  async x => x,
  10,
)(11) // => Promise { true }

gt(
  x => x,
  10,
)(9) // => false

gt(2, 1)() // => true
gt(1, 1)() // => false
gt(0, 1)() // => false

lt

tests left less than right

y = lt(left, right)(x)

x is anything

left is a non-function value or a function that expects one argument x

right is a non-function value or a function that expects one argument x

leftCompare is left if left is a non-function value, else left(x)

rightCompare is right if right is a non-function value, else right(x)

y is true if leftCompare is less than rightCompare

y is wrapped in a Promise if any of the following are true:

  • left is asynchronous
  • right is asynchronous
lt(
  x => x,
  10,
)(9) // => true

lt(
  async x => x,
  10,
)(9) // => Promise { true }

lt(
  x => x,
  10,
)(11) // => false

lt(0, 1)() // => true
lt(1, 1)() // => false
lt(2, 1)() // => false

gte

tests left greater than or equal right

y = gte(left, right)(x)

x is anything

left is a non-function value or a function that expects one argument x

right is a non-function value or a function that expects one argument x

leftCompare is left if left is a non-function value, else left(x)

rightCompare is right if right is a non-function value, else right(x)

y is true if leftCompare is greater than or equal to rightCompare

y is wrapped in a Promise if any of the following are true:

  • left is asynchronous
  • right is asynchronous
gte(
  x => x,
  10,
)(11) // => true

gte(
  async x => x,
  10,
)(11) // => Promise { true }

gte(
  x => x,
  10,
)(9) // => false

gte(2, 1)() // => true
gte(1, 1)() // => true
gte(0, 1)() // => false

lte

tests left less than or equal right

y = lte(left, right)(x)

x is anything

left is a non-function value or a function that expects one argument x

right is a non-function value or a function that expects one argument x

leftCompare is left if left is a non-function value, else left(x)

rightCompare is right if right is a non-function value, else right(x)

y is true if leftCompare is less than or equal to rightCompare

y is wrapped in a Promise if any of the following are true:

  • left is asynchronous
  • right is asynchronous
lte(
  x => x,
  10,
)(9) // => true

lte(
  async x => x,
  10,
)(9) // => Promise { true }

lte(
  x => x,
  10,
)(11) // => false

lte(0, 1)() // => true
lte(1, 1)() // => true
lte(2, 1)() // => false

get

accesses a property by path

y = get(path, defaultValue)(x)

x is an object

path is a number, string, a dot-delimited string, or an array

defaultValue is optional; if not provided, it is undefined

y depends on path:

  • if path is a number or string, y is x[path]
  • if path is a dot-delimited string 'p.a...t.h', y is x['p']['a']...['t']['h']
  • if path is an array ['p', 'a', ..., 't', 'h'], y is x['p']['a']...['t']['h']
  • if path is not found in x, y is defaultValue
get('a')({ a: 1, b: 2 }) // => 1

get('a')({}) // => undefined

get('a', 10)({}) // => 10

get(0)(['hello', 'world']) // => 'hello'

get('a.b.c')({ a: { b: { c: 'hey' } } }) // => 'hey'

get([0, 'user', 'id'])([
  { user: { id: '1' } },
  { user: { id: '2' } },
]) // => '1'

pick

constructs a new object from input composed of the provided properties

y = pick(properties)(x)

x is an object

properties is an array of strings

y is an object composed of all properties enumerated in properties and defined in x

pick(['a', 'b'])({ a: 1, b: 2, c: 3 }) // => { a: 1, b: 2 }

pick(['d'])({ a: 1, b: 2, c: 3 }) // => {}

omit

constructs a new object from input without the provided properties

y = omit(properties)(x)

x is an object

properties is an array of strings

y is an object composed of every property in x except for those enumerated in properties

omit(['a', 'b'])({ a: 1, b: 2, c: 3 }) // => { c: 3 }

omit(['d'])({ a: 1, b: 2, c: 3 }) // => { a: 1, b: 2, c: 3 }

Transducers

Transducers enable us to wrangle very large or infinite streams of data in a
composable and memory efficient way. Say you had veryBigData in an array

veryBigData = [...]
veryBigFilteredData = veryBigData.filter(datum => datum.isBig === true)
veryBigProcessedData = veryBigFilteredData.map(memoryIntensiveProcess)
console.log(veryBigProcessedData)

The above is not very memory efficient because of the intermediate arrays veryBigFilteredData
and veryBigProcessedData. We're also logging out a large quantity of data at once to the console.

With rubico, you could express the above transformation as a single pass
without incurring a memory penalty

veryBigData = [...]
transform(pipe([
  filter(datum => datum.isBig === true),
  map(memoryIntensiveProcess),
]), process.stdout)(veryBigData)

In this case, pipe([filter(...), map(...)]) is a transducer, and we're writing each datum
to the console via process.stdout. transform consumes our pipe([filter(...), map(...)])
transducer and supplies it with veryBigData.

Behind the scenes, transform is calling reduce with a reducing function suitable for writing
to process.stdout converted from the transducer pipe([filter(...), map(...)])

reducer is an alias for reducing function, very much the same as the one supplied to reduce

y = reduce(reducer)(x)

A reducer takes two arguments: an aggregate y and an iterative value xi.
It can be something like (y, xi) => doSomethingWith(y, xi)

A transducer is a function that takes a reducer and returns another reducer

transducer = reducer => (y, xi) => reducer(doSomethingWith(y, xi))

The transducer above, when passed a reducer, returns another reducer that will do something
with y and xi, then pass it to the input reducer

We can create a chained reducer by passing a reducer to a chain of transducers

Imagine dominos falling over. The reducer you pass to a chain of transducers is called last.
Because of this implementation detail,

if x is a function, pipe chains functions from right to left

You can use pipe to construct chains of transducers. Pipe will read left to right in all cases.

There are two other functions you'll need to get started with transducers, map and filter.

given x is a reducer, f is a mapping function; map(f)(x) is a transduced reducer
that applies f to each element in the final transform pipeline.

given x is a reducer, f is a predicate function; filter(f)(x) is a transduced reducer
that filters each element in the final transform pipeline based on f

The following transformations isOdd, square, and squaredOdds are used as transducers

const concat = (y, xi) => y.concat([xi])

const isOdd = filter(x => x % 2 === 1)

transform(isOdd, [])([1, 2, 3, 4, 5]) // => [1, 3, 5]
reduce(
  isOdd(concat),
  [],
)([1, 2, 3, 4, 5]) // => [1, 3, 5]

const square = map(x => x ** 2)

transform(square, [])([1, 2, 3, 4, 5]) // => [1, 4, 9, 16, 25]
reduce(
  square(concat),
  [],
)([1, 2, 3, 4, 5]) // => [1, 4, 9, 16, 25]

const squaredOdds = pipe([isOdd, square])

transform(squaredOdds, [])([1, 2, 3, 4, 5]) // => [1, 9, 25]
reduce(
  squaredOdds(concat),
  [],
)([1, 2, 3, 4, 5]) // => [1, 9, 25]

The following transformations isOdd, square, and squaredOdds are not used as transducers

const isOdd = filter(x => x % 2 === 1)

isOdd([1, 2, 3, 4, 5]) // => [1, 3, 5]

const square = map(x => x ** 2)

square([1, 2, 3, 4, 5]) // => [1, 4, 9, 16, 25]

const squaredOdds = pipe([isOdd, square])

squaredOdds([1, 2, 3, 4, 5]) // => [1, 9, 25]

More Examples

A webserver using map, transform, and https://deno.land/std/http serve

import { serve } from "https://deno.land/std/http/server.ts";
import { map, transform } from "https://deno.land/x/rubico/rubico.js"
const s = serve({ port: 8001 });
console.log("http://localhost:8001/");
transform(map(req => {
  req.respond({ body: "Hello World\n" });
}), null)(s);

A server with middleware

import { serve } from 'https://deno.land/std/http/server.ts'
import {
  pipe, fork, assign, tap, tryCatch, switchCase,
  map, filter, reduce, transform,
  any, all, and, or, not,
  eq, gt, lt, gte, lte,
  get, pick, omit,
} from 'https://deno.land/x/rubico/rubico.js'

const join = delim => x => x.join(delim)

const addServerTime = req => {
  req.serverTime = (new Date()).toJSON()
  return req
}

const traceRequest = pipe([
  fork([
    pipe([get('serverTime'), x => '[' + x + ']']),
    get('method'),
    get('url'),
  ]),
  join(' '),
  console.log,
])

const respondWithHelloWorld = req => {
  req.respond({ body: 'Hello World\n' })
}

const respondWithServerTime = req => {
  req.respond({ body: `The server time is ${req.serverTime}\n` })
}

const respondWithNotFound = req => {
  req.respond({ body: 'Not Found\n' })
}

const route = switchCase([
  eq('/', get('url')), respondWithHelloWorld,
  eq('/time', get('url')), respondWithServerTime,
  respondWithNotFound,
])

const onRequest = pipe([
  addServerTime,
  tap(traceRequest),
  route,
])

const s = serve({ port: 8001 })
console.log('http://localhost:8001/')
transform(map(onRequest), null)(s)