rubico

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

[a]synchronous functional syntax; the [a] is for optionally asynchronous

Motivation

A note from the author

At a certain point in my career, I grew frustrated with the entanglement of my own code. While looking for something better, I found functional programming. I was excited by the idea of functional composition, but disillusioned by the complex hierarchy of effectful types. I started this project to capitalize on the prior while rebuking the latter. Many iterations since then, the library has grown into something I personally enjoy using, and continue to use to this day.

rubico's value resides at the intersection of the following principles:

When you use this library, you obtain the freedom that comes only from having those three points fulfilled. Just as a compiler would optimize the performance of the code it compiles, each method in this library strives to optimize the JavaScript it exports. The result is something you may enjoy.

Introduction

Tour

Installation

Documentation

Transducers

Examples

programs written with rubico follow a point-free style, otherwise known as data last.

This is data first

[1, 2, 3, 4, 5].map(number => number * 2) // => [2, 4, 6, 8, 10]

This is data last

map(number => number * 2)([1, 2, 3, 4, 5]) // => [2, 4, 6, 8, 10]

Data last enables modularity by function composition. This is a powerful concept for code reuse.

const square = x => x ** 2

const isOdd = x => x % 2 === 1

const squaredOdds = pipe([
  filter(isOdd),
  map(square),
])

Installation

with npm

npm i rubico

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'

browser script, global rubico

<script src="https://unpkg.com/rubico/index.js" crossorigin></script>

browser module

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Your Title Here</title>
    <script type="module">
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'

// your code here
    </script>
</head>
<body></body>
</html>

System Requirements

  • minimum node version: 10.3
  • minimum Chrome version: 63
  • minimum Firefox version: 57
  • minimum Edge version: 79
  • minimum Safari version: 11.1

Documentation

rubico aims to hit the sweet spot between expressivity and interface surface area. There are 23 functions at the moment; this number is not expected to go up much more or at all. Instead, some methods will have property functions that represent the same signature (i.e. map vs map.series) but exhibit differences in behavior (i.e. map executes in parallel while map.series executes in series).

[series] and [parallel] are tags to denote the asynchronous behavior of methods that accept multiple functions.

  • [series]: execute functions one at a time. If order is not implied, it is left to the implementation. (i.e. iterating an Object)
  • [parallel]: execute functions in parallel.

All higher order functions accept sync or async functions; if all provided functions are synchronous, the entire execution is synchronous.

function composition

  • pipe [series] - chain functions together
  • tap - spy on data
  • tryCatch [series] - try a function, catch with another
  • switchCase [series] - control flow

function + data composition

  • fork [parallel] - multiply data by functions
    • fork.series [series]
  • assign [parallel] - set properties on data by functions

data transformation

  • map [parallel] - apply a function to data
    • map.pool [parallel] - map with asynchronous limit
    • map.withIndex [parallel] - map with index
    • map.series [series]
    • map.sleep [series] - map.series with delay in between todo
  • filter [parallel] - exclude data by predicate
    • filter.withIndex [parallel] - filter with index
  • reduce [series] - execute data transformation (powerful)
  • transform [series] - execute data transformation (convenient)

predicate composition

  • any [parallel] - is function of any data truthy?
  • all [parallel] - is function of all data truthy?
  • and [parallel] - any functions of data truthy?
  • or [parallel] - all functions of data truthy?
  • not - not(equals)(x) is !equals(x)

comparison

  • eq [parallel] - left equals right?
  • gt [parallel] - left > right?
  • lt [parallel] - left < right?
  • gte [parallel] - left >= right?
  • lte [parallel] - left <= right?

property + index access

  • get - access a value by path or index

data composition

  • pick - only allow provided properties
    • pick.range - only allow provided range todo
  • omit - exclude provided properties

pipe

chains functions from left to right in series

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' }

back to documentation

fork

parallelizes functions with data, 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' } }

fork.series executes functions with data in series, retaining functions' type and shape

y = fork.series(functions)(x)

back to documentation

assign

parallelizes functions with data, merging output into data

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' }

back to documentation

tap

calls a function with data, returning data

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]

back to documentation

tryCatch

tries a function with data, 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' }

back to documentation

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' }

back to documentation

map

applies a function to each element of data in parallel, retaining data 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' }

map.pool Apply a function to every element of data in parallel with limited concurrency

y = map.pool(size, f)(x)

map.withIndex Apply a function to every element of data in parallel with index and reference to data

y = map.withIndex(f)(x); yi = f(xi, i, x)

map.series Apply a function to every element of data in series

y = map.series(f)(x)

map.seriesWithIndex Apply a function to every element of data in series with index and reference to data

 = map.seriesWithIndex(f)(x); yi = f(xi, i, x)

back to documentation

filter

filters elements out of data in parallel 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 }

filter.withIndex Filter, but with each predicate called with index and reference to data

y = filter.withIndex(f)(x); yi = f(xi, i, x)

back to documentation

reduce

transforms data in series 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]

back to documentation

transform

transforms data in series 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 }

back to documentation

any

applies a function to each element of data parallel, 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

back to documentation

all

applies a function to each element of data in parallel, 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

back to documentation

and

applies each function of functions in parallel to data, 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

back to documentation

or

applies each function of functions in parallel to data, 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

back to documentation

not

applies a function to data, 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

back to documentation

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

back to documentation

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

back to documentation

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

back to documentation

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

back to documentation

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

back to documentation

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'

back to documentation

pick

constructs a new object from data 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 }) // => {}

back to documentation

omit

constructs a new object from data 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]

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)