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Tiny but powerful goodies for Continuation-Passing-Style (CPS) functions

npm install tiny-cps

No dependency policy. For maximum security, this package is intended not to have any dependencies ever.

CPS function

Any function

//cb1, cb2, ... are called any number of times with any (possibly varying each time) number of arguments
const cpsFn = (cb1, cb2, ...) => { ... } 

that expects to be called with several (possibly zero) functions (callbacks) as arguments. The number of callbacks may vary each time cpsFn is called. Once called and running, cpsFn may call any of the callbacks cbn any (possibly zero) number of times with any number m of arguments (x1, ..., xm), where m may also vary from call to call. The m-tuple (vector) (x1, ..., xm) is regarded as the output of cpsFn passed to the nthe callback:

// (x1, ..., xm) becomes an output from the `n`th callback whenever
cbn(x1, ..., xm)  // is called

In other words, a CPS function receives any number of callbacks that it may call in any order any number of times at any moments immediately or in the future with any number of arguments.

API in brief

const { map, chain, filter, scan, CPS, pipeline } = require('tiny-cps')

Each of the map, chain, filter, scan operators can be used in 3 ways:

// 'map' as curried function
// 'map' method provided by the 'CPS' wrapper
// 'cpsFn' is passed as value ("piped") into 'map(f)' via 'pipeline' operator


map(f1, f2, ...)(cpsFn)
CPS(cpsFn).map(f1, f2, ...)
pipeline(cpsFn)(map(f1, f2, ...))

For each n, apply fn to each output from the nth callback of cpsFn. The result is the new CPS function that calls its nth callback cbn as

cbn(fn(x1, x2, ...))

whenever cpsFn calls its nth callback.

const fs = require('fs')
const readFile = (file, encoding) =>
  cb => fs.readFile(file, encoding, cb)   // CPS function
// read file and convert all letters to uppercase
const getCaps = map(str => str.toUpperCase())(
  readFile('message.txt', 'utf8')
// or
const getCaps = CPS(readFile('message.txt', 'utf8'))
  .map(str => str.toUpperCase())
// or
const getCaps = pipeline(readFile('message.txt', 'utf8'))(
  map(str => str.toUpperCase())
// getCaps is a CPS function, so we just call it with any callback
  (err, data) => err ? console.error(err) : console.log(data)
) // => file content is capitalized and printed to console


chain(f1, f2, ...)(cpsFn)
CPS(cpsFn).chain(f1, f2, ...)
pipeline(cpsFn)(chain(f1, f2, ...))

where each fn is a curried function

// fn(x1, x2, ...) is expected to return a CPS function
const fn = (x1, x2, ...) => (cb1, cb2, ...) => { ... }

The chain operator applies each fn to each output from the nth callback of cpsFn, however, the CPS ouptup of fn is passed ahead instead of the return value. The chain operator returns the new CPS function newCpsFn that calls fn(x1, x2, ...) whenever cpsFn passes output (x1, x2, ...) into its nth callback, and collects all outputs from all callbacks of all fns. Then for each fixed m, outputs from the mth callbacks of all fns are collected and passed into the mth callback cbm of newCpsFn:

cbm(y1, y2, ...)  // is called whenever 
cbmFn(y1, y2, ...)  // is called where
// cbmFn is the mth callback of fn
const writeFile = (file, encoding, content) =>
  cb => fs.readFile(file, encoding, content, cb)   // CPS function
const copy = chain(
  // function that returns CPS function
  text => writFile('target.txt', 'utf8', text)
  readFile('source.txt', 'utf8')  // CPS function
// or
const copy = CPS(readFile('source.txt', 'utf8'))
  .chain(text => writFile('target.txt', 'utf8', text))
// or
const copy = pipeline(readFile('source.txt', 'utf8'))(
  chain(text => writFile('target.txt', 'utf8', text))
// copy is a CPS function, so we just call it with any callback
  (err, data) => err ? console.error(err) : console.log(data)
) // => file content is capitalized and printed to console


filter(pred1, pred2, ...)(cpsFn)
CPS(cpsFn).filter(pred1, pred2, ...)
pipeline(cpsFn)(filter(pred1, pred2, ...))

where each predn is the nth predicate function used to filter output from the nth callback of cpsFn. The result is the new CPS function that calls its nth callback cbn(x1, x2, ...) whenever (x1, x2, ...) is an output from the nth callback of cpsFun and

predn(x1, x2, ...) == true
// only copy if text is not empty
const copyNotEmpty = CPS(readFile('source.txt', 'utf8'))
  .filter(text => text.length > 0)
  .chain(text => writFile('target.txt', 'utf8', text))


Similar to reduce, except that all partial accumulated values are passed into callback whenever there is new output.

scan(red1, red2, ...)(x1, x2, ...)(cpsFn)
(cpsFn).scan(red1, red2, ...)(x1, x2, ...)
pipeline(cpsFn)(scan(red1, red2, ...)(x1, x2, ...))

where each redn is a reducer

// take accumulated value 'acc' and input (y1, y2, ...) and return new acc. value
const redn = (acc, y1, y2, ...) => ... 

The result is the new CPS function whose output from the nthe callback is the nth accumulated value accn. Upon each output (y1, y2, ...), the new acculated value redn(accn, y1, y2, ...) is computed and passed into the callback. The nth value xn serves in place of acc at the start, similar to reduce. Note that the initial values (x1, x2, ...) must be passed as curried arguments to avoid getting mixed with reducers.

// CPS function with 2 callbacks
// each click on one of the buttons sends '1' into respective callback
const getVotes = (onUpvote, onDownvote) => {
  upvoteButton.addEventListener('click', ev => onUpvote(1))
  downvoteButton.addEventListener('click', ev => onDownvote(1))  
const add = (acc, x) => acc + x
// count numbers of up- and downvotes and pass into respective callbacks
const countVotes = scan(add, add)(0, 0)(getVotes) // or
const countVotes = CPS(getVotes).scan(add, add)(0, 0)
// countVotes is CPS function that we can call with any pair of callbacks
  upvotes => console.log(upvotes, ' people voted for'),
  downvotes => console.log(downvotes, ' people voted against'),

More details?

This is kept minimal to reduce the package size. For more human introduction, motivation, use cases and other details, please see DOCUMENTATION.


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