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Tiny but powerful goodies for Continuation-Passing-Style (CPS) functions with functional composability backed by category theory foundations.

npm install cpsfy

(Or pnpm install cpsfy to save disc space.)

No dependency policy. For maximum security, this package is intended to be kept minimal and transparent with no dependencies ever.

Quick demo

We want to read the content of name.txt into string str and remove spaces from both ends of str. If the resulting str is nonempty, we read the content of the file with that name into string content, otherwise do nothing. Finally we split the content string into array of lines. If there are any errors on the way, we want to handle them at the very end in a separate function without any change to our main code.

//function returning CPS function with 2 callbacks
const readFileCps = file => (onRes, onErr) =>  
  require('fs').readFile(file, (err, content) => {
    err ? onErr(err) : onRes(content)

// the provided `CPS` operator wraps a CPS function to provide the API methods
const getLines = CPS(readFileCps('name.txt'))
  // map applies function to the file content
  .map(file => file.trim()) 
  .filter(file => file.length > 0)
  // chain applies function that returns a CPS function
  .chain(file => readFileCps(file))
  .map(text => text.split('\n'))
// => CPS function with 2 callbacks

// To use, simply pass callbacks in the same order
  lines => console.log(lines),  // onRes callback
  err => console.error(err)  // onErr callback

Note how we handle error at the end without affecting the main logic!

But can't I do it with promises?

Ok, let us have another example where you can't.😉 Reading from static files is easy but boring. Data is rarely static. What if we have to react to data changing in real time? Like our file names arriving as data stream? Let us use the popular websocket library:

const WebSocket = require('ws')
// general purpose CPS function listening to websocket
const wsMessageListenerCps = url => cb => 
  new WebSocket(url).on('message', cb)

And here is the crux:

wsMessageListenerCps(url) is just another CPS function!

So we can simply drop it instead of readFileCps('name.txt') into exactly the same code and be done with it:

const getLinesFromWS = CPS(wsMessageListenerCps(someUrl))
  .map(file => file.trim()) 
  .filter(file => file.length > 0)
  .chain(file => readFileCps(file))
  .map(text => text.split('\n'))

The new CPS function has only one callback, while the old one had two! Yet we have used exactly the same code! How so? Because we haven't done anything to other callbacks. The only difference is in how the final function is called - with one callback instead of two. As wsMessageListenerCps(url) accepts one callback, so does getLinesFromWS when we call it:

getLinesFromWS(lines => console.log(lines))

That will print all lines for all files whose names we receive from our websocket. And if we feel overwhelmed and only want to see lines containing say "breakfast", nothing can be easier:

// just add a filter
const breakfastLines = CPS(getLinesFromWS)
  .filter(line => /[Bb]reakfast/.test(line))
// call it exactly the same way
breakfastLines(lines => console.log(lines))

And from now on we'll never miss a breakfast.😄

CPS function

Any function

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 its callbacks 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 from the nth callback cbn:

// (x1, ..., xm) becomes output from nth callback whenever
cbn(x1, ..., xm)  // is called, where n = 1, 2, ..., m

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, ap, CPS, pipeline } 
  = require('cpsfy')

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 piped into 'map(f)' via 'pipeline' operator

The wrapped CPS function CPS(cpsFn) has all operators available as methods, while it remains a plain CPS function, i.e. can be called with the same callbacks:

CPS(cpsFn)(f1, f2, ...) // is equivalent to
cpsFn(f1, f2, ...)


Pass any number of arguments to a sequence of functions, one after another, similar to the UNIX pipe (x1, ..., xn) | f1 | f2 | ... | fm. Allows to write functional composition in the intiutive linear way.

Examples of pipeline

pipeline(1, 2)(
  (x, y) => x + y,
  sum => sum * 2,
  doubleSum => -doubleSum
) //=> -6


The CPS factory provides the same methods for simple chaining:


However, the need to wrap a plain function might feel as overhead. The pipeline operator allows to write the same code in functional style without the need to wrap:

// pass cpsFn directly as argument

But the most important advantage of the pipeline style is that you can drop there arbitrary functions without any need to patch object prototypes. For instance, using the above example, we can start our pipe with url or even insert some intermediate function to compute the correct url for us:

pipeline(path)(               // begin with path
  path => 'https://' + path  // compute url on the spot
  url => {console.log(url); return url} // check for debugging
  wsMessageListenerCps,       // return CPS function
  map(f),                     // use CPS operators as usual


// these are equivalent
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.

Result of applying map

New CPS function that calls its nth callback cbn as

cbn(fn(x1, x2, ...))

whenever cpsFn calls its nth callback.

Example of map

Using readFileCps as above.

// read file and convert all letters to uppercase
const getCaps = map(str => str.toUpperCase())(
// or
const getCaps = CPS(readFileCps('message.txt'))
  .map(str => str.toUpperCase())
// or
const getCaps = pipeline(readFileCps('message.txt'))(
  map(str => str.toUpperCase())

// getCaps is CPS function, call with any callback
getCaps((err, data) => err 
  ? console.error(err) 
  : console.log(data)
) // => file content is capitalized and printed


// these are equivalent
chain(f1, f2, ...)(cpsFn)
CPS(cpsFn).chain(f1, f2, ...)
pipeline(cpsFn)(chain(f1, f2, ...))

where each fn is a curried function

// fn 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.

Result of applying chain

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

Example of chain

Using readFileCps as above.

// write version of readFileCps
const writeFileCps = (file, content) => (onRes, onErr) =>  
  require('fs').writeFile(file, content, (err, message) => {
    err ? onErr(err) : onRes(message)

const copy = chain(
  // function that returns CPS function
  text => writeFileCps('target.txt', text)
  readFileCps('source.txt')  // CPS function
// or as method
const copy = CPS(readFileCps('source.txt'))
  .chain(text => writeFileCps('target.txt', text))
// or with pipeline operator
const copy = pipeline(readFileCps('source.txt'))(
  chain(text => writeFileCps('target.txt', text))

// copy is a CPS function, call it with any callback
copy((err, data) => err 
  ? console.error(err) 
  : console.log(data)
) // => file content is capitalized and printed


// these are equivalent
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.

Result of applying filter

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

Example of filter

Using readFileCps and writeFileCps as above.

// only copy text if it is not empty
const copyNotEmpty = CPS(readFileCps('source.txt'))
  .filter(text => text.length > 0)
  .chain(text => writeFileCps('target.txt', text))

// copyNotEmpty is CPS function, call with any callback
copyNotEmpty(err => console.error(err))


Similar to reduce, except that each accumulated values is passed into its callback whenever there is new output.

// these are equivalent
scan(ini1, init2, ...)(red1, red2, ...)(cpsFn)
(cpsFn).scan(ini1, init2, ...)(red1, red2, ...)
pipeline(cpsFn)(scan(ini1, init2, ...)(red1, red2, ...))

where each redn is a reducer and init is the initial accumulated value.

// compute new accumulator value from the old one 
// and the tuple of current values (y1, y2, ...)
const redn = (acc, y1, y2, ...) => ... 

Result of applying scan

New CPS function whose output from the first callback is the accumulated value. For each output (y1, y2, ...) from the nth callback, the nth reducer redn is used to compute the new acculated value redn(acc, y1, y2, ...), where acc starts with init, similar to reduce.

Example of scan

// CPS function with 2 callbacks, clicking on one
// of the buttons sends '1' into respective callback
const getVotes = (onUpvote, onDownvote) => {
    ev => onUpvote(1)
    ev => onDownvote(1)
// count numbers of up- and downvotes and 
// pass into respective callbacks
const countVotes = CPS(getVotes)
    ([up, down], upvote) => [up + upvote, down], 
    ([up, down], downvote) => [up, down + downvote]

// countVotes is CPS function that we can call 
// with any callback
  upvotes => console.log('Total upvotes: ', upvotes),
  downvotes => console.log('Total downvotes: ', downvotes),


See running CPS functions in parallel. Inspired by the Applicative Functor interface, see e.g. https://funkia.github.io/jabz/#ap

lift(...functions)(cpsFunction) (TODO)

See lifting functions of multiple arguments The "sister" of ap, apply functions with multiple arguments to outputs of CPS functions running in parallel, derived from ap, see e.g. https://funkia.github.io/jabz/#lift

merge(...cpsFunctions) (TODO)

See CPS.merge. Merge outputs from multiple CPS functions, separately in each callback. E.g. separately merge results and errors from multiple promises running in parallel.

More details?

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


MIT © Dmitri Zaitsev

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