//  ) )     
    ___      ___      ___   __//__         
  //   ) ) //   ) ) ((   ) ) //   //   / / 
 //       //___/ /   \ \    //   ((___/ /  
((____   //       //   ) ) //        / /   

(Generated with http://patorjk.com/software/taag)

cpsfy

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.

Why?

• Functions are among the most basic and powerful objects in JavaScript.
• Callbacks are prominent for events and asynchronous functions, but they don't make composition convenient (leading to the so-called "callback hell").
• Promises are more convenient to compose but introduce overheads, such as conditionally calling then and consequently do not conform to functor or monad laws and thus are not safe for compositional refactoring.
• Promises introduce limitations of being able to return only one value only once, that makes it difficult to update them or use uniformly along with streams.
• Promises provide only one error handling callback, forcing to handle all errors in the same function, and thus making writing smaller focused functions and separating concerns more difficult.
• The recent async/await notation retains the overheads of promises, in addition to "new and exciting ways to shoot yourself in the foot".
• The present cpsfy library aims to provide unified and simple operators for all callback-based (aka Continuation-Passing-Style aka CPS) functions to make their composition as simple as with promises, while addressing the above limitations and overheads.

Quick demo

We want to read the content of the file 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.

const fs = require('fs')
// function returning CPS function with 2 callbacks
const readFile = file => (onRes, onErr) =>  
  fs.readFile(file, (e, name) => { // read file as string
    e ? onErr(e) : onRes(name)
  })

// CPS wraps a CPS function to provide the methods
const getLines = CPS(readFile('name.txt'))
  // map applies function to the file content
  .map(file => file.trim()) 
  .filter(file => file.length > 0)// only pass if nonempty
  // chain applies function that returns CPS function
  .chain(file => readFile(file))  // read file content
  .map(text => text.split('\n'))  // split into lines
// => CPS function with 2 callbacks

// To use, simply pass callbacks in the same order
getLines(
  lines => console.log(lines),  // result callback
  err => console.error(err)  // error callback
)
// Note how we handle error at the end 
// without affecting the main logic!

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

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

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

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

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

chaining

// as methods
CPS(cpsFn).map(f).chain(g).filter(h)

// or equivalently with 'pipeline' operator
pipeline(cpsFn)(
  map(f),
  chain(g),
  filter(h)
)

map(...functions)(cpsFunction)

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

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

chain(...functions)(cpsFunction)

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.

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

const writeFile = (file, encoding, content) =>
  // CPS function
  cb => fs.readFile(file, encoding, content, cb)

const copy = chain(
  // function that returns CPS function
  text => writFile('target.txt', 'utf8', text)
)(
  readFile('source.txt', 'utf8')  // CPS function
)
// or as method
const copy = CPS(readFile('source.txt', 'utf8'))
  .chain(text => writFile('target.txt', 'utf8', text))
// or with pipeline operator
const copy = pipeline(readFile('source.txt', 'utf8'))(
  chain(text => writFile('target.txt', 'utf8', 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

filter(...predicates)(cpsFunction)

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

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

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

scan(...reducers)(...initialValues)(cpsFunction)

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

// 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 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.

Example of scan

// CPS function with 2 callbacks, a 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
countVotes(
  upvotes => console.log(upvotes, ' votes for'),
  downvotes => console.log(downvotes, ' votes against'),
)

ap(...cpsFunctions)(cpsFunction) (TODO)

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.

License

MIT © Dmitri Zaitsev