Js Deferred/Promise/Future compared to functional languages like Scala

Question

I\'m mostly using programming languages like Scala and JavaScript. I\'m trying to understand the similarities and differences in how async reactive programming is used in bo

Answer 1

In contrast to Scala, the JS Promise is not a monad, due to the implicit "thenable" unwrapping breaking monadic law. You can, however, implement a callback-based monadic semantics and functionality, serving the same purpose.

See e.g. the cpsfy library.

In addition, there is a structural difference due to .then accepting 2 functions, while .chain accepts only one. However, a chain accepting 2 or even any number of argument functions can be implemented, like e.g. with CPS wrapper from cpsfy:

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

// CPS 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 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
getLines(
  lines => console.log(lines),  // onRes callback
  err => console.error(err)  // onErr callback
)